Positive electrode active material and preparation method thereof, positive electrode plate, sodium battery and energy storage device

Abstract

The invention provides a positive active material and a preparation method thereof, a positive pole piece, a sodium battery and an energy storage device. The positive electrode active material disclosed by the embodiment of the invention comprises ferric sodium phosphate pyrophosphate particles and a carbon coating layer, wherein the carbon coating layer is coated on the surfaces of the ferric sodium phosphate pyrophosphate particles; in the positive electrode active material, the content of potassium element is less than or equal to 100ppm.

Description

Technical Field

[0001] The present application relates to the field of energy storage, and specifically to a positive electrode active material and a preparation method thereof, a positive electrode plate, a sodium battery and an energy storage device. Background Art

[0002] Sodium iron phosphate pyrophosphate (Na4Fe3(PO4)2P2O7), which has a three-dimensional sodium ion diffusion channel and a sodium superion conductor structure, has high voltage platform, high capacity, and excellent rate and cycle stability, making it a promising cathode material for large-scale production in sodium batteries. However, Na4Fe3(PO4)2P2O7 powder has high resistivity and poor conductivity. When used in sodium batteries, this leads to excessive internal resistance and reduced cycle life. Summary of the Invention

[0003] The embodiments of the present application provide a positive electrode active material having low powder resistivity.

[0004] In a first aspect, an embodiment of the present application provides a positive electrode active material, which includes sodium iron phosphate pyrophosphate particles and a carbon coating layer, wherein the carbon coating layer is wrapped around the surface of the sodium iron phosphate pyrophosphate particles; in the positive electrode active material, the content of potassium element is less than or equal to 100 ppm.

[0005] Furthermore, the content of boron in the positive electrode active material is less than or equal to 100 ppm.

[0006] Furthermore, the content of sulfur in the positive electrode active material is less than or equal to 200 ppm.

[0007] Furthermore, in the X-ray diffraction pattern of the positive electrode active material, the maximum intensity of the diffraction peak at a diffraction angle 2θ of 10.5° to 10.9° is I1, the maximum intensity of the diffraction peak at a diffraction angle 2θ of 32.8° to 33.2° is I2, and the maximum intensity of the diffraction peak at a diffraction angle 2θ of 33.3° to 33.8° is I3, then the positive electrode active material satisfies the relationship: P = (I1+I2) / I3≤0.1.

[0008] Furthermore, the powder resistivity R of the positive electrode active material is in the range of 70Ω·cm≤R≤800Ω·cm.

[0009] In a second aspect, an embodiment of the present application provides a method for preparing a positive electrode active material, which comprises:

[0010] Provide sodium source, phosphorus source, iron source and carbon source;

[0011] mixing the sodium source, the phosphorus source, the iron source, and the carbon source in a solvent to obtain a slurry;

[0012] spray drying the slurry to obtain a precursor powder; and

[0013] The precursor powder is sintered to obtain the positive electrode active material; the positive electrode active material includes sodium iron phosphate pyrophosphate particles and a carbon coating layer, and the carbon coating layer is wrapped around the surface of the sodium iron phosphate pyrophosphate particles; the content of potassium in the positive electrode active material is less than or equal to 100 ppm.

[0014] Furthermore, the content of potassium in the sodium source is less than or equal to 20 ppm, the content of potassium in the phosphorus source is less than or equal to 20 ppm, and the content of potassium in the iron source is less than or equal to 60 ppm.

[0015] Furthermore, the content of boron in the sodium source is less than or equal to 20 ppm, the content of boron in the phosphorus source is less than or equal to 20 ppm, and the content of boron in the iron source is less than or equal to 60 ppm.

[0016] Furthermore, the sulfur content in the sodium source is less than or equal to 40 ppm, the sulfur content in the phosphorus source is less than or equal to 40 ppm, and the sulfur content in the iron source is less than or equal to 120 ppm.

[0017] Furthermore, the step of sintering the precursor powder to obtain the positive electrode active material comprises:

[0018] The precursor powder is sintered at a temperature of 580° C. to 630° C. to obtain the positive electrode active material.

[0019] In a third aspect, an embodiment of the present application provides a positive electrode plate, the positive electrode plate comprising:

[0020] a positive electrode current collector; and

[0021] The positive electrode active layer comprises the positive electrode active material described in the first aspect of the present application or the positive electrode active material prepared by the preparation method of the positive electrode active material described in the second aspect of the present application.

[0022] In a fourth aspect, an embodiment of the present application provides a sodium battery, which includes: an electrolyte, the positive electrode plate described in the third aspect of the present application, a separator, and a negative electrode plate.

[0023] In a fifth aspect, an embodiment of the present application provides an energy storage device, comprising:

[0024] cabinet; and

[0025] The sodium battery described in the fourth aspect of the present application is housed in the box.

[0026] The positive electrode active material of the embodiment of the present application includes positive electrode particles, the positive electrode particles include sodium iron phosphate pyrophosphate particles and a carbon coating layer, the carbon coating layer is wrapped around the surface of the sodium iron phosphate pyrophosphate particles; the content of potassium in the positive electrode active material is less than or equal to 100 ppm. By controlling the content of potassium in the positive electrode active material to be less than or equal to 100 ppm, the decomposition of sodium iron phosphate pyrophosphate can be well suppressed during sintering during the preparation of the positive electrode active material, the decomposition temperature of sodium iron phosphate pyrophosphate can be increased, the formation of sodium iron phosphate and sodium iron pyrophosphate impurities can be better reduced, and the gram capacity of the positive electrode active material can be increased. In addition, the positive electrode active material can be sintered at a higher temperature during the preparation process, so that the carbon coating layer of the positive electrode active material has a higher electron transfer rate and a lower powder resistivity. Furthermore, since the positive electrode active material has a lower powder resistivity, when the positive electrode active material is applied to the positive electrode sheet, while ensuring that the resistivity of the positive electrode sheet remains unchanged, the amount of expensive positive electrode conductive agents such as carbon nanotubes in the positive electrode active layer of the positive electrode sheet can be reduced, thereby greatly reducing the cost of the positive electrode sheet. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present application. For ordinary technicians in this field, other drawings can be obtained based on these drawings without creative work.

[0028] Figure 1 Schematic diagram of the structure of the positive electrode active material according to one embodiment of the present application.

[0029] Figure 2 Schematic diagram of a process for preparing a positive electrode active material according to an embodiment of the present application.

[0030] Figure 3 It is a structural schematic diagram of the positive electrode plate of an embodiment of the present application.

[0031] Figure 4 Schematic diagram of the structure of a sodium battery according to an embodiment of the present application.

[0032] Figure 5 This is a sodium battery according to an embodiment of the present application. Figure 4 Schematic diagram of the cross-sectional structure in the AA direction.

[0033] Figure 6 Schematic diagram of the structure of the negative electrode sheet of one embodiment of the present application.

[0034] Figure 7 It is a structural diagram of an energy storage device according to an embodiment of the present application.

[0035] Figure 8 This is a structural block diagram of an energy storage system according to an embodiment of the present application.

[0036] Figure 9 This is an application scenario diagram of the energy storage system of one embodiment of the present application.

[0037] Description of reference numerals:

[0038] 100-positive electrode active material, 10-sodium iron pyrophosphate particles, 20-carbon coating layer, 200-positive electrode plate, 210-positive electrode current collector, 220-positive electrode active layer, 300-sodium battery, 320-diaphragm, 330-negative electrode plate, 331-negative electrode current collector, 332-negative electrode active layer, 340-shell, 350-end cover assembly, 400-energy storage device, 410-case, 500-energy storage system, 510-electric energy conversion device. DETAILED DESCRIPTION

[0039] In order to enable those skilled in the art to better understand the present invention, the following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of this application.

[0040] The terms "first," "second," and the like in the specification and claims of this application and the accompanying drawings are used to distinguish between different objects, not to describe a particular order. Furthermore, the terms "including," "having," and any variations thereof, are intended to cover non-exclusive inclusions. For example, a process, method, system, product, or apparatus comprising a series of steps or elements is not limited to the listed steps or elements but may optionally include steps or elements not listed, or may optionally include other steps or elements inherent to the process, method, product, or apparatus.

[0041] The technical solutions in the embodiments of the present application will be described below with reference to the accompanying drawings.

[0042] It should be noted that, for the convenience of explanation, in the embodiments of the present application, the same reference numerals represent the same components, and for the sake of brevity, detailed descriptions of the same components are omitted in different embodiments.

[0043] Currently, the generation of green electricity generally relies on photovoltaics, wind power, and hydropower. However, wind and solar energy are generally intermittent and highly volatile, which can cause grid instability, insufficient electricity during peak hours, and excessive electricity during off-peak hours. Unstable voltage can also damage electricity. Therefore, insufficient electricity demand or insufficient grid capacity may lead to the problem of "wind and solar power curtailment". To solve these problems, we must rely on energy storage. This means converting electrical energy into other forms of energy through physical or chemical means and storing them. When needed, this energy is converted into electrical energy and released. Simply put, energy storage is like a large "power bank", storing electricity when photovoltaic and wind energy are sufficient and releasing the stored electricity when needed.

[0044] Batteries are the smallest energy storage unit in energy storage devices and systems. Their performance directly impacts the performance and applications of these devices and systems. Batteries include lithium batteries and sodium batteries.

[0045] Sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7, which has a three-dimensional sodium ion diffusion channel and a sodium superion conductor structure, has the characteristics of a high voltage platform, high capacity, excellent rate and cycle stability, and has great potential to become a positive electrode material for large-scale production in sodium batteries. However, the powder resistivity of Na4Fe3(PO4)2P2O7 is high and the conductivity is poor. When applied to sodium batteries, the internal resistance of the sodium battery is too large, which reduces the cycle life of the sodium battery. In the related art, the powder resistivity of sodium iron phosphate pyrophosphate is reduced by carbon coating, and the electron transfer efficiency of sodium iron phosphate pyrophosphate is improved. However, the conductivity of the carbon coating layer depends on the carbonization temperature. However, if the carbonization temperature is too high, the sodium iron phosphate pyrophosphate is easily decomposed to generate low-activity iron phosphate nano-hybrid phase and sodium iron pyrophosphate hybrid phase, resulting in a significant reduction in the gram capacity of sodium iron phosphate pyrophosphate. In view of this, the embodiment of the present application provides a positive electrode active material.

[0046] See Figure 1 , an embodiment of the present application provides a positive electrode active material 100, the positive electrode active material 100 includes sodium iron phosphate pyrophosphate particles 10 and a carbon coating layer 20, the carbon coating layer 20 is wrapped around the surface of the sodium iron phosphate pyrophosphate particles 10; in the positive electrode active material 100, the content of potassium element is less than or equal to 100 ppm.

[0047] The positive electrode active material 100 of the embodiment of the present application can be applied to a sodium battery (such as a sodium ion battery) as an active material for a positive electrode sheet of the sodium battery.

[0048] It should be noted that the sodium iron pyrophosphate particles 10 and the carbon coating layer 20 form a core-shell structure, that is, the sodium iron pyrophosphate particles 10 are the core and the carbon coating layer 20 is the shell. In other words, the positive electrode particles have a core-shell structure.

[0049] Specifically, the content of potassium element in the positive electrode active material 100 can be, but is not limited to, less than or equal to 100 ppm, less than or equal to 95 ppm, less than or equal to 90 ppm, less than or equal to 85 ppm, less than or equal to 80 ppm, less than or equal to 75 ppm, less than or equal to 70 ppm, less than or equal to 65 ppm, less than or equal to 60 ppm, less than or equal to 55 ppm, less than or equal to 50 ppm, less than or equal to 45 ppm, less than or equal to 40 ppm, etc.

[0050] When the potassium content in the positive electrode active material 100 is low, the decomposition of sodium iron phosphate pyrophosphate during the synthesis of the positive electrode active material 100 can be effectively inhibited, the decomposition temperature of sodium iron phosphate pyrophosphate can be increased, the generation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities can be better reduced, and the gram capacity of the positive electrode active material 100 can be increased. In addition, the positive electrode active material 100 can be sintered at a higher temperature, so that the carbon coating layer 20 of the positive electrode active material 100 has a higher electron transfer rate (as the sintering temperature increases, the carbonization degree of the carbon coating layer 20 increases, and the electrical conductivity increases) and has a lower powder resistivity; however, if the potassium content in the positive electrode active material 100 is too low, the purity of the raw materials and the preparation process conditions of the positive electrode active material 100 are too stringent, which increases the preparation cost of the positive electrode active material 100. If the potassium content in the positive electrode active material 100 is too high, the probability of decomposition of sodium iron phosphate pyrophosphate increases during the synthesis of the positive electrode active material 100, thereby increasing the content of the mixed phase (sodium iron phosphate mixed phase and sodium iron pyrophosphate mixed phase) in the sodium iron phosphate pyrophosphate particles 10, thereby reducing the gram capacity of the positive electrode active material 100; in addition, when the potassium content in the positive electrode active material 100 is too high, since the decomposition ratio of sodium iron phosphate pyrophosphate increases, it is necessary to lower the sintering temperature when preparing the positive electrode active material 100 to ensure that the synthesized positive electrode active material 100 has a higher proportion of sodium iron phosphate pyrophosphate phase and the positive electrode active material 100 has a higher gram capacity; however, lowering the sintering temperature will reduce the electron transfer rate of the carbon coating layer 20 and reduce the conductivity of the carbon coating layer 20, thereby increasing the powder resistivity of the positive electrode active material 100, which reduces the cycle life of the sodium battery when used in a sodium battery.

[0051] The positive electrode active material 100 of the embodiment of the present application includes positive electrode particles, each including sodium iron phosphate pyrophosphate particles 10 and a carbon coating layer 20, wherein the carbon coating layer 20 is coated on the surface of the sodium iron phosphate pyrophosphate particles 10. The potassium content in the positive electrode active material 100 is less than or equal to 100 ppm. By controlling the potassium content in the positive electrode active material 100 to be less than or equal to 100 ppm, the decomposition of sodium iron phosphate pyrophosphate can be effectively suppressed during sintering during the preparation of the positive electrode active material 100, the decomposition temperature of sodium iron phosphate pyrophosphate can be increased, the formation of sodium iron phosphate and sodium iron pyrophosphate impurities can be effectively reduced, and the gram capacity of the positive electrode active material 100 can be increased. In addition, the positive electrode active material 100 can be sintered at a higher temperature during the preparation process, so that the carbon coating layer 20 of the positive electrode active material 100 has a higher electron transfer rate and a lower powder resistivity. Furthermore, since the positive electrode active material 100 has a lower powder resistivity, when the positive electrode active material 100 is applied to the positive electrode sheet, while ensuring that the resistivity of the positive electrode sheet remains unchanged, the amount of expensive positive electrode conductive agents such as carbon nanotubes in the positive electrode active layer of the positive electrode sheet can be reduced, thereby greatly reducing the cost of the positive electrode sheet.

[0052] In some embodiments, the boron content in the positive electrode active material 100 is less than or equal to 100 ppm.

[0053] Specifically, the content of boron element in the positive electrode active material 100 can be, but is not limited to, less than or equal to 100 ppm, less than or equal to 95 ppm, less than or equal to 90 ppm, less than or equal to 85 ppm, less than or equal to 80 ppm, less than or equal to 75 ppm, less than or equal to 70 ppm, less than or equal to 65 ppm, less than or equal to 60 ppm, less than or equal to 55 ppm, less than or equal to 50 ppm, less than or equal to 45 ppm, less than or equal to 40 ppm, etc.

[0054] In this embodiment, when the boron content in the positive electrode active material 100 is low, the decomposition of sodium iron phosphate pyrophosphate during the synthesis of the positive electrode active material 100 can be effectively suppressed, the decomposition temperature of sodium iron phosphate pyrophosphate can be increased, the generation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities can be better reduced, and the gram capacity of the positive electrode active material 100 can be increased. In addition, the positive electrode active material 100 can be sintered at a higher temperature, so that the carbon coating layer 20 of the positive electrode active material 100 has a higher electron transfer rate and a lower powder resistivity; however, if the boron content in the positive electrode active material 100 is too low, the purity of the raw materials and the preparation process conditions of the positive electrode active material 100 are too stringent, which increases the preparation cost of the positive electrode active material 100. If the boron content in the positive electrode active material 100 is too high, the probability of decomposition of sodium ferric phosphate pyrophosphate increases during the synthesis of the positive electrode active material 100, thereby increasing the content of the mixed phase (sodium ferric phosphate mixed phase and sodium ferric pyrophosphate mixed phase) in the sodium ferric phosphate pyrophosphate particles 10, thereby reducing the gram capacity of the positive electrode active material 100; in addition, when the boron content in the positive electrode active material 100 is too high, since the decomposition ratio of sodium ferric phosphate pyrophosphate increases, it is necessary to lower the sintering temperature when preparing the positive electrode active material 100 to ensure that the synthesized positive electrode active material 100 has a higher proportion of sodium ferric phosphate pyrophosphate phase and the positive electrode active material 100 has a higher gram capacity; however, lowering the sintering temperature will reduce the electron transfer rate of the carbon coating layer 20 and reduce the conductivity of the carbon coating layer 20, thereby increasing the powder resistivity of the positive electrode active material 100, which reduces the cycle life of the sodium battery when used in a sodium battery.

[0055] In some embodiments, the content of sulfur in the positive electrode active material 100 is less than or equal to 200 ppm.

[0056] Specifically, the content of sulfur element in the positive electrode active material 100 can be, but is not limited to, less than or equal to 200 ppm, less than or equal to 190 ppm, less than or equal to 180 ppm, less than or equal to 170 ppm, less than or equal to 160 ppm, less than or equal to 150 ppm, less than or equal to 140 ppm, less than or equal to 130 ppm, less than or equal to 120 ppm, less than or equal to 110 ppm, less than or equal to 100 ppm, less than or equal to 90 ppm, less than or equal to 80 ppm, less than or equal to 70 ppm, less than or equal to 60 ppm, less than or equal to 50 ppm, etc.

[0057] In this embodiment, when the sulfur content in the positive electrode active material 100 is low, the decomposition of sodium iron phosphate pyrophosphate during the synthesis of the positive electrode active material 100 can be effectively suppressed, the decomposition temperature of sodium iron phosphate pyrophosphate can be increased, the generation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities can be better reduced, and the gram capacity of the positive electrode active material 100 can be increased. In addition, the positive electrode active material 100 can be sintered at a higher temperature, so that the carbon coating layer 20 of the positive electrode active material 100 has a higher electron transfer rate and a lower powder resistivity; however, if the sulfur content in the positive electrode active material 100 is too low, the purity of the raw materials and the preparation process conditions of the positive electrode active material 100 are too stringent, which increases the preparation cost of the positive electrode active material 100. If the sulfur content in the positive electrode active material 100 is too high, the probability of decomposition of sodium iron phosphate pyrophosphate increases during the synthesis of the positive electrode active material 100, thereby increasing the content of the mixed phase (sodium iron phosphate mixed phase and sodium iron pyrophosphate mixed phase) in the sodium iron phosphate pyrophosphate particles 10, thereby reducing the gram capacity of the positive electrode active material 100; in addition, when the sulfur content in the positive electrode active material 100 is too high, since the decomposition ratio of sodium iron phosphate pyrophosphate increases, it is necessary to lower the sintering temperature when preparing the positive electrode active material 100 to ensure that the synthesized positive electrode active material 100 has a higher proportion of sodium iron phosphate pyrophosphate phase and the positive electrode active material 100 has a higher gram capacity; however, lowering the sintering temperature will reduce the electron transfer rate of the carbon coating layer 20 and reduce the conductivity of the carbon coating layer 20, thereby increasing the powder resistivity of the positive electrode active material 100, which reduces the cycle life of the sodium battery when used in a sodium battery.

[0058] It can be understood that when the positive electrode active material 100 satisfies at least one of the following conditions: the potassium content is less than or equal to 100 ppm, the boron content is less than or equal to 100 ppm, and the sulfur content is less than or equal to 200 ppm, the powder resistivity of the positive electrode active material 100 can be reduced and the gram capacity of the positive electrode active material 100 can be increased.

[0059] In addition, compared to the embodiment in which only one of the potassium content is less than or equal to 100 ppm, the boron content is less than or equal to 100 ppm, and the sulfur content is less than or equal to 200 ppm is met, when the positive electrode active material 100 satisfies at least two or three of the potassium content is less than or equal to 100 ppm, the boron content is less than or equal to 100 ppm, and the sulfur content is less than or equal to 200 ppm, the synergistic effect of the two or three can better increase the decomposition temperature of sodium iron pyrophosphate and increase the gram capacity of the positive electrode active particles; it can also increase the sintering temperature, so that the carbon coating layer 20 of the positive electrode active material 100 has a higher electron transfer rate and a lower powder resistivity.

[0060] It should be noted that the contents of potassium, boron, and sulfur in the positive electrode active material 100 and its raw materials (such as sodium source, phosphorus source, and iron source) are measured using an inductively coupled plasma optical emission spectrometer (ICP-OES). Specifically, 0.2 to 0.3 g of the positive electrode active material 100 powder is dissolved in a strong acid, and the resulting solution is then tested in an ICP-OES instrument to determine the content of each impurity element.

[0061] In some embodiments, in the X-ray diffraction pattern (XRD pattern) of the positive electrode active material 100, the maximum intensity of the diffraction peak at a diffraction angle 2θ of 10.5° to 10.9° is I1, the maximum intensity of the diffraction peak at a diffraction angle 2θ of 32.8° to 33.2° is I2, and the maximum intensity of the diffraction peak at a diffraction angle 2θ of 33.3° to 33.8° is I3, then the positive electrode active material 100 satisfies the relationship: P = (I1 + I2) / I3 ≤ 0.1.

[0062] It should be noted that, in the X-ray diffraction pattern, the diffraction peak at a diffraction angle 2θ of 10.5° to 10.9° is the diffraction peak of sodium ferric pyrophosphate; the diffraction peak at a diffraction angle 2θ of 32.8° to 33.2° is the diffraction peak of sodium ferric phosphate; and the diffraction peak at a diffraction angle 2θ of 33.3° to 33.8° is the diffraction peak of sodium ferric pyrophosphate.

[0063] It should be noted that P can be understood as the proportion factor of the impurity phase in the positive electrode active material 100, that is, the impurity phase factor. The smaller P is, the less the content of the impurity phase in the positive electrode active material 100 is, and the positive electrode active material 100 has a higher gram capacity. However, when P is too small, the sintering temperature during the preparation of the positive electrode active material 100 needs to be set lower, which will reduce the conductivity of the carbon coating layer 20; conversely, the larger P is, the higher the content of the impurity phase in the positive electrode active material 100 is, and the positive electrode active material 100 has a lower gram capacity.

[0064] Specifically, the value of (I1+I2) / I3 can be, but is not limited to, less than or equal to 0.1, less than or equal to 0.095, less than or equal to 0.09, less than or equal to 0.085, less than or equal to 0.08, less than or equal to 0.075, less than or equal to 0.07, less than or equal to 0.065, less than or equal to 0.06, less than or equal to 0.055, less than or equal to 0.05, less than or equal to 0.045, less than or equal to 0.04, less than or equal to 0.035, less than or equal to 0.03, less than or equal to 0.025, less than or equal to 0.02, etc.

[0065] In this embodiment, when (I1+I2) / I3 is too large, the content of the sodium iron phosphate mixed phase and the sodium iron pyrophosphate mixed phase in the positive electrode active material 100 is too high, thereby reducing the gram capacity of the positive electrode active material 100.

[0066] Optionally, the positive electrode active material 100 further satisfies the relationship: P1 = I1 / I3 ≤ 0.05. Specifically, the value of I1 / I3 can be, but is not limited to, less than or equal to 0.05, less than or equal to 0.045, less than or equal to 0.04, less than or equal to 0.035, less than or equal to 0.03, less than or equal to 0.025, less than or equal to 0.02, less than or equal to 0.015, less than or equal to 0.01, etc. If the value of I1 / I3 is too large, it indicates that the content of the sodium iron pyrophosphate impurity phase in the positive electrode active material 100 is too high, thereby reducing the gram capacity of the positive electrode active material 100.

[0067] Optionally, the positive electrode active material 100 further satisfies the relationship: P2 = I2 / I3 ≤ 0.05. Specifically, the value of I2 / I3 can be, but is not limited to, less than or equal to 0.05, less than or equal to 0.045, less than or equal to 0.04, less than or equal to 0.035, less than or equal to 0.03, less than or equal to 0.025, less than or equal to 0.02, less than or equal to 0.015, less than or equal to 0.01, etc. If the value of I2 / I3 is too large, it indicates that the content of the sodium iron phosphate impurity phase in the positive electrode active material 100 is too high, thereby reducing the gram capacity of the positive electrode active material 100.

[0068] In some embodiments, the powder resistivity R of the positive electrode active material 100 is in the range of 70 Ω·cm≤R≤800 Ω·cm.

[0069] It should be noted that the powder resistivity R of the positive electrode active material 100 in the examples of the present application was measured using a two-probe method. Specifically, 2 to 3 g of the positive electrode active material 100 powder was added to a mold of a 13 mm diameter powder resistance meter. After pressurization to 200 MPa, the powder resistivity was measured.

[0070] Specifically, the powder resistivity R of the positive electrode active material 100 may be, but is not limited to, 70Ω·cm, 80Ω·cm, 90Ω·cm, 100Ω·cm, 150Ω·cm, 200Ω·cm, 250Ω·cm, 300Ω·cm, 350Ω·cm, 400Ω·cm, 450Ω·cm, 500Ω·cm, 550Ω·cm, 600Ω·cm, 650Ω·cm, 700Ω·cm, 750Ω·cm, 800Ω·cm, etc.

[0071] In this embodiment, if the powder resistivity R of the positive electrode active material 100 is too low, a higher sintering temperature is required during the preparation of the positive electrode active material 100 to achieve a lower powder resistivity. However, an excessively high sintering temperature will increase the decomposition ratio of sodium ferric pyrophosphate and reduce the gram capacity of the positive electrode active material 100. If the powder resistivity R of the positive electrode active material 100 is too high, when used in a sodium battery, the internal resistance of the sodium battery will be too large, thereby reducing the cycle life of the sodium battery.

[0072] Optionally, the D10 of the positive electrode active material 100 is in the range of 0.5 μm≤D10≤3 μm.

[0073] The term “D10” refers to the particle size corresponding to when the cumulative distribution reaches 10% in a particle size distribution test of the positive electrode active material 100 .

[0074] Specifically, the D10 of the positive electrode active material 100 may be, but is not limited to, 0.5 μm, 0.8 μm, 1.0 μm, 1.3 μm, 1.5 μm, 1.8 μm, 2.0 μm, 2.3 μm, 2.5 μm, 2.8 μm, 3 μm, etc. If the D10 of the positive electrode active material 100 is too small, the viscosity of the positive electrode slurry is too high when preparing the positive electrode slurry, making the slurry coating uneven or even impossible to coat; if the D10 of the positive electrode active material 100 is too large, the positive electrode active material 100 may not be deposited well on the positive electrode current collector when the slurry is coated on the positive electrode current collector, resulting in excessive pressure on the positive electrode current collector during the rolling process of the positive electrode sheet, reducing the ionic conductivity of the positive electrode sheet and reducing the discharge capacity of the resulting sodium battery at high current.

[0075] Optionally, the D50 of the positive electrode active material 100 is in the range of 4 μm≤D50≤8 μm.

[0076] The term “D50” refers to the particle size corresponding to when the cumulative distribution reaches 50% in a particle size distribution test of the positive electrode active material 100 .

[0077] Specifically, the D50 of the positive electrode active material 100 may be, but is not limited to, 4 μm, 4.5 μm, 5 μm, 5.5 μm, 6 μm, 6.5 μm, 7 μm, 7.5 μm, 8 μm, etc. If the D50 of the positive electrode active material 100 is too small, the viscosity of the positive electrode slurry will be too high when preparing the positive electrode slurry, making the slurry coating uneven or even impossible to coat; if the D50 of the positive electrode active material 100 is too large, the positive electrode active material 100 will not be deposited well on the positive electrode current collector when the slurry is coated on the positive electrode current collector, and excessive pressure will be applied to the positive electrode current collector during the rolling process of the positive electrode sheet, thereby reducing the ionic conductivity of the positive electrode sheet and reducing the discharge capacity of the resulting sodium battery at high current.

[0078] Optionally, the D90 of the positive electrode active material 100 is in the range of 9 μm≤D90≤13 μm.

[0079] The term “D90” refers to the particle size corresponding to 90% of the cumulative distribution in a particle size distribution test of the positive electrode active material 100 .

[0080] Specifically, the D90 of the positive electrode active material 100 may be, but is not limited to, 9 μm, 9.5 μm, 10 μm, 10.5 μm, 11 μm, 11.5 μm, 12 μm, 12.5 μm, 13 μm, etc. If the D90 of the positive electrode active material 100 is too small, the viscosity of the positive electrode slurry is too high when preparing the positive electrode slurry, making the slurry coating uneven or even impossible to coat. If the D90 of the positive electrode active material 100 is too large, the positive electrode active material 100 may not be deposited well on the positive electrode current collector when the slurry is coated on the positive electrode current collector. This may result in excessive pressure on the positive electrode current collector during the rolling process of the positive electrode sheet, reducing the ionic conductivity of the positive electrode sheet and the discharge capacity of the resulting sodium battery at high currents.

[0081] The D10, D50, and D90 of the examples of the present application were tested in the following manner: 0.2 g to 0.3 g of the positive electrode active material 100 powder was dispersed in 100 ml of deionized water by external ultrasonication for 5 minutes to obtain a dispersion; the dispersion was then tested using a laser particle size analyzer, with each sample tested three times. The test results were averaged over the three tests to obtain the particle size distribution curve of the positive electrode active material 100 and D10, D50, and D90.

[0082] The positive electrode active material 100 of the embodiments of the present application can be prepared by the methods described in the following embodiments of the present application. In addition, it can also be prepared by other methods. The preparation methods of the embodiments of the present application are merely one or more preparation methods of the positive electrode active material 100 of the present application and should not be understood as limiting the positive electrode active material 100 provided in the embodiments of the present application.

[0083] See Figure 2 , an embodiment of the present application provides a method for preparing a positive electrode active material 100, which includes:

[0084] S201, providing a sodium source, a phosphorus source, an iron source and a carbon source;

[0085] Optionally, the sodium source may include but is not limited to at least one of sodium dihydrogen phosphate, sodium pyrophosphate, sodium carbonate, sodium acetate and compounds containing crystal water.

[0086] Optionally, the phosphorus source may include but is not limited to at least one of sodium dihydrogen phosphate, sodium pyrophosphate, ammonium dihydrogen phosphate and compounds containing crystal water.

[0087] Optionally, the iron source may include but is not limited to at least one of ferrous oxalate, ferric nitrate, ferrous sulfate and compounds thereof containing crystal water, such as ferrous oxalate dihydrate.

[0088] Optionally, the carbon source is at least one of glucose, sucrose, starch, polyethylene glycol, ascorbic acid, citric acid, etc.

[0089] S202, mixing a sodium source, a phosphorus source, an iron source, and a carbon source in a solvent to obtain a slurry;

[0090] S203, spray drying the slurry to obtain a precursor powder; and

[0091] S204, sintering the precursor powder to obtain the positive electrode active material 100; the positive electrode active material 100 includes sodium iron phosphate pyrophosphate particles 10 and a carbon coating layer 20, and the carbon coating layer 20 is wrapped around the surface of the sodium iron phosphate pyrophosphate particles 10; the content of potassium in the positive electrode active material 100 is less than or equal to 100 ppm.

[0092] In the preparation method of the positive electrode active material 100 of the embodiment of the present application, by controlling the potassium content in the sodium source, phosphorus source, and iron source to be low, the decomposition of sodium ferric pyrophosphate can be effectively suppressed during sintering during the preparation of the positive electrode active material 100, the decomposition temperature of sodium ferric pyrophosphate can be increased, the formation of sodium ferric phosphate and sodium ferric pyrophosphate impurities can be effectively reduced, and the gram capacity of the prepared positive electrode active material 100 can be increased. In addition, the decomposition temperature of sodium ferric pyrophosphate is increased, and the positive electrode active material 100 can be sintered at a higher temperature during the preparation process, thereby achieving a higher electron transfer rate and a lower powder resistivity in the carbon coating layer 20 of the positive electrode active material 100. Furthermore, because the positive electrode active material 100 has a lower powder resistivity, when the positive electrode active material 100 is used in a positive electrode sheet, the amount of expensive positive electrode conductive agents such as carbon nanotubes in the positive active layer of the positive electrode sheet can be reduced while maintaining the resistivity of the positive electrode sheet, thereby significantly reducing the cost of the positive electrode sheet.

[0093] In some embodiments, the potassium content in the sodium source is less than or equal to 20 ppm, the potassium content in the phosphorus source is less than or equal to 20 ppm, and the potassium content in the iron source is less than or equal to 60 ppm.

[0094] Specifically, the potassium content in the sodium source may be, but is not limited to, less than or equal to 20 ppm, less than or equal to 18 ppm, less than or equal to 16 ppm, less than or equal to 14 ppm, less than or equal to 12 ppm, less than or equal to 10 ppm, less than or equal to 8 ppm, less than or equal to 6 ppm, less than or equal to 4 ppm, etc. When the potassium content in the sodium source is low, the decomposition of sodium ferric phosphate pyrophosphate can be effectively suppressed during sintering, the decomposition temperature of sodium ferric phosphate pyrophosphate can be increased, the formation of sodium ferric phosphate and sodium ferric pyrophosphate impurities can be effectively reduced, and the gram capacity of the positive electrode active material 100 can be increased. In addition, since the decomposition temperature of sodium ferric phosphate pyrophosphate is increased, the positive electrode active material 100 can be sintered at a higher temperature, thereby making the carbon coating layer 20 of the prepared positive electrode active material 100 have a higher electron transfer rate and a lower powder resistivity. However, if the potassium content in the sodium source is too low, the cost of the sodium source will be increased. If the potassium content in the sodium source is too high, the decomposition temperature of sodium ferric phosphate pyrophosphate is reduced during sintering, and the probability of decomposition of sodium ferric phosphate pyrophosphate is increased, thereby increasing the content of the mixed phase (sodium ferric phosphate mixed phase and sodium ferric pyrophosphate mixed phase) in the sodium ferric phosphate pyrophosphate particles 10, thereby reducing the gram capacity of the positive electrode active material 100; in addition, when the potassium content in the sodium source is too high, since the decomposition temperature of sodium ferric phosphate pyrophosphate is reduced, it is necessary to reduce the sintering temperature to ensure that the sodium ferric phosphate pyrophosphate phase has a higher proportion in the prepared positive electrode active material 100, and the positive electrode active material 100 has a higher gram capacity; however, reducing the sintering temperature will reduce the electron transfer rate of the carbon coating layer 20 and reduce the conductivity of the carbon coating layer 20, thereby increasing the powder resistivity of the positive electrode active material 100, and reducing the cycle life of the sodium battery when used in a sodium battery.

[0095] Specifically, the potassium content in the phosphorus source may be, but is not limited to, less than or equal to 20 ppm, less than or equal to 18 ppm, less than or equal to 16 ppm, less than or equal to 14 ppm, less than or equal to 12 ppm, less than or equal to 10 ppm, less than or equal to 8 ppm, less than or equal to 6 ppm, less than or equal to 4 ppm, etc. When the potassium content in the phosphorus source is low, the decomposition of sodium ferric phosphate pyrophosphate can be effectively suppressed during sintering, the decomposition temperature of sodium ferric phosphate pyrophosphate can be increased, the formation of sodium ferric phosphate and sodium ferric pyrophosphate impurities can be effectively reduced, and the gram capacity of the positive electrode active material 100 can be increased. In addition, since the decomposition temperature of sodium ferric phosphate pyrophosphate is increased, the positive electrode active material 100 can be sintered at a higher temperature, thereby making the carbon coating layer 20 of the prepared positive electrode active material 100 have a higher electron transfer rate and a lower powder resistivity. However, if the potassium content in the phosphorus source is too low, the cost of the phosphorus source will be increased. If the potassium content in the phosphorus source is too high, the decomposition temperature of the sodium ferric phosphate pyrophosphate is reduced during sintering, and the probability of decomposition of the sodium ferric phosphate pyrophosphate is increased, thereby increasing the content of the mixed phase (sodium ferric phosphate mixed phase and sodium ferric pyrophosphate mixed phase) in the sodium ferric phosphate pyrophosphate particles 10, thereby reducing the gram capacity of the positive electrode active material 100; in addition, when the potassium content in the phosphorus source is too high, since the decomposition temperature of the sodium ferric phosphate pyrophosphate is reduced, it is necessary to reduce the sintering temperature to ensure that the sodium ferric phosphate pyrophosphate phase has a higher proportion in the prepared positive electrode active material 100, and the positive electrode active material 100 has a higher gram capacity; however, lowering the sintering temperature will reduce the electron transfer rate of the carbon coating layer 20 and reduce the conductivity of the carbon coating layer 20, thereby increasing the powder resistivity of the positive electrode active material 100, and reducing the cycle life of the sodium battery when used in a sodium battery.

[0096] Specifically, the content of potassium in the iron source may be, but is not limited to, less than or equal to 60 ppm, less than or equal to 55 ppm, less than or equal to 50 ppm, less than or equal to 45 ppm, less than or equal to 40 ppm, less than or equal to 35 ppm, less than or equal to 30 ppm, less than or equal to 25 ppm, less than or equal to 20 ppm, less than or equal to 15 ppm, less than or equal to 10 ppm, etc. When the content of potassium in the iron source is low, the decomposition of sodium ferric phosphate pyrophosphate can be effectively suppressed during sintering, the decomposition temperature of sodium ferric phosphate pyrophosphate can be increased, the formation of sodium ferric phosphate and sodium ferric pyrophosphate impurities can be better reduced, and the gram capacity of the positive electrode active material 100 can be increased. In addition, since the decomposition temperature of sodium ferric phosphate pyrophosphate is increased, the positive electrode active material 100 can be sintered at a higher temperature, so that the carbon coating layer 20 of the prepared positive electrode active material 100 has a higher electron transfer rate and a lower powder resistivity. However, if the content of potassium in the iron source is too low, the cost of the iron source will be increased. If the potassium content in the iron source is too high, the decomposition temperature of the sodium ferric phosphate pyrophosphate is reduced during sintering, and the probability of decomposition of the sodium ferric phosphate pyrophosphate is increased, thereby increasing the content of the mixed phase (sodium ferric phosphate mixed phase and sodium ferric pyrophosphate mixed phase) in the sodium ferric phosphate pyrophosphate particles 10, thereby reducing the gram capacity of the positive electrode active material 100; in addition, when the potassium content in the iron source is too high, since the decomposition temperature of the sodium ferric phosphate pyrophosphate is reduced, it is necessary to reduce the sintering temperature to ensure that the sodium ferric phosphate pyrophosphate phase has a higher proportion in the prepared positive electrode active material 100, and the positive electrode active material 100 has a higher gram capacity; however, lowering the sintering temperature will reduce the electron transfer rate of the carbon coating layer 20 and reduce the conductivity of the carbon coating layer 20, thereby increasing the powder resistivity of the positive electrode active material 100, and reducing the cycle life of the sodium battery when applied to the sodium battery.

[0097] In some embodiments, the boron content in the sodium source is less than or equal to 20 ppm, the boron content in the phosphorus source is less than or equal to 20 ppm, and the boron content in the iron source is less than or equal to 60 ppm.

[0098] Specifically, the boron content in the sodium source can be, but is not limited to, less than or equal to 20 ppm, less than or equal to 18 ppm, less than or equal to 16 ppm, less than or equal to 14 ppm, less than or equal to 12 ppm, less than or equal to 10 ppm, less than or equal to 8 ppm, less than or equal to 6 ppm, less than or equal to 4 ppm, etc. When the boron content in the sodium source is low, the decomposition of sodium ferric phosphate pyrophosphate can be effectively suppressed during sintering, the decomposition temperature of sodium ferric phosphate pyrophosphate can be increased, the formation of sodium ferric phosphate and sodium ferric pyrophosphate impurities can be effectively reduced, and the gram capacity of the positive electrode active material 100 can be increased. In addition, due to the increased decomposition temperature of sodium ferric phosphate pyrophosphate, the positive electrode active material 100 can be sintered at a higher temperature, thereby making the carbon coating layer 20 of the prepared positive electrode active material 100 have a higher electron transfer rate and a lower powder resistivity. However, if the boron content in the sodium source is too low, the cost of the sodium source will be increased. If the boron content in the sodium source is too high, the decomposition temperature of sodium ferric phosphate pyrophosphate is reduced during sintering, and the probability of decomposition of sodium ferric phosphate pyrophosphate is increased, thereby increasing the content of the mixed phase (sodium ferric phosphate mixed phase and sodium ferric pyrophosphate mixed phase) in the sodium ferric phosphate pyrophosphate particles 10, thereby reducing the gram capacity of the positive electrode active material 100; in addition, when the boron content in the sodium source is too high, since the decomposition temperature of sodium ferric phosphate pyrophosphate is reduced, it is necessary to reduce the sintering temperature to ensure that the sodium ferric phosphate pyrophosphate phase has a higher proportion in the prepared positive electrode active material 100, and the positive electrode active material 100 has a higher gram capacity; however, reducing the sintering temperature will reduce the electron transfer rate of the carbon coating layer 20 and reduce the conductivity of the carbon coating layer 20, thereby increasing the powder resistivity of the positive electrode active material 100, and reducing the cycle life of the sodium battery when used in a sodium battery.

[0099] Specifically, the boron content in the phosphorus source may be, but is not limited to, less than or equal to 20 ppm, less than or equal to 18 ppm, less than or equal to 16 ppm, less than or equal to 14 ppm, less than or equal to 12 ppm, less than or equal to 10 ppm, less than or equal to 8 ppm, less than or equal to 6 ppm, less than or equal to 4 ppm, etc. When the boron content in the phosphorus source is low, the decomposition of sodium ferric phosphate pyrophosphate can be effectively suppressed during sintering, the decomposition temperature of sodium ferric phosphate pyrophosphate can be increased, the formation of sodium ferric phosphate and sodium ferric pyrophosphate impurities can be effectively reduced, and the gram capacity of the positive electrode active material 100 can be increased. In addition, since the decomposition temperature of sodium ferric phosphate pyrophosphate is increased, the positive electrode active material 100 can be sintered at a higher temperature, thereby making the carbon coating layer 20 of the prepared positive electrode active material 100 have a higher electron transfer rate and a lower powder resistivity. However, if the boron content in the phosphorus source is too low, the cost of the phosphorus source will be increased. If the boron content in the phosphorus source is too high, the decomposition temperature of sodium ferric phosphate pyrophosphate is reduced during sintering, and the probability of decomposition of sodium ferric phosphate pyrophosphate is increased, thereby increasing the content of the mixed phase (sodium ferric phosphate mixed phase and sodium ferric pyrophosphate mixed phase) in the sodium ferric phosphate pyrophosphate particles 10, thereby reducing the gram capacity of the positive electrode active material 100; in addition, when the boron content in the phosphorus source is too high, since the decomposition temperature of sodium ferric phosphate pyrophosphate is reduced, it is necessary to reduce the sintering temperature to ensure that the sodium ferric phosphate pyrophosphate phase has a higher proportion in the prepared positive electrode active material 100, and the positive electrode active material 100 has a higher gram capacity; however, lowering the sintering temperature will reduce the electron transfer rate of the carbon coating layer 20 and reduce the conductivity of the carbon coating layer 20, thereby increasing the powder resistivity of the positive electrode active material 100, and reducing the cycle life of the sodium battery when used in a sodium battery.

[0100] Specifically, the boron content in the iron source may be, but is not limited to, less than or equal to 60 ppm, less than or equal to 55 ppm, less than or equal to 50 ppm, less than or equal to 45 ppm, less than or equal to 40 ppm, less than or equal to 35 ppm, less than or equal to 30 ppm, less than or equal to 25 ppm, less than or equal to 20 ppm, less than or equal to 15 ppm, less than or equal to 10 ppm, etc. When the boron content in the iron source is low, the decomposition of sodium ferric phosphate pyrophosphate can be effectively suppressed during sintering, the decomposition temperature of sodium ferric phosphate pyrophosphate can be increased, the formation of sodium ferric phosphate and sodium ferric pyrophosphate impurities can be better reduced, and the gram capacity of the positive electrode active material 100 can be increased. In addition, since the decomposition temperature of sodium ferric phosphate pyrophosphate is increased, the positive electrode active material 100 can be sintered at a higher temperature, thereby making the carbon coating layer 20 of the prepared positive electrode active material 100 have a higher electron transfer rate and a lower powder resistivity. However, if the boron content in the iron source is too low, the cost of the iron source will be increased. If the boron content in the iron source is too high, the decomposition temperature of the sodium ferric phosphate pyrophosphate is reduced during sintering, and the probability of decomposition of the sodium ferric phosphate pyrophosphate is increased, thereby increasing the content of the mixed phase (sodium ferric phosphate mixed phase and sodium ferric pyrophosphate mixed phase) in the sodium ferric phosphate pyrophosphate particles 10, thereby reducing the gram capacity of the positive electrode active material 100; in addition, when the boron content in the iron source is too high, since the decomposition temperature of the sodium ferric phosphate pyrophosphate is reduced, it is necessary to reduce the sintering temperature to ensure that the sodium ferric phosphate pyrophosphate phase has a higher proportion in the prepared positive electrode active material 100, and the positive electrode active material 100 has a higher gram capacity; however, reducing the sintering temperature will reduce the electron transfer rate of the carbon coating layer 20 and reduce the conductivity of the carbon coating layer 20, thereby increasing the powder resistivity of the positive electrode active material 100, and reducing the cycle life of the sodium battery when applied to the sodium battery.

[0101] In some embodiments, the sulfur content in the sodium source is less than or equal to 40 ppm, the sulfur content in the phosphorus source is less than or equal to 40 ppm, and the sulfur content in the iron source is less than or equal to 120 ppm.

[0102] Specifically, the sulfur content in the sodium source can be, but is not limited to, less than or equal to 40 ppm, less than or equal to 38 ppm, less than or equal to 35 ppm, less than or equal to 33 ppm, less than or equal to 30 ppm, less than or equal to 28 ppm, less than or equal to 25 ppm, less than or equal to 23 ppm, less than or equal to 20 ppm, less than or equal to 18 ppm, less than or equal to 15 ppm, less than or equal to 13 ppm, less than or equal to 10 ppm, etc. When the sulfur content in the sodium source is low, the decomposition of sodium iron phosphate pyrophosphate can be well suppressed during sintering, the decomposition temperature of sodium iron phosphate pyrophosphate can be increased, the generation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities can be better reduced, and the gram capacity of the positive electrode active material 100 can be increased; in addition, since the decomposition temperature of sodium iron phosphate pyrophosphate is increased, the positive electrode active material 100 can be sintered at a higher temperature, so that the carbon coating layer 20 of the prepared positive electrode active material 100 has a higher electron transfer rate and a lower powder resistivity; however, the sulfur content in the sodium source is too low, which increases the cost of the sodium source. If the sulfur content in the sodium source is too high, the decomposition temperature of sodium ferric phosphate pyrophosphate is reduced during sintering, and the probability of decomposition of sodium ferric phosphate pyrophosphate is increased, thereby increasing the content of the mixed phase (sodium ferric phosphate mixed phase and sodium ferric pyrophosphate mixed phase) in the sodium ferric phosphate pyrophosphate particles 10, thereby reducing the gram capacity of the positive electrode active material 100; in addition, when the sulfur content in the sodium source is too high, since the decomposition temperature of sodium ferric phosphate pyrophosphate is reduced, it is necessary to reduce the sintering temperature to ensure that the sodium ferric phosphate pyrophosphate phase has a higher proportion in the prepared positive electrode active material 100, and the positive electrode active material 100 has a higher gram capacity; however, lowering the sintering temperature will reduce the electron transfer rate of the carbon coating layer 20 and reduce the conductivity of the carbon coating layer 20, thereby increasing the powder resistivity of the positive electrode active material 100, and reducing the cycle life of the sodium battery when applied to the sodium battery.

[0103] Specifically, the content of sulfur element in the phosphorus source can be, but is not limited to, less than or equal to 40 ppm, less than or equal to 38 ppm, less than or equal to 35 ppm, less than or equal to 33 ppm, less than or equal to 30 ppm, less than or equal to 28 ppm, less than or equal to 25 ppm, less than or equal to 23 ppm, less than or equal to 20 ppm, less than or equal to 18 ppm, less than or equal to 15 ppm, less than or equal to 13 ppm, less than or equal to 10 ppm, etc. When the sulfur content in the phosphorus source is low, the decomposition of sodium iron phosphate pyrophosphate can be effectively suppressed during sintering, the decomposition temperature of sodium iron phosphate pyrophosphate can be increased, the generation of sodium iron phosphate impurity phase and sodium iron pyrophosphate impurity phase can be better reduced, and the gram capacity of the positive electrode active material 100 can be increased; in addition, since the decomposition temperature of sodium iron phosphate pyrophosphate is increased, the positive electrode active material 100 can be sintered at a higher temperature, so that the carbon coating layer 20 of the prepared positive electrode active material 100 has a higher electron transfer rate and a lower powder resistivity; however, the sulfur content in the phosphorus source is too low, which increases the cost of the phosphorus source. If the sulfur content in the phosphorus source is too high, the decomposition temperature of sodium ferric phosphate pyrophosphate is reduced during sintering, and the probability of decomposition of sodium ferric phosphate pyrophosphate is increased, thereby increasing the content of the mixed phase (sodium ferric phosphate mixed phase and sodium ferric pyrophosphate mixed phase) in the sodium ferric phosphate pyrophosphate particles 10, thereby reducing the gram capacity of the positive electrode active material 100; in addition, when the sulfur content in the phosphorus source is too high, since the decomposition temperature of sodium ferric phosphate pyrophosphate is reduced, it is necessary to reduce the sintering temperature to ensure that the sodium ferric phosphate pyrophosphate phase has a higher proportion in the prepared positive electrode active material 100, and the positive electrode active material 100 has a higher gram capacity; however, lowering the sintering temperature will reduce the electron transfer rate of the carbon coating layer 20 and reduce the conductivity of the carbon coating layer 20, thereby increasing the powder resistivity of the positive electrode active material 100, and reducing the cycle life of the sodium battery when used in a sodium battery.

[0104] Specifically, the sulfur content in the iron source may be, but is not limited to, less than or equal to 120 ppm, less than or equal to 110 ppm, less than or equal to 100 ppm, less than or equal to 90 ppm, less than or equal to 80 ppm, less than or equal to 70 ppm, less than or equal to 60 ppm, less than or equal to 50 ppm, less than or equal to 30 ppm, less than or equal to 20 ppm, less than or equal to 10 ppm, etc. When the sulfur content in the iron source is low, the decomposition of sodium ferric phosphate pyrophosphate can be effectively suppressed during sintering, the decomposition temperature of sodium ferric phosphate pyrophosphate can be increased, the formation of sodium ferric phosphate and sodium ferric pyrophosphate impurities can be better reduced, and the gram capacity of the positive electrode active material 100 can be increased. In addition, since the decomposition temperature of sodium ferric phosphate pyrophosphate is increased, the positive electrode active material 100 can be sintered at a higher temperature, so that the carbon coating layer 20 of the prepared positive electrode active material 100 has a higher electron transfer rate and a lower powder resistivity. However, if the sulfur content in the iron source is too low, the cost of the iron source will be increased. If the sulfur content in the iron source is too high, the decomposition temperature of the sodium ferric phosphate pyrophosphate is reduced during sintering, and the probability of decomposition of the sodium ferric phosphate pyrophosphate is increased, thereby increasing the content of the mixed phase (sodium ferric phosphate mixed phase and sodium ferric pyrophosphate mixed phase) in the sodium ferric phosphate pyrophosphate particles 10, thereby reducing the gram capacity of the positive electrode active material 100; in addition, when the sulfur content in the iron source is too high, since the decomposition temperature of the sodium ferric phosphate pyrophosphate is reduced, it is necessary to reduce the sintering temperature to ensure that the sodium ferric phosphate pyrophosphate phase has a higher proportion in the prepared positive electrode active material 100, and the positive electrode active material 100 has a higher gram capacity; however, lowering the sintering temperature will reduce the electron transfer rate of the carbon coating layer 20 and reduce the conductivity of the carbon coating layer 20, thereby increasing the powder resistivity of the positive electrode active material 100, and reducing the cycle life of the sodium battery when applied to the sodium battery.

[0105] In some embodiments, in S202, the sodium source, phosphorus source, iron source and carbon source are mixed in a solvent to obtain a slurry, including: mixing the sodium source, phosphorus source, iron source and carbon source in water, and sand milling to obtain a slurry, wherein the sand milling time is 0.5h to 4h, and the sand milling speed is 1000rpm to 4000rpm.

[0106] In the embodiments of the present application, when a numerical value range from a to b is involved, unless otherwise specified, it means that the numerical value can be any numerical value between a and b, including the endpoint numerical value a and the endpoint numerical value b.

[0107] It can be understood that the solvent may be, but is not limited to, water.

[0108] In this embodiment, before spray drying, the raw materials (i.e., sodium source, phosphorus source, iron source and carbon source) are first sand-milled and mixed. Sand milling can reduce the particle size of the raw materials, providing a prerequisite for the formation of sodium ferric pyrophosphate; in addition, sand milling can fully mix the soluble and insoluble substances in the raw materials, thereby avoiding uneven mixing of the raw materials when there are multiple insoluble raw materials in the raw materials, and increasing the probability of generating impurities in the finally formed sodium ferric pyrophosphate.

[0109] Specifically, the sand milling time can be, but is not limited to, 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, etc. If the sand milling time is too short, the particle size of the insoluble raw materials in the sodium source, phosphorus source, iron source, and carbon source will be too large, so that during the subsequent sintering process, local crystal growth will occur, resulting in a decrease in the sphericity of the prepared positive electrode active material 100; if the sand milling time is too long, the production efficiency will be reduced.

[0110] Specifically, the speed of the sand milling process may be, but is not limited to, 1000 rpm, 1500 rpm, 2000 rpm, 2500 rpm, 3000 rpm, 3500 rpm, 4000 rpm, etc. If the speed of the sand milling process is too low, the particle size of the insoluble raw materials in the sodium source, phosphorus source, iron source, and carbon source may be too large, so that during the subsequent sintering process, crystals may grow locally, resulting in a decrease in the sphericity of the prepared positive electrode active material 100; if the speed of the sand milling process is too high, the slurry may easily splash.

[0111] In some embodiments, the solid content of the slurry ranges from 20% to 40%. Specifically, the solid content of the slurry can be, but is not limited to, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, etc. In this embodiment, if the solid content of the slurry is too low, the material cannot be quickly and effectively sphericalized into a spherical morphology during the spray drying stage, affecting the sphericity of the final positive electrode active material 100. If the solid content of the slurry is too high, the sand milling process will be greatly difficult. In addition, the material will form balls too quickly during the spray drying stage, and there will not be enough time to form high-sphericity balls, thereby reducing the sphericity of the resulting precursor powder, and further reducing the sphericity of the final positive electrode active material 100.

[0112] In some embodiments, in S203 , spray-drying the slurry to obtain a precursor powder includes: spray-drying the slurry at a temperature of 102° C. ≤ T1 ≤ 120° C. to obtain a precursor powder.

[0113] Specifically, the spray drying temperature can be, but is not limited to, 102°C, 103°C, 104°C, 105°C, 106°C, 107°C, 108°C, 110°C, 112°C, 114°C, 116°C, 118°C, 119°C, 120°C, etc.

[0114] In this embodiment, if the temperature for spray drying the slurry is too low, the evaporation rate of the solvent (such as water) is too slow, which makes the precursor powder form balls too slowly, and easily forms a hollow spherical morphology or a collapsed spherical morphology, affecting the processing performance and compaction density of the final positive electrode active material 100; if the temperature for spray drying the slurry is too high, the evaporation rate of the solvent (such as water) is too fast, which makes the precursor powder form balls too quickly, and the sphericity of the formed precursor powder is reduced.

[0115] In some embodiments, in S204 , sintering the precursor powder to obtain the positive electrode active material 100 includes: sintering the precursor powder at a temperature of 580° C. to 630° C. to obtain the positive electrode active material 100 .

[0116] Optionally, the sintering is performed under the protection of an inert atmosphere. Optionally, the inert atmosphere includes at least one of helium, nitrogen, and argon.

[0117] Specifically, the temperature for sintering the precursor powder can be, but is not limited to, 580° C., 585° C., 590° C., 595° C., 600° C., 605° C., 610° C., 615° C., 620° C., 625° C., 630° C., etc. If the temperature for sintering the precursor powder is too low, the carbonization degree of the carbon coating layer 20 in the prepared positive electrode active material 100 is too low, and the powder resistivity of the positive electrode active material 100 is too high. When used in a sodium battery, the internal resistance of the sodium battery is too large, which reduces the cycle life of the sodium battery. If the temperature for sintering the precursor powder is too high, the control of the impurity elements (potassium, boron, sulfur) in the sodium source, iron source, and phosphorus source on the decomposition of sodium pyrophosphate is insufficient to withstand the decomposition of sodium iron pyrophosphate at high temperature. Sodium iron pyrophosphate is easily decomposed to form sodium iron phosphate and sodium iron pyrophosphate mixed phases, thereby reducing the specific capacity of the positive electrode active material 100.

[0118] Furthermore, the temperature range for sintering the precursor powder is 600° C. to 610° C. This allows the carbon coating layer 20 of the prepared positive electrode active material 100 to have a higher electron transfer rate, the positive electrode active material 100 to have a lower powder resistivity, and the content of sodium iron phosphate and sodium iron pyrophosphate impurities in the prepared positive electrode active particles to be lower, thereby having a higher specific capacity.

[0119] Optionally, the precursor powder can be sintered for 2 hours to 48 hours. Specifically, the precursor powder can be sintered for, but is not limited to, 2 hours, 4 hours, 8 hours, 12 hours, 16 hours, 20 hours, 24 hours, 28 hours, 32 hours, 36 hours, 40 hours, 44 hours, 48 hours, etc. If the precursor powder is sintered for too short a time, the carbonization degree of the carbon coating layer 20 of the prepared positive electrode active material 100 is too low, thereby increasing the powder resistivity of the positive electrode active material 100; if the precursor powder is sintered for too long a time, the probability of decomposition of the sodium iron pyrophosphate increases, thereby increasing the content of the sodium iron phosphate impurity phase and the sodium iron pyrophosphate impurity phase in the positive electrode active material 100, thereby reducing the gram capacity of the positive electrode active material 100.

[0120] See Figure 3 The embodiment of the present application further provides a positive electrode plate 200 , which includes a positive electrode current collector 210 and a positive electrode active layer 220 , and the positive electrode active layer 220 includes the positive electrode active material 100 described in the embodiment of the present application.

[0121] Optionally, the positive electrode current collector 210 may be, but is not limited to, aluminum foil or aluminum sheet.

[0122] Optionally, the positive electrode active layer 220 further includes a positive electrode conductor and a positive electrode binder.

[0123] Optionally, the positive electrode conductive agent may be, but is not limited to, at least one of conductive carbon black (SP), acetylene black, carbon nanotubes (CNT), carbon fibers, graphene, and the like.

[0124] Optionally, the positive electrode binder may be, but is not limited to, at least one of polyvinylidene fluoride (PVDF), polyamide (PA), polyacrylonitrile (PAN), polyacrylate, polyvinylether, polymethyl methacrylate (PMMA), and polyhexafluoropropylene.

[0125] The positive electrode sheet 200 of the embodiment of the present application includes a positive electrode active material 100, wherein the positive electrode active material 100 includes positive electrode particles, wherein the positive electrode particles include sodium iron phosphate pyrophosphate particles 10 and a carbon coating layer 20, wherein the carbon coating layer 20 is coated on the surface of the sodium iron phosphate pyrophosphate particles 10; the potassium content in the positive electrode active material 100 is less than or equal to 100 ppm. By controlling the potassium content in the positive electrode active material 100 to be less than or equal to 100 ppm, the decomposition of the sodium iron phosphate pyrophosphate can be effectively suppressed during sintering during the preparation process of the positive electrode active material 100, the decomposition temperature of the sodium iron phosphate pyrophosphate can be increased, the formation of sodium iron phosphate and sodium iron pyrophosphate impurities can be effectively reduced, and the gram capacity of the positive electrode active material 100 can be increased. In addition, the positive electrode active material 100 can be sintered at a higher temperature during the preparation process, thereby enabling the carbon coating layer 20 of the positive electrode active material 100 to have a higher electron transfer rate and a lower powder resistivity. Furthermore, since the positive electrode active material 100 has a lower powder resistivity, the amount of expensive positive electrode conductive agents such as carbon nanotubes in the positive electrode active layer 220 of the positive electrode sheet 200 can be reduced while ensuring that the resistivity of the positive electrode sheet 200 remains unchanged, thereby greatly reducing the cost of the positive electrode sheet 200.

[0126] See Figure 4 and Figure 5 The embodiment of the present application further provides a sodium battery 300 , which includes an electrolyte, the positive electrode plate 200 described in the embodiment of the present application, a separator 320 and a negative electrode plate 330 .

[0127] It should be noted that the sodium battery 300 of the embodiment of the present application can be, but is not limited to, at least one of a sodium ion battery, a sodium metal battery, a lithium-sodium hybrid battery, and the like.

[0128] Optionally, the sodium battery 300 may be, but is not limited to, at least one of a cylindrical sodium battery, a square sodium battery, and a blade sodium battery. The drawings in this application merely illustrate one or more possible forms of the sodium battery 300 and should not be construed as limiting the sodium battery 300 of the embodiments of this application, nor should they be construed as limiting the positive electrode active material 100 of the embodiments of this application.

[0129] It can be understood that the positive electrode sheet 200 and the negative electrode sheet 330 are respectively located on opposite sides of the separator 320 , that is, the separator 320 is located between the positive electrode sheet 200 and the negative electrode sheet 330 to separate the positive electrode sheet 200 from the negative electrode sheet 330 .

[0130] It should be noted that the positive electrode sheet 200 , the separator 320 and the negative electrode sheet 330 are at least partially immersed in the electrolyte.

[0131] Optionally, the electrolyte includes an electrolyte salt, an organic solvent and a film-forming additive.

[0132] Alternatively, the electrolyte salt may include, but is not limited to, a sodium salt. Alternatively, the sodium salt may be, but is not limited to, at least one of sodium hexafluorophosphate (NaPF6), sodium tetrafluoroborate (NaBF4), sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium bisoxalatoborate, sodium difluorobisoxalatophosphate, sodium difluorooxalatoborate, sodium difluorophosphate (NaPO2F2), sodium trifluoromethanesulfonate (CF3SO3Na), and the like.

[0133] Alternatively, the organic solvent may include at least one of a cyclic carbonate and a chain carbonate. Alternatively, the cyclic carbonate may include but is not limited to at least one of ethylene carbonate (EC), propylene carbonate (PC), etc. The dielectric constant of ethylene carbonate is much greater than that of propylene carbonate, and ethylene carbonate can better promote the formation of a solid electrolyte interface membrane (SEI). Alternatively, the chain carbonate may include but is not limited to at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), etc. Alternatively, the organic solvent may also include at least one of ethyl acetate, propyl acetate, ethyl propionate, propyl propionate, γ-butyrolactone, and 2,2-difluoroethyl acetate.

[0134] Optionally, the film-forming additive may include but is not limited to at least one of propargylbenzenesulfonic acid, vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), methylene methanedisulfonate (MMDS), butane sultone (BS), 1,3-propylene sultone (PST), etc.

[0135] Optionally, the diaphragm 320 may be, but is not limited to, at least one of a polypropylene film (PP film), a polyethylene film (PE film), a ceramic diaphragm 320, etc. Optionally, the thickness of the diaphragm 320 is 10 μm to 18 μm. Specifically, the thickness of the diaphragm 320 may be, but is not limited to, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 14.5 μm, 15 μm, 15.5 μm, 16 μm, 16.5 μm, 17 μm, 17.5 μm, and 18 μm.

[0136] Optionally, the compaction density of the positive active layer 220 of the positive electrode sheet 200 is in the range of 2.2 g / cm 3 Up to 2.4g / cm 3 Specifically, it can be but not limited to 2.20g / cm 3, 2.25g / cm 3 , 2.30g / cm 3 , 2.35g / cm 3 , 2.4g / cm 3 wait.

[0137] The compaction density test method of the positive electrode active layer 220 of the present application is as follows: the thickness of the aluminum foil and the mass of the aluminum foil cut into 12 mm round pieces are measured in advance, and then the thickness of the positive electrode plate 200 made of the positive electrode active material 100 is measured and the positive electrode plate 200 is cut into 12 mm round pieces and weighed, thereby calculating the mass and volume of the positive electrode active material 100 on the positive electrode plate 200, and then calculating the compaction density of the positive electrode active material 100 on the positive electrode plate 200.

[0138] See Figure 6 Optionally, the negative electrode sheet 330 includes a negative electrode current collector 331 and a negative electrode active layer 332. The negative electrode current collector 331 may be, but is not limited to, copper foil, copper sheet, aluminum foil, or aluminum sheet. The negative electrode active layer 332 includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder, and a negative electrode thickener.

[0139] Alternatively, the negative electrode active material may be, but is not limited to, hard carbon.

[0140] Optionally, the negative electrode conductive agent may be, but is not limited to, at least one of conductive carbon black (SP for short), acetylene black, carbon nanotubes, carbon fibers, graphene, and the like.

[0141] Optionally, the negative electrode binder may be, but is not limited to, at least one of polyvinylidene fluoride, polyamide, polyacrylonitrile, polyacrylate, polyvinyl ether, polymethyl methacrylate, polyhexafluoropropylene, sodium carboxymethyl cellulose (CMC for short), styrene-butadiene rubber, and the like.

[0142] Optionally, the negative electrode thickener may be, but is not limited to, at least one of polyacrylamide (PAM) and polymethacrylate (PMA).

[0143] Optionally, the sodium battery 300 further includes a housing 340 and an end cap assembly 350. The housing 340 and the end cap assembly 350 enclose a closed receiving chamber (not shown) for accommodating the electrolyte, the positive electrode sheet 200, the separator 320, and the negative electrode sheet 330. It is understood that the end cap assembly 350 electrically connects the positive electrode sheet 200 and the negative electrode sheet 330, respectively, and leads the positive electrode sheet 200 and the negative electrode sheet 330 out for electrical connection to external devices or other sodium batteries 300.

[0144] The positive electrode active material 100 of the present application is further described below through specific examples.

[0145] Examples 1 to 7, Comparative Examples 1 to 6

[0146] The positive electrode active material 100 of each embodiment and comparative example was prepared by the following steps:

[0147] (1) 178 g of sodium pyrophosphate (sodium source), 150 g of ammonium dihydrogen phosphate (phosphorus source), 340 g of ferrous oxalate dihydrate (iron source), 32 g of glucose (carbon source) and 2.5 kg of water (solvent) were stirred uniformly to obtain a slurry, wherein the solid content of the slurry was 35%; the contents of potassium, boron and sulfur in the raw materials of each embodiment and comparative example are shown in Table 1 below.

[0148] (2) spray drying the slurry at 105° C. to obtain a precursor powder;

[0149] (3) The precursor powder was sintered in a sintering furnace under nitrogen protection at a temperature of 610° C. for 12 h, and the positive electrode active material 100 was obtained after cooling.

[0150] Table 1 Performance parameters of sodium source, phosphorus source and iron source of Examples 1 to 7 and Comparative Examples 1 to 6

[0151]

[0152] The following performance tests were performed on the positive electrode active materials 100 prepared in Examples 1 to 7 and Comparative Examples 1 to 6.

[0153] (1) Powder resistivity R of the positive electrode active material 100: Measured using a two-probe method. Specifically, 2 to 3 g of the positive electrode active material 100 powder was placed in a mold of a 13 mm diameter powder resistance meter. After pressurizing to 200 MPa, the powder resistivity was measured.

[0154] (2) Gram capacity test of positive electrode active material 100: The positive electrode active material 100 is mixed with conductive carbon black (SP) and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1 and prepared into a positive electrode slurry using a defoamer. PVDF is dissolved in N-methylpyrrolidone (NMP) at a mass fraction of 5%. The prepared positive electrode slurry is evenly coated on an aluminum foil current collector (positive electrode current collector 210) with a scraper, and then vacuum dried and cut to obtain a positive electrode plate 200. A metal sodium plate is used as a counter electrode, and a glass fiber separator 320 is used as a separator 320. The positive electrode plate 200, the separator 320, the sodium plate, the electrolyte and the shell are assembled into a button-type sodium battery 300. The gram capacity of the sodium battery 300 is then measured using a blue electric tester for constant current charge and discharge testing. The current density is 12 mA / g (the actual current value is the current density multiplied by the mass of the positive electrode active material 100 of the positive electrode plate 200). The upper and lower limits of the test voltage are 1.5V-3.5V. The tested discharge gram capacity is the gram capacity of the positive electrode active material 100.

[0155] (3) Resistivity M of the positive electrode sheet 200: The positive electrode sheet 200 coated on both sides with the positive electrode slurry in (2) is cut into 5 small square positive electrode sheets 200 of 5 cm × 5 cm in size, and the thickness of the positive electrode sheet 200 is measured with a micrometer (the average value is taken after measuring five points), and then the membrane resistance of each small square positive electrode sheet 200 is measured with a resistance meter (the thickness value is input, and the measuring pressure is adjusted to maintain at 0.4 tons), and finally the average value of the resistivity measurement values of the five square positive electrode sheets 200 is taken as the resistivity M of the positive electrode sheet 200.

[0156] (4) Test of heterogeneous phase factor P: measured using an X-ray diffractometer.

[0157] (5) Content of impurity elements (potassium, boron, and sulfur) in the positive electrode active material 100: Measured using an inductively coupled plasma optical emission spectrometer (ICP-OES). Specifically, 0.2 to 0.3 g of the positive electrode active material 100 powder was dissolved in a strong acid, and the resulting solution was then tested in an ICP-OES instrument to determine the content of each impurity element.

[0158] Various performance parameters of the positive electrode active materials 100 of Examples 1 to 7 and Comparative Examples 1 to 3 are shown in Table 2 below.

[0159] Table 2 Performance parameters of the positive electrode active materials 100 of Examples 1 to 7 and Comparative Examples 1 to 3

[0160]

[0161] From the test results of Examples 1 to 3 and Comparative Example 1, it can be seen that as the content of potassium in the sodium source, phosphorus source, and iron source decreases, the heterogeneous phase factor P of the prepared positive electrode active material 100 gradually decreases. As the content of potassium in the sodium source, phosphorus source, and iron source decreases, the amount of sodium iron pyrophosphate heterogeneous phase and sodium iron phosphate heterogeneous phase generated by the positive electrode active material 100 during the preparation process gradually decreases, so that as the content of potassium in the sodium source, phosphorus source, and iron source decreases, the gram capacity of the positive electrode active material 100 gradually increases. This shows that controlling the sodium source, phosphorus source, and iron source to have less potassium can effectively inhibit the generation of sodium iron pyrophosphate heterogeneous phase and sodium iron phosphate heterogeneous phase during the preparation of the positive electrode active material 100, thereby improving the gram capacity of the positive electrode active material 100. However, the change in the content of potassium in the sodium source, phosphorus source, and iron source has little effect on the powder resistivity of the positive electrode active material 100 and the resistivity of the positive electrode sheet 200.

[0162] From the test results of Example 2, Example 4, Example 5 and Comparative Example 2, it can be seen that as the content of boron in the sodium source, phosphorus source and iron source decreases, the heterogeneous phase factor P of the prepared positive electrode active material 100 gradually decreases. As the content of boron in the sodium source, phosphorus source and iron source decreases, the amount of sodium iron pyrophosphate heterogeneous phase and sodium iron phosphate heterogeneous phase generated by the positive electrode active material 100 during the preparation process gradually decreases, so that as the content of boron in the sodium source, phosphorus source and iron source decreases, the gram capacity of the positive electrode active material 100 gradually increases. This shows that controlling the sodium source, phosphorus source and iron source to have less boron can effectively inhibit the generation of sodium iron pyrophosphate heterogeneous phase and sodium iron phosphate heterogeneous phase during the preparation of the positive electrode active material 100, and improve the gram capacity of the positive electrode active material 100. However, the change in the content of boron in the sodium source, phosphorus source and iron source has little effect on the powder resistivity of the positive electrode active material 100 and the resistivity of the positive electrode sheet 200.

[0163] From the test results of Example 2, Example 6, Example 7 and Comparative Example 3, it can be seen that as the content of sulfur in the sodium source, phosphorus source, and iron source decreases, the heterogeneous phase factor P of the prepared positive electrode active material 100 gradually decreases. As the sulfur content in the sodium source, phosphorus source, and iron source decreases, the amount of sodium iron pyrophosphate heterogeneous phase and sodium iron phosphate heterogeneous phase generated in the preparation process of the positive electrode active material 100 gradually decreases, so that as the sulfur content in the sodium source, phosphorus source, and iron source decreases, the gram capacity of the positive electrode active material 100 gradually increases. This shows that controlling the sodium source, phosphorus source, and iron source to have less sulfur can effectively inhibit the generation of sodium iron pyrophosphate heterogeneous phase and sodium iron phosphate heterogeneous phase during the preparation of the positive electrode active material 100, thereby improving the gram capacity of the positive electrode active material 100. However, the change in the sulfur content in the sodium source, phosphorus source, and iron source has little effect on the powder resistivity of the positive electrode active material 100 and the resistivity of the positive electrode sheet 200.

[0164] The test results of Comparative Example 4 show that the content of boron and sulfur in the iron source in Comparative Example 4 is too high, which greatly increases the amount of sodium iron pyrophosphate and sodium iron phosphate mixed phases generated during the preparation of the positive electrode active material 100, thereby reducing the gram capacity of the positive electrode active material 100.

[0165] From the test results of Comparative Example 5, it can be seen that the content of boron element in the sodium source and the boron element in the phosphorus source in Comparative Example 5 is too large, which greatly increases the amount of sodium iron pyrophosphate mixed phase and sodium iron phosphate mixed phase generated in the preparation process of the positive electrode active material 100. The content of boron element in the prepared positive electrode active material 100 is also greatly increased, which reduces the gram capacity of the positive electrode active material 100.

[0166] From the test results of Comparative Example 6, it can be seen that the potassium content in the sodium source in Comparative Example 6 is too high, which greatly increases the amount of sodium iron pyrophosphate mixed phase and sodium iron phosphate mixed phase generated during the preparation of the positive electrode active material 100. The potassium content in the prepared positive electrode active material 100 is also greatly increased, reducing the gram capacity of the positive electrode active material 100.

[0167] Examples 8 to 12, Comparative Examples 7 to 8

[0168] The positive electrode active material 100 of each embodiment and comparative example was prepared by the following steps:

[0169] (1) 178 g of sodium pyrophosphate (sodium source), 150 g of ammonium dihydrogen phosphate (phosphorus source), 340 g of ferrous oxalate dihydrate (iron source), 32 g of glucose (carbon source) and 2.5 kg of water (solvent) were stirred uniformly to obtain a slurry, wherein the solid content of the slurry was 35%; in each embodiment and comparative example, the content of potassium in the sodium source was less than or equal to 15 ppm, the content of potassium in the phosphorus source was less than or equal to 15 ppm, the content of potassium in the iron source was less than or equal to 50 ppm, the content of boron in the sodium source was less than or equal to 15 ppm, the content of boron in the phosphorus source was less than or equal to 15 ppm, the content of boron in the iron source was less than or equal to 50 ppm, the content of sulfur in the sodium source was less than or equal to 30 ppm, the content of sulfur in the phosphorus source was less than or equal to 30 ppm, and the content of sulfur in the iron source was less than or equal to 100 ppm.

[0170] (2) spray drying the slurry at 105° C. to obtain a precursor powder;

[0171] (3) The precursor powder was sintered in a sintering furnace under nitrogen protection for 12 hours, and the positive electrode active material 100 was obtained after cooling. The sintering temperatures of the embodiments and comparative examples are shown in Table 3 below.

[0172] Various performance tests were performed on the positive electrode active materials 100 prepared in Examples 8 to 12 and Comparative Examples 7 and 8. The test results are shown in Table 3 below.

[0173] Table 3 Performance parameters of the positive electrode active materials 100 of Examples 8 to 12 and Comparative Examples 7 to 8

[0174]

[0175] The test results of Examples 8 to 12, and Comparative Examples 7 and 8, show that when the potassium, boron, and sulfur content in the sodium, phosphorus, and iron sources remains constant, the powder resistivity of the resulting positive electrode active material 100 gradually decreases as the sintering temperature increases, as does the film resistivity of the positive electrode plate 200. As the sintering temperature increases, the heterogeneous phase factor P gradually increases, the content of the sodium iron pyrophosphate and sodium iron phosphate heterogeneous phases in the positive electrode active material 100 gradually increases, and the gram capacity of the positive electrode active material 100 gradually decreases.

[0176] It can be seen from the test results of the above embodiments and comparative examples that during the preparation process of the positive electrode active material 100 of the embodiment of the present application, by reducing the content of potassium, boron and sulfur in the sodium source, phosphorus source and iron source, the decomposition of sodium iron pyrophosphate during the sintering process can be suppressed, and the formation of sodium iron pyrophosphate impurity phase and sodium iron phosphate impurity phase can be suppressed. Furthermore, while ensuring that the positive electrode active material 100 has a high gram capacity, the powder resistivity of the positive electrode active material 100 and the film resistivity of the positive electrode sheet 200 can be greatly reduced by appropriately increasing the sintering temperature.

[0177] See Figure 7 The embodiment of the present application further provides an energy storage device 400 , which includes a box 410 and the sodium battery 300 described in the embodiment of the present application, wherein the sodium battery 300 is accommodated in the box 410 .

[0178] The energy storage device 400 of the present application can be applied to, but is not limited to, energy storage on the power generation side, energy storage on the grid side, and energy storage on the power consumption side.

[0179] Optionally, the energy storage device 400 may include, but is not limited to, a sodium battery module, a sodium battery pack, a sodium battery system, an energy storage box, an energy storage cabinet, an energy storage container, and the like. The actual application form of the energy storage device 400 provided in the embodiments of the present application may be, but is not limited to, the products listed above, and may also be other application forms. The embodiments of the present application do not impose strict restrictions on the application form of the energy storage device 400. The drawings of the embodiments of the present application illustrate only an energy storage device 400 including multiple sodium batteries 300, and should not be construed as limiting the energy storage device 400 of the embodiments of the present application.

[0180] Optionally, the number of sodium batteries 300 may be, but is not limited to, one or more. When there are multiple sodium batteries 300, the multiple sodium batteries 300 are stacked within the housing 410. It is understood that the stacking of the multiple sodium batteries 300 can be such that the multiple sodium batteries 300 are arranged side by side, or such that the multiple sodium batteries 300 are arranged side by side with intervals between them. Furthermore, the multiple sodium batteries 300 can be stacked in a transverse direction (e.g., horizontal direction) or a longitudinal direction (e.g., direction of gravity). The stacking method and stacking direction of the multiple sodium batteries 300 can be designed based on actual conditions and are not specifically limited in this application.

[0181] The term "plurality" means greater than or equal to two.

[0182] It is understandable that the multiple sodium batteries 300 of the energy storage device 400 can be connected in parallel with each other; or in series with each other; or partially in parallel and partially in series (in other words, mixed connection). This application does not specifically limit the connection method of the multiple sodium batteries 300 of the same energy storage device 400.

[0183] It is understood that the housing 410 has a receiving cavity, and one or more sodium batteries 300 are received in the receiving cavity. In some embodiments, each receiving cavity receives one sodium battery 300. In other embodiments, each receiving cavity receives multiple sodium batteries 300.

[0184] See Figure 8 and Figure 9 , an embodiment of the present application also provides an energy storage system 500, which includes the energy storage device 400 described in the embodiment of the present application; and an electric energy conversion device 510, the electric energy conversion device 510 is electrically connected to the energy storage device 400, the electric energy conversion device 510 is used to convert other forms of energy into electric energy, and the energy storage device 400 is used to store the electric energy.

[0185] It should be noted that energy storage (i.e., energy storage) has a wide range of application scenarios, including power generation-side energy storage, grid-side energy storage, and power consumption-side energy storage. The energy storage system 500 of the embodiment of the present application is described in detail using power generation-side energy storage as an example. This description should not be construed as limiting the energy storage system 500 of the embodiment of the present application, nor should it be construed as limiting the energy storage device 400, sodium battery 300, and positive electrode active material 100 of the embodiment of the present application.

[0186] During operation, the power conversion device 510 converts other forms of energy into electrical energy and stores it in the energy storage device 400. This stored energy can be used to supply loads such as streetlights and household appliances during peak electricity prices, or to provide power during power outages. The energy generated by the power conversion device 510 can also be supplied to the grid via high-voltage cables to alleviate pressure on the grid during peak hours.

[0187] Optionally, the electric energy conversion device 510 can convert at least one other form of energy such as solar energy, light energy, wind energy, thermal energy, tidal energy, biomass energy and mechanical energy into electric energy.

[0188] Optionally, the number of the electric energy conversion devices 510 may be one or more. When there are multiple electric energy conversion devices 510, the multiple electric energy conversion devices 510 may be connected in series, in parallel, or in mixed connection, which is not specifically limited in this application.

[0189] Optionally, the electric energy conversion device 510 may be, but is not limited to, at least one of a photovoltaic panel, a wind power generation device, a hydropower generation device, and the like.

[0190] Optionally, the number of the energy storage devices 400 may be one or more. When the number of the energy storage devices 400 is multiple, the multiple energy storage devices 400 are connected in series or in parallel, which is not specifically limited in this application.

[0191] References to "embodiments" and "implementation methods" in this application mean that the specific features, structures or characteristics described in conjunction with the embodiments may be included in at least one embodiment of the present application. The appearance of the phrases in various locations in the specification does not necessarily refer to the same embodiment, nor are they independent or alternative embodiments that are mutually exclusive with other embodiments. It is understood explicitly and implicitly by those skilled in the art that the embodiments described in this application can be combined with other embodiments. In addition, it should be understood that the features, structures or characteristics described in the various embodiments of this application can be arbitrarily combined to form another embodiment that does not deviate from the spirit and scope of the technical solution of this application, unless there is a contradiction between them.

[0192] Finally, it should be noted that the above implementation modes are only used to illustrate the technical solutions of the present application and are not limiting. Although the present application has been described in detail with reference to the above preferred implementation modes, ordinary technicians in this field should understand that the technical solutions of the present application can be modified or replaced by equivalents without departing from the spirit and scope of the technical solutions of the present application.

Claims

1. A positive electrode active material, characterized in that The positive electrode active material includes sodium iron phosphate pyrophosphate particles and a carbon coating layer, wherein the carbon coating layer is wrapped around the surface of the sodium iron phosphate pyrophosphate particles; the content of potassium in the positive electrode active material is less than or equal to 100 ppm.

2. The positive electrode active material according to claim 1, characterized in that The content of boron in the positive electrode active material is less than or equal to 100 ppm.

3. The positive electrode active material according to claim 1, characterized in that The content of sulfur in the positive electrode active material is less than or equal to 200 ppm.

4. The positive electrode active material according to claim 1, characterized in that In the X-ray diffraction pattern of the positive electrode active material, the maximum intensity of the diffraction peak at a diffraction angle 2θ of 10.5° to 10.9° is I1, the maximum intensity of the diffraction peak at a diffraction angle 2θ of 32.8° to 33.2° is I2, and the maximum intensity of the diffraction peak at a diffraction angle 2θ of 33.3° to 33.8° is I3. The positive electrode active material satisfies the relationship: P = (I1 + I2) / I3 ≤ 0.

1.

5. The positive electrode active material according to claim 1, characterized in that The powder resistivity R of the positive electrode active material is in the range of 70Ω·cm≤R≤800Ω·cm.

6. A method for preparing a positive electrode active material, characterized in that: include: Provide sodium source, phosphorus source, iron source and carbon source; mixing the sodium source, the phosphorus source, the iron source, and the carbon source in a solvent to obtain a slurry; spray drying the slurry to obtain a precursor powder; and The precursor powder is sintered to obtain the positive electrode active material; the positive electrode active material includes sodium iron phosphate pyrophosphate particles and a carbon coating layer, and the carbon coating layer is wrapped around the surface of the sodium iron phosphate pyrophosphate particles; the content of potassium in the positive electrode active material is less than or equal to 100 ppm.

7. The method for preparing a positive electrode active material according to claim 6, wherein: in, The content of potassium in the sodium source is less than or equal to 20 ppm, the content of potassium in the phosphorus source is less than or equal to 20 ppm, and the content of potassium in the iron source is less than or equal to 60 ppm.

8. The method for preparing a positive electrode active material according to claim 6, wherein: The content of boron in the sodium source is less than or equal to 20 ppm, the content of boron in the phosphorus source is less than or equal to 20 ppm, and the content of boron in the iron source is less than or equal to 60 ppm.

9. The method for preparing a positive electrode active material according to claim 6, wherein: The sulfur content in the sodium source is less than or equal to 40 ppm, the sulfur content in the phosphorus source is less than or equal to 40 ppm, and the sulfur content in the iron source is less than or equal to 120 ppm.

10. The method for preparing a positive electrode active material according to any one of claims 6 to 9, characterized in that: The step of sintering the precursor powder to obtain the positive electrode active material comprises: The precursor powder is sintered at a temperature of 580° C. to 630° C. to obtain the positive electrode active material.

11. A positive electrode plate, characterized in that: The positive electrode plate comprises: a positive electrode current collector; and A positive electrode active layer, wherein the positive electrode active layer comprises the positive electrode active material according to any one of claims 1 to 5 or the positive electrode active material prepared by the preparation method of the positive electrode active material according to any one of claims 6 to 10.

12. A sodium battery, characterized in that: include: An electrolyte, a positive electrode sheet according to claim 11, a separator, and a negative electrode sheet.

13. An energy storage device, characterized in that: include: Box; as well as The sodium battery according to claim 12, wherein the sodium battery is housed in the casing.

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