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

Abstract

The application provides a positive electrode active material and a preparation method thereof, a positive electrode sheet, a sodium battery and an energy storage device. The positive electrode active material of the application comprises sodium iron pyrophosphite phosphate particles and a carbon coating layer, the carbon coating layer is wrapped on the surface of the sodium iron pyrophosphite phosphate particles, and the content of potassium elements in the positive electrode active material is less than or equal to 100 ppm.

Description

Technical Field

[0001] This application relates to the field of energy storage, specifically to a positive electrode active material and its preparation method, a positive electrode sheet, a sodium battery, and an energy storage device. Background Technology

[0002] Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7), with its three-dimensional sodium ion diffusion channels and sodium superionic conductor structure, exhibits high voltage plateau, high capacity, excellent rate capability, and excellent 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, which, when applied to sodium batteries, results in excessively high internal resistance and reduces cycle life. Summary of the Invention

[0003] This application provides a positive electrode active material with low powder resistivity.

[0004] In a first aspect, embodiments of this application provide a positive electrode active material, the positive electrode active material comprising sodium iron pyrophosphate particles and a carbon coating layer, the carbon coating layer being coated on the surface of the sodium iron pyrophosphate particles; the content of potassium element in the positive electrode active material is less than or equal to 100 ppm.

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

[0006] Furthermore, the sulfur content 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 resistivity R of the positive electrode active material powder is in the range of 70Ω·cm≤R≤800Ω·cm.

[0009] Secondly, embodiments of this application provide a method for preparing a positive electrode active material, comprising:

[0010] It provides sodium, phosphorus, iron, and carbon sources;

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

[0012] The slurry was spray-dried to obtain precursor powder; and

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

[0014] Furthermore, 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.

[0015] Furthermore, 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.

[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] Further, the sintering of the precursor powder to obtain the positive electrode active material includes:

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

[0019] Thirdly, embodiments of this application provide a positive electrode sheet, the positive electrode sheet comprising:

[0020] Positive current collector; and

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

[0022] Fourthly, embodiments of this application provide a sodium battery, which includes: an electrolyte, a positive electrode as described in the third aspect of this application, a separator, and a negative electrode.

[0023] Fifthly, embodiments of this application provide an energy storage device, which includes:

[0024] Box; and

[0025] The sodium battery described in the fourth aspect of this application is housed within the casing.

[0026] The positive electrode active material described in this embodiment includes positive electrode particles, which comprise sodium iron pyrophosphate particles and a carbon coating layer. The carbon coating layer coats the surface of the sodium iron pyrophosphate particles. The potassium content in the positive electrode active material is less than or equal to 100 ppm. By controlling the potassium content in the positive electrode active material to be less than or equal to 100 ppm, the decomposition of sodium iron pyrophosphate can be effectively suppressed during sintering, increasing the decomposition temperature of sodium iron pyrophosphate and further reducing the formation of sodium iron phosphate and sodium iron pyrophosphate impurities, thereby increasing the specific capacity of the positive electrode active material. Furthermore, sintering can be performed at a higher temperature during the preparation of the positive electrode active material, resulting in a higher electron transport rate and lower powder resistivity in the carbon coating layer. 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, 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 while ensuring that the resistivity of the positive electrode sheet remains unchanged, thereby greatly reducing the cost of the positive electrode sheet. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0028] Figure 1 This is a schematic diagram of the structure of a positive electrode active material according to an embodiment of this application.

[0029] Figure 2 This is a schematic flowchart of a method for preparing a positive electrode active material according to an embodiment of this application.

[0030] Figure 3 This is a schematic diagram of the structure of the positive electrode sheet according to an embodiment of this application.

[0031] Figure 4 This is a schematic diagram of the structure of a sodium battery according to an embodiment of this application.

[0032] Figure 5 This application describes a sodium battery according to an embodiment of the present application. Figure 4 A schematic diagram of the cross-sectional structure along the AA direction.

[0033] Figure 6 This is a schematic diagram of the structure of the negative electrode sheet according to an embodiment of this application.

[0034] Figure 7 This is a schematic diagram of the structure of an energy storage device according to an embodiment of this application.

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

[0036] Figure 9 This is an application scenario diagram of an energy storage system according to an embodiment of this application.

[0037] Explanation of reference numerals in the attached figures:

[0038] 100-Positive electrode active material, 10-Sodium iron pyrophosphate particles, 20-Carbon coating layer, 200-Positive electrode sheet, 210-Positive electrode current collector, 220-Positive electrode active layer, 300-Sodium battery, 320-Separator, 330-Negative electrode sheet, 331-Negative electrode current collector, 332-Negative electrode active layer, 340-Shell, 350-End cap assembly, 400-Energy storage device, 410-Box, 500-Energy storage system, 510-Electric power conversion device. Detailed Implementation

[0039] To enable those skilled in the art to better understand the present application, the technical solutions in the embodiments of the present application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present application, and not all embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present application.

[0040] The terms "first," "second," etc., in the specification, claims, and accompanying drawings of this application are used to distinguish different objects, not to describe a specific order. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover non-exclusive inclusion. For example, a process, method, system, product, or apparatus that includes a series of steps or units is not limited to the listed steps or units, but may optionally include steps or units not listed, or may optionally include other steps or units inherent to these processes, methods, products, or apparatuses.

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

[0042] It should be noted that, for ease of explanation, the same reference numerals denote the same components in the embodiments of this application, 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 solar, wind, and hydropower. However, wind and solar power are generally characterized by strong intermittency and large fluctuations, which can cause grid instability, insufficient power during peak demand periods, and excessive power during off-peak periods. Unstable voltage can also damage the power grid. Therefore, insufficient electricity demand or insufficient grid capacity may lead to the problem of "wind and solar curtailment." Solving these problems requires energy storage. This involves converting electrical energy into other forms of energy through physical or chemical means and storing it. When needed, this energy can be converted back into electrical energy and released. Simply put, energy storage is like a large "power bank," storing electrical energy when solar and wind power are abundant and releasing the stored electricity when needed.

[0044] Batteries are the smallest energy storage unit in energy storage devices and systems, and their performance directly affects the performance and application of these devices and systems. Batteries include lithium batteries and sodium batteries.

[0045] Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7), possessing a three-dimensional sodium ion diffusion channel and a sodium superionic conductor structure, exhibits high voltage plateau, high capacity, excellent rate capability, and excellent 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, resulting in excessive internal resistance and reduced cycle life in sodium batteries. Related technologies utilize carbon coating to reduce the powder resistivity of sodium iron pyrophosphate and improve its electron transport efficiency. However, the conductivity of the carbon coating layer depends on the carbonization temperature; excessively high carbonization temperatures can easily cause sodium iron pyrophosphate to decompose, generating low-activity sodium iron phosphate impurities and sodium iron pyrophosphate impurities, leading to a significant reduction in the specific capacity of sodium iron pyrophosphate. Therefore, this application provides a cathode active material.

[0046] Please see Figure 1 This application provides a positive electrode active material 100, which includes sodium iron pyrophosphate particles 10 and a carbon coating layer 20. The carbon coating layer 20 is wrapped around the surface of the sodium iron pyrophosphate particles 10. The content of potassium in the positive electrode active material 100 is less than or equal to 100 ppm.

[0047] The positive electrode active material 100 of this application embodiment can be applied to sodium batteries (such as sodium-ion batteries) as the active material of the positive electrode sheet of sodium batteries.

[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 outer shell. In other words, the cathode particles have a core-shell structure.

[0049] Specifically, the potassium content 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, it can effectively suppress the decomposition of sodium iron pyrophosphate during the synthesis of the positive electrode active material 100, increase the decomposition temperature of sodium iron pyrophosphate, better reduce the formation of sodium iron phosphate impurity phase and sodium iron pyrophosphate impurity phase, and improve the specific capacity of the positive electrode active material 100. In addition, the positive electrode active material 100 can be sintered at a higher temperature, thereby making the carbon coating layer 20 of the positive electrode active material 100 have a higher electron transport rate (the carbonization degree of the carbon coating layer 20 increases and the conductivity increases as the sintering temperature increases), and has a lower powder resistivity. However, if the potassium content in the positive electrode active material 100 is too low, the requirements for the purity of the raw materials and the preparation process conditions of the positive electrode active material 100 will be too stringent, increasing 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 sodium iron pyrophosphate decomposition increases during synthesis, leading to an increase in the content of impurities (sodium iron phosphate impurities and sodium iron pyrophosphate impurities) in the sodium iron pyrophosphate particles 10, thus reducing the specific capacity of the positive electrode active material 100. Furthermore, the high potassium content in the positive electrode active material 100 increases the proportion of sodium iron pyrophosphate decomposition. Therefore, during the preparation of the positive electrode active material 100, the sintering temperature needs to be lowered to ensure a high proportion of sodium iron pyrophosphate phase and a high specific capacity in the synthesized positive electrode active material 100. However, lowering the sintering temperature reduces the electron transport rate and 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 sodium batteries.

[0051] The positive electrode active material 100 of this application embodiment includes positive electrode particles, which include sodium iron pyrophosphate particles 10 and a carbon coating layer 20. The carbon coating layer 20 is wrapped around the surface of the sodium iron 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 pyrophosphate can be effectively suppressed during sintering during the preparation of the positive electrode active material 100, thereby increasing the decomposition temperature of sodium iron pyrophosphate, further reducing the formation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities, and improving the specific capacity of the positive electrode active material 100. In addition, the positive electrode active material 100 can be sintered at a higher temperature during preparation, thereby giving the carbon coating layer 20 of the positive electrode active material 100 a higher electron transport 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, 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 while ensuring that the resistivity of the positive electrode sheet remains unchanged, 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 boron content 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 pyrophosphate during the synthesis of the positive electrode active material 100 can be effectively suppressed, the decomposition temperature of sodium iron pyrophosphate can be increased, the formation of sodium iron phosphate impurity phase and sodium iron pyrophosphate impurity phase can be better reduced, and the specific 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, thereby making the carbon coating layer 20 of the positive electrode active material 100 have a higher electron transport rate and a lower powder resistivity. However, if the boron content in the positive electrode active material 100 is too low, the requirements for the purity of the raw materials and the preparation process conditions of the positive electrode active material 100 will be too stringent, increasing 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 iron pyrophosphate increases during its synthesis. This leads to an increase in the content of impurities (sodium iron phosphate impurities and sodium iron pyrophosphate impurities) in the sodium iron pyrophosphate particles 10, thereby reducing the specific capacity of the positive electrode active material 100. Furthermore, when the boron content in the positive electrode active material 100 is too high, the increased proportion of sodium iron pyrophosphate decomposition necessitates a lower sintering temperature during the preparation of the positive electrode active material 100 to ensure a high proportion of sodium iron pyrophosphate phase and a high specific capacity. However, lowering the sintering temperature reduces the electron transport rate and 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 sodium batteries.

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

[0056] Specifically, the sulfur content 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 pyrophosphate during the synthesis of the positive electrode active material 100 can be effectively suppressed, the decomposition temperature of sodium iron pyrophosphate can be increased, the formation of sodium iron phosphate impurity phase and sodium iron pyrophosphate impurity phase can be better reduced, and the specific 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, thereby making the carbon coating layer 20 of the positive electrode active material 100 have a higher electron transport rate and a lower powder resistivity. However, if the sulfur content in the positive electrode active material 100 is too low, the requirements for the purity of the raw materials and the preparation process conditions of the positive electrode active material 100 will be too stringent, increasing 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 sodium iron pyrophosphate decomposition increases during its synthesis. This leads to an increase in the content of impurities (sodium iron phosphate impurities and sodium iron pyrophosphate impurities) in the sodium iron pyrophosphate particles 10, thereby reducing the specific capacity of the positive electrode active material 100. Furthermore, because the proportion of sodium iron pyrophosphate decomposition increases when the sulfur content in the positive electrode active material 100 is too high, the sintering temperature needs to be lowered during its preparation to ensure a higher proportion of sodium iron pyrophosphate phase in the synthesized positive electrode active material 100 and a higher specific capacity. However, lowering the sintering temperature reduces the electron transport rate and 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 sodium batteries.

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

[0059] Furthermore, compared to embodiments where only one of the following conditions is met: potassium content less than or equal to 100 ppm, boron content less than or equal to 100 ppm, and sulfur content less than or equal to 200 ppm, when the positive electrode active material 100 meets at least two or three of the following conditions: potassium content less than or equal to 100 ppm, boron content less than or equal to 100 ppm, and sulfur content less than or equal to 200 ppm, the synergistic effect of two or three of these conditions can better increase the decomposition temperature of sodium iron pyrophosphate and increase the specific capacity of the positive electrode active particles; it can also increase the sintering temperature, thereby giving the carbon coating layer 20 of the positive electrode active material 100 a higher electron transport 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) of this application were measured using inductively coupled plasma optical emission spectrometry (ICP-OES). Specifically, 0.2g to 0.3g of the positive electrode active material 100 powder was dissolved in a strong acid, and then the dissolved solution was added to the ICP-OES instrument for testing to obtain 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 peaks at diffraction angles 2θ of 10.5° to 10.9° are diffraction peaks of sodium iron pyrophosphate; the diffraction peaks at diffraction angles 2θ of 32.8° to 33.2° are diffraction peaks of sodium iron phosphate; and the diffraction peaks at diffraction angles 2θ of 33.3° to 33.8° are diffraction peaks of sodium iron 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, i.e., the impurity phase factor. The smaller P is, the less impurity phase is in the positive electrode active material 100, and the higher the specific capacity of the positive electrode active material 100. However, if 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 impurity phase is in the positive electrode active material 100, and the lower the specific capacity of the positive electrode active material 100.

[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 sodium iron phosphate impurity phase and sodium iron pyrophosphate impurity phase in the positive electrode active material 100 is too high, which reduces the specific capacity of the positive electrode active material 100.

[0066] Optionally, the positive electrode active material 100 also 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. When the value of I1 / I3 is too large, it indicates that the content of sodium iron pyrophosphate impurity phase in the positive electrode active material 100 is too high, which reduces the specific capacity of the positive electrode active material 100.

[0067] Optionally, the positive electrode active material 100 also 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. When the value of I2 / I3 is too large, it indicates that the content of sodium iron phosphate impurity phase in the positive electrode active material 100 is too high, which reduces the specific 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 this embodiment of the application is measured using the two-probe method. Specifically, 2g to 3g of positive electrode active material 100 powder is added to the mold of a powder resistivity instrument with a diameter of 13mm, and after being pressurized to 200MPa, the powder resistivity value is measured.

[0070] Specifically, the powder resistivity R of the positive electrode active material 100 can be, but is not limited to, 70 Ω·cm, 80 Ω·cm, 90 Ω·cm, 100 Ω·cm, 150 Ω·cm, 200 Ω·cm, 250 Ω·cm, 300 Ω·cm, 350 Ω·cm, 400 Ω·cm, 450 Ω·cm, 500 Ω·cm, 550 Ω·cm, 600 Ω·cm, 650 Ω·cm, 700 Ω·cm, 750 Ω·cm, 800 Ω·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 proportion of sodium iron pyrophosphate decomposition, reducing the specific capacity of the positive electrode active material 100. If the powder resistivity R of the positive electrode active material 100 is too high, when applied to a sodium battery, it will result in excessive internal resistance of the sodium battery, reducing the cycle life of the sodium battery.

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

[0073] The term "D10" refers to the particle size at which the cumulative distribution of the positive electrode active material 100 reaches 10% in the particle size distribution test.

[0074] Specifically, the D10 of the positive electrode active material 100 can be, but is not limited to, 0.5μm, 0.8μm, 1.0μm, 1.3μm, 1.5μm, 1.8μm, 2.0μm, 2.3μm, 2.5μm, 2.8μm, 3μm, etc. If the D10 of the positive electrode active material 100 is too small, the viscosity of the positive electrode slurry will be too high during preparation, making the slurry coating uneven or even impossible. If the D10 of the positive electrode active material 100 is too large, the deposition effect of the positive electrode active material 100 on the positive electrode current collector will be poor when the slurry of the positive electrode active material 100 is coated on the positive electrode current collector. Excessive pressure will be generated 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 sodium battery under high current.

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

[0076] The term "D50" refers to the particle size at which the cumulative distribution of the positive electrode active material 100 reaches 50% in the particle size distribution test.

[0077] Specifically, the D50 of the positive electrode active material 100 can be, but is not limited to, 4μm, 4.5μm, 5μm, 5.5μm, 6μm, 6.5μm, 7μm, 7.5μm, 8μm, 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 during preparation, making the slurry coating uneven or even impossible. If the D50 of the positive electrode active material 100 is too large, the deposition effect of the positive electrode active material 100 on the positive electrode current collector will be poor when the positive electrode active material 100 is coated onto the positive electrode current collector. Excessive pressure will be generated on the positive electrode current collector during the rolling process of the positive electrode sheet, reducing the ion conductivity of the positive electrode sheet and reducing the discharge capacity of the sodium battery under high current.

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

[0079] The term "D90" refers to the particle size at which the cumulative distribution of the positive electrode active material 100 reaches 90% in the particle size distribution test.

[0080] Specifically, the D90 of the positive electrode active material 100 can be, but is not limited to, 9μm, 9.5μm, 10μm, 10.5μm, 11μm, 11.5μm, 12μm, 12.5μm, 13μm, etc. If the D90 of the positive electrode active material 100 is too small, the viscosity of the positive electrode slurry will be too high during preparation, making the slurry coating uneven or even impossible. If the D90 of the positive electrode active material 100 is too large, the deposition effect of the positive electrode active material 100 on the positive electrode current collector will be poor when the slurry of the positive electrode active material 100 is coated on the positive electrode current collector. Excessive pressure will be generated on the positive electrode current collector during the rolling process of the positive electrode sheet, reducing the ion conductivity of the positive electrode sheet and reducing the discharge capacity of the sodium battery under high current.

[0081] The D10, D50, and D90 values ​​of this embodiment were tested as follows: 0.2g to 0.3g of positive electrode active material 100 powder was taken and dispersed in 100ml of deionized water by external ultrasonication for 5min to obtain a dispersion. The dispersion was then tested using a laser particle size analyzer. Each sample was tested three times, and the average value of the three tests was taken to obtain the particle size distribution curve of positive electrode active material 100 and the D10, D50, and D90 values.

[0082] The positive electrode active material 100 of this application embodiment can be prepared by the method described in the following embodiments of this application. In addition, it can also be prepared by other methods. The preparation method of this application embodiment is only one or more preparation methods of the positive electrode active material 100 of this application, and should not be construed as a limitation on the positive electrode active material 100 provided in the embodiments of this application.

[0083] Please see Figure 2 This application provides a method for preparing a positive electrode active material 100, which includes:

[0084] S201 provides sodium, phosphorus, iron and carbon sources;

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

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

[0087] Optionally, the iron source may include, but is not limited to, at least one of these compounds, including ferrous oxalate, ferric nitrate, ferrous sulfate, and compounds containing water of crystallization. For example, 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, sodium source, phosphorus source, iron source and carbon source are mixed in a solvent to obtain a slurry;

[0090] S203, the slurry is spray-dried to obtain precursor powder; and

[0091] S204, the precursor powder is sintered to obtain the positive electrode active material 100; the positive electrode active material 100 includes sodium iron pyrophosphate particles 10 and a carbon coating layer 20, the carbon coating layer 20 is wrapped around the surface of the sodium iron pyrophosphate particles 10; the potassium content 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 described in this application embodiment, by controlling the potassium content in the sodium, phosphorus, and iron sources to be relatively low, the decomposition of sodium iron pyrophosphate can be effectively suppressed during sintering during the preparation of the positive electrode active material 100, thereby increasing the decomposition temperature of sodium iron pyrophosphate and further reducing the formation of sodium iron phosphate and sodium iron pyrophosphate impurities, thus increasing the specific capacity of the obtained positive electrode active material 100. Furthermore, the increased decomposition temperature of sodium iron pyrophosphate allows for sintering at higher temperatures during the preparation of the positive electrode active material 100, resulting in a higher electron transport rate and lower powder resistivity in the carbon coating layer 20 of the positive electrode active material 100. Moreover, because the positive electrode active material 100 has a lower powder resistivity, when applied to a positive electrode sheet, the amount of expensive positive electrode conductive agents such as carbon nanotubes in the positive electrode active layer can be reduced while maintaining the same resistivity, 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 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 potassium content in the sodium source is low, the decomposition of sodium iron pyrophosphate can be effectively suppressed during sintering, increasing the decomposition temperature of sodium iron pyrophosphate, better reducing the formation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities, and increasing the specific capacity of the positive electrode active material 100. In addition, since the decomposition temperature of sodium iron 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 obtained positive electrode active material 100 have a higher electron transport 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 increase. If the potassium content in the sodium source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered during sintering, increasing the probability of decomposition. This leads to an increase in the content of impurities (sodium iron phosphate impurities and sodium iron pyrophosphate impurities) in the sodium iron pyrophosphate particles 10, thereby reducing the specific capacity of the positive electrode active material 100. Furthermore, when the potassium content in the sodium source is too high, the decomposition temperature of sodium iron pyrophosphate is lowered, requiring a lower sintering temperature to ensure a higher proportion of the sodium iron pyrophosphate phase in the obtained positive electrode active material 100, resulting in a higher specific capacity. However, lowering the sintering temperature will reduce the electron transport rate of the carbon coating layer 20, decreasing its conductivity and thus increasing the powder resistivity of the positive electrode active material 100. When applied to sodium batteries, this reduces the cycle life of the sodium battery.

[0095] Specifically, the potassium content in the phosphorus 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 potassium content in the phosphorus source is low, the decomposition of sodium iron pyrophosphate can be effectively suppressed during sintering, increasing the decomposition temperature of sodium iron pyrophosphate, better reducing the formation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities, and increasing the specific capacity of the positive electrode active material 100. In addition, since the decomposition temperature of sodium iron 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 obtained positive electrode active material 100 have a higher electron transport 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 increase. If the potassium content in the phosphorus source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered during sintering, increasing the probability of decomposition. This leads to an increase in the content of impurities (sodium iron phosphate impurities and sodium iron pyrophosphate impurities) in the sodium iron pyrophosphate particles 10, thereby reducing the specific capacity of the positive electrode active material 100. Furthermore, when the potassium content in the phosphorus source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered, requiring a reduction in the sintering temperature to ensure a high proportion of sodium iron pyrophosphate phase in the obtained positive electrode active material 100, resulting in a high specific capacity. However, lowering the sintering temperature will reduce the electron transport rate of the carbon coating layer 20, decreasing its conductivity and thus increasing the powder resistivity of the positive electrode active material 100. When applied to sodium batteries, this reduces the cycle life of the sodium battery.

[0096] Specifically, the potassium content in the iron source can 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 potassium content in the iron source is low, the decomposition of sodium iron pyrophosphate can be effectively suppressed during sintering, increasing the decomposition temperature of sodium iron pyrophosphate, better reducing the formation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities, and increasing the specific capacity of the positive electrode active material 100. In addition, since the decomposition temperature of sodium iron 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 obtained positive electrode active material 100 have a higher electron transport rate and a lower powder resistivity. However, if the potassium content in the iron source is too low, the cost of the iron source will increase. If the potassium content in the iron source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered during sintering, increasing the probability of decomposition. This leads to an increase in the content of impurities (sodium iron phosphate impurities and sodium iron pyrophosphate impurities) in the sodium iron pyrophosphate particles 10, thereby reducing the specific capacity of the positive electrode active material 100. Furthermore, when the potassium content in the iron source is too high, the decomposition temperature of sodium iron pyrophosphate is lowered, requiring a lower sintering temperature to ensure a higher proportion of the sodium iron pyrophosphate phase in the obtained positive electrode active material 100, resulting in a higher specific capacity. However, lowering the sintering temperature will reduce the electron transport rate of the carbon coating layer 20, decreasing its conductivity and thus increasing the powder resistivity of the positive electrode active material 100. When applied to sodium batteries, this reduces the cycle life of 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 iron pyrophosphate can be effectively suppressed during sintering, increasing the decomposition temperature of sodium iron pyrophosphate, better reducing the formation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities, and increasing the specific capacity of the positive electrode active material 100. In addition, since the decomposition temperature of sodium iron 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 obtained positive electrode active material 100 have a higher electron transport 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 increase. If the boron content in the sodium source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered during sintering, increasing the probability of decomposition. This leads to an increase in the content of impurities (sodium iron phosphate impurities and sodium iron pyrophosphate impurities) in the sodium iron pyrophosphate particles 10, thereby reducing the specific capacity of the positive electrode active material 100. Furthermore, when the boron content in the sodium source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered, requiring a reduction in the sintering temperature to ensure a high proportion of sodium iron pyrophosphate phase in the obtained positive electrode active material 100, resulting in a high specific capacity. However, lowering the sintering temperature will reduce the electron transport rate of the carbon coating layer 20, decreasing its conductivity and thus increasing the powder resistivity of the positive electrode active material 100. When applied to sodium batteries, this reduces the cycle life of the sodium battery.

[0099] Specifically, the boron content in the phosphorus 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 phosphorus source is low, the decomposition of sodium iron pyrophosphate can be effectively suppressed during sintering, increasing the decomposition temperature of sodium iron pyrophosphate, better reducing the formation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities, and increasing the specific capacity of the positive electrode active material 100. In addition, since the decomposition temperature of sodium iron 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 obtained positive electrode active material 100 have a higher electron transport 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 increase. If the boron content in the phosphorus source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered during sintering, increasing the probability of decomposition. This leads to an increase in the content of impurities (sodium iron phosphate impurities and sodium iron pyrophosphate impurities) in the sodium iron pyrophosphate particles 10, thereby reducing the specific capacity of the positive electrode active material 100. Furthermore, when the boron content in the phosphorus source is too high, the decomposition temperature of sodium iron pyrophosphate is lowered, requiring a lower sintering temperature to ensure a higher proportion of the sodium iron pyrophosphate phase in the obtained positive electrode active material 100, resulting in a higher specific capacity. However, lowering the sintering temperature will reduce the electron transport rate of the carbon coating layer 20, decreasing its conductivity and thus increasing the powder resistivity of the positive electrode active material 100. When applied to sodium batteries, this reduces the cycle life of the sodium battery.

[0100] Specifically, the boron content in the iron source can 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 iron pyrophosphate can be effectively suppressed during sintering, increasing the decomposition temperature of sodium iron pyrophosphate, better reducing the formation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities, and increasing the specific capacity of the positive electrode active material 100. In addition, since the decomposition temperature of sodium iron 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 obtained positive electrode active material 100 have a higher electron transport 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 increase. If the boron content in the iron source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered during sintering, increasing the probability of decomposition. This leads to an increase in the content of impurities (sodium iron phosphate impurities and sodium iron pyrophosphate impurities) in the sodium iron pyrophosphate particles 10, thereby reducing the specific capacity of the positive electrode active material 100. Furthermore, when the boron content in the iron source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered, requiring a reduction in the sintering temperature to ensure a high proportion of sodium iron pyrophosphate phase in the obtained positive electrode active material 100, resulting in a high specific capacity. However, lowering the sintering temperature will reduce the electron transport rate of the carbon coating layer 20, decreasing its conductivity and thus increasing the powder resistivity of the positive electrode active material 100. When applied to sodium batteries, this reduces the cycle life of 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 pyrophosphate can be effectively suppressed during sintering, increasing the decomposition temperature of sodium iron pyrophosphate and further reducing the formation of sodium iron phosphate and sodium iron pyrophosphate impurities, thereby increasing the specific capacity of the positive electrode active material 100. Furthermore, because the decomposition temperature of sodium iron pyrophosphate is increased, the positive electrode active material 100 can be sintered at a higher temperature, resulting in a carbon coating layer 20 with a higher electron transport rate and lower powder resistivity. However, if the sulfur content in the sodium source is too low, the cost of the sodium source increases. If the sulfur content in the sodium source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered during sintering, increasing the probability of decomposition. This leads to an increase in the content of impurities (sodium iron phosphate impurities and sodium iron pyrophosphate impurities) in the sodium iron pyrophosphate particles 10, thereby reducing the specific capacity of the positive electrode active material 100. Furthermore, when the sulfur content in the sodium source is too high, the decomposition temperature of sodium iron pyrophosphate is lowered, requiring a lower sintering temperature to ensure a higher proportion of the sodium iron pyrophosphate phase in the obtained positive electrode active material 100, resulting in a higher specific capacity. However, lowering the sintering temperature will reduce the electron transport rate of the carbon coating layer 20, decreasing its conductivity and thus increasing the powder resistivity of the positive electrode active material 100. When applied to sodium batteries, this reduces the cycle life of the sodium battery.

[0103] Specifically, the sulfur content 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 pyrophosphate can be effectively suppressed during sintering, increasing the decomposition temperature of sodium iron pyrophosphate and further reducing the formation of sodium iron phosphate and sodium iron pyrophosphate impurities, thereby increasing the specific capacity of the positive electrode active material 100. Furthermore, because the decomposition temperature of sodium iron pyrophosphate is increased, the positive electrode active material 100 can be sintered at a higher temperature, resulting in a carbon coating layer 20 with a higher electron transport rate and lower powder resistivity. However, if the sulfur content in the phosphorus source is too low, the cost of the phosphorus source increases. If the sulfur content in the phosphorus source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered during sintering, increasing the probability of sodium iron pyrophosphate decomposition. This will increase the content of impurities (sodium iron phosphate impurities and sodium iron pyrophosphate impurities) in the sodium iron pyrophosphate particles 10, thereby reducing the specific capacity of the positive electrode active material 100. In addition, when the sulfur content in the phosphorus source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered, requiring a lower sintering temperature to ensure that the sodium iron pyrophosphate phase has a high proportion in the obtained positive electrode active material 100, and that the positive electrode active material 100 has a high specific capacity. However, lowering the sintering temperature will reduce the electron transport 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. When applied to sodium batteries, this will reduce the cycle life of the sodium battery.

[0104] Specifically, the sulfur content in the iron source can 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 iron pyrophosphate can be effectively suppressed during sintering, increasing the decomposition temperature of sodium iron pyrophosphate, better reducing the formation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities, and increasing the specific capacity of the positive electrode active material 100. In addition, since the decomposition temperature of sodium iron 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 obtained positive electrode active material 100 have a higher electron transport 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 increase. If the sulfur content in the iron source is too high, the decomposition temperature of sodium iron pyrophosphate will be lowered during sintering, increasing the probability of decomposition. This leads to an increase in the content of impurities (sodium iron phosphate impurities and sodium iron pyrophosphate impurities) in the sodium iron pyrophosphate particles 10, thereby reducing the specific capacity of the positive electrode active material 100. Furthermore, when the sulfur content in the iron source is too high, the decomposition temperature of sodium iron pyrophosphate is lowered, requiring a lower sintering temperature to ensure a higher proportion of the sodium iron pyrophosphate phase in the obtained positive electrode active material 100, resulting in a higher specific capacity. However, lowering the sintering temperature will reduce the electron transport rate of the carbon coating layer 20, decreasing its conductivity and thus increasing the powder resistivity of the positive electrode active material 100. When applied to sodium batteries, this reduces the cycle life of the sodium battery.

[0105] In some embodiments, in S202, the step of mixing the sodium source, phosphorus source, iron source and carbon source in a solvent to obtain a slurry includes: mixing the sodium source, phosphorus source, iron source and carbon source in water and performing a sand milling process 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 this embodiment of the application, when the numerical range a to b is involved, unless otherwise specified, the numerical value can be any value between a and b, including the endpoint value a and the endpoint value b.

[0107] Understandably, 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 mixed by sand milling. Sand milling can reduce the particle size of the raw materials, providing a prerequisite for the formation of sodium iron pyrophosphate. In addition, sand milling can fully mix the soluble and insoluble substances in the raw materials, avoiding uneven mixing of the raw materials when there are multiple insoluble raw materials, which would increase the probability of impurity phases in the final sodium iron pyrophosphate.

[0109] Specifically, the milling time can be, but is not limited to, 0.5h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, etc. If the milling time is too short, the particle size of the insoluble raw materials in the sodium, phosphorus, iron, and carbon sources will be too large, resulting in localized growth and crystallization during the subsequent sintering process, thus reducing the sphericity of the obtained positive electrode active material 100. If the milling time is too long, it will reduce production efficiency.

[0110] Specifically, the milling speed can be, but is not limited to, 1000 rpm, 1500 rpm, 2000 rpm, 2500 rpm, 3000 rpm, 3500 rpm, 4000 rpm, etc. If the milling speed is too low, the particle size of the insoluble raw materials in the sodium, phosphorus, iron, and carbon sources will be too large, resulting in localized protruding crystal growth during the subsequent sintering process, which reduces the sphericity of the obtained positive electrode active material 100. If the milling speed is too high, the slurry is prone to splashing.

[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 quickly and effectively shrink into a spherical shape 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, it will cause great difficulties in the sand milling stage. In addition, if the material forms spheres too quickly during the spray drying stage, there is not enough time to form spheres with high sphericity, thereby reducing the sphericity of the obtained precursor powder, which in turn leads to a reduction in the sphericity of the final positive electrode active material 100.

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

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

[0114] In this embodiment, if the spray drying temperature of the slurry is too low, the solvent (such as water) evaporates too slowly, resulting in a slow spherical formation rate of the precursor powder. This can easily lead to the formation of hollow or collapsed spherical shapes, affecting the processing performance and compaction density of the final positive electrode active material 100. If the spray drying temperature of the slurry is too high, the solvent (such as water) evaporates too quickly, resulting in a fast spherical formation rate of the precursor powder and a decrease in the sphericity of the formed precursor powder.

[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, sintering is carried out under an inert atmosphere. Optionally, the inert atmosphere includes at least one of helium, nitrogen, and argon.

[0117] Specifically, the sintering temperature of the precursor powder can be, but is not limited to, 580℃, 585℃, 590℃, 595℃, 600℃, 605℃, 610℃, 615℃, 620℃, 625℃, 630℃, etc. If the sintering temperature of the precursor powder is too low, the carbonization degree of the carbon coating layer 20 in the obtained positive electrode active material 100 will be too low, and the powder resistivity of the positive electrode active material 100 will be too high. When applied to sodium batteries, this will result in excessive internal resistance of the sodium battery and reduce the cycle life of the sodium battery. If the sintering temperature of the precursor powder is too high, the control of impurity elements (potassium, boron, sulfur) in the sodium, iron, and phosphorus sources will not be sufficient to inhibit the decomposition of sodium pyrophosphate at high temperatures. Sodium pyrophosphate is prone to decomposition to generate sodium iron phosphate impurity phase and sodium pyrophosphate impurity phase, thereby reducing the specific capacity of the positive electrode active material 100.

[0118] Furthermore, the sintering temperature of the precursor powder is in the range of 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 transport rate and a lower powder resistivity, while also reducing the content of sodium iron phosphate impurity phase and sodium iron pyrophosphate impurity phase in the prepared positive electrode active particles, thereby achieving a higher specific capacity.

[0119] Optionally, the sintering time of the precursor powder can be from 2 hours to 48 hours. Specifically, the sintering time of the precursor powder can be, 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 sintering time of the precursor powder is too short, the carbonization degree of the carbon coating layer 20 of the obtained positive electrode active material 100 will be too low, increasing the powder resistivity of the positive electrode active material 100; if the sintering time of the precursor powder is too long, the probability of decomposition of sodium iron pyrophosphate will increase, increasing the content of sodium iron phosphate impurities and sodium iron pyrophosphate impurities in the positive electrode active material 100, and reducing the specific capacity of the positive electrode active material 100.

[0120] Please see Figure 3 This application embodiment also provides a positive electrode 200, which includes a positive current collector 210 and a positive active layer 220, wherein the positive active layer 220 includes the positive active material 100 described in this application embodiment.

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

[0122] Optionally, the positive electrode active layer 220 may also include a positive electrode conductive agent and a positive electrode binder.

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

[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, polyvinyl ether, polymethyl methacrylate (PMMA), and polyhexafluoropropylene.

[0125] The positive electrode 200 of this application embodiment includes a positive electrode active material 100, which includes positive electrode particles. The positive electrode particles include sodium iron pyrophosphate particles 10 and a carbon coating layer 20, with the carbon coating layer 20 covering the surface of the sodium iron 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 pyrophosphate can be effectively suppressed during sintering during the preparation of the positive electrode active material 100, increasing the decomposition temperature of sodium iron pyrophosphate, further reducing the formation of sodium iron phosphate impurities and sodium iron pyrophosphate impurities, and improving the specific capacity of the positive electrode active material 100. In addition, the positive electrode active material 100 can be sintered at a higher temperature during preparation, resulting in the carbon coating layer 20 of the positive electrode active material 100 having a higher electron transport 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 200 can be reduced while ensuring that the resistivity of the positive electrode 200 remains unchanged, thereby greatly reducing the cost of the positive electrode 200.

[0126] Please see Figure 4 and Figure 5 This application also provides a sodium battery 300, which includes an electrolyte, a positive electrode 200, a separator 320, and a negative electrode 330 as described in this application embodiment.

[0127] It should be noted that the sodium battery 300 in this application embodiment can be, but is not limited to, at least one of sodium-ion batteries, sodium metal batteries, lithium-sodium hybrid batteries, etc.

[0128] Optionally, the sodium battery 300 may be, but is not limited to, at least one of cylindrical sodium batteries, prismatic sodium batteries, and blade sodium batteries. The accompanying drawings of 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] Understandably, the positive electrode 200 and the negative electrode 330 are located on opposite sides of the separator 320, that is, the separator 320 is located between the positive electrode 200 and the negative electrode 330, separating the positive electrode 200 and the negative electrode 330.

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

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

[0132] Optionally, the electrolyte salt may include, but is not limited to, sodium salts. Optionally, 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 bis(oxalatoborate), sodium difluorodioxalatophosphate, sodium difluorooxalatoborate, sodium difluorophosphate (NaPO2F2), sodium trifluoromethanesulfonate (CF3SO3Na), etc.

[0133] Optionally, the organic solvent may include at least one of cyclic carbonates and chain carbonates. Optionally, the cyclic carbonate may include, but is not limited to, at least one of ethylene carbonate (EC) and propylene carbonate (PC). Ethylene carbonate has a much higher dielectric constant than propylene carbonate, and can better promote the formation of a solid electrolyte interface membrane (SEI). Optionally, the chain carbonate may include, but is not limited to, at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), and methyl ethyl carbonate (EMC). Optionally, 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 the following: propargylbenzenesulfonic acid, vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), methanedisulfonate (MMDS), butyl sulfonate lactone (BS), and 1,3-propenyl sulfonate lactone (PST).

[0135] Optionally, the diaphragm 320 can be, but is not limited to, at least one of a polypropylene membrane (PP membrane), a polyethylene membrane (PE membrane), and a ceramic diaphragm 320. Optionally, the thickness of the diaphragm 320 is from 10 μm to 18 μm, specifically, the thickness of the diaphragm 320 can 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 electrode active layer 220 of the positive electrode sheet 200 is in the range of 2.2 g / cm³. 3 Up to 2.4 g / cm 3 Specifically, it can be, but is not limited to, 2.20 g / cm³. 32.25g / cm 3 2.30g / cm 3 2.35g / cm 3 2.4g / cm 3 wait.

[0137] The method for testing the compaction density of the positive electrode active layer 220 in this application is as follows: the thickness of the aluminum foil and the mass of the aluminum foil cut into 12mm round pieces are measured in advance. Then, the thickness of the positive electrode sheet 200 made from the positive electrode active material 100 is measured, and the positive electrode sheet 200 is cut into 12mm round pieces and weighed. The mass and volume of the positive electrode active material 100 on the positive electrode sheet 200 are calculated, and then the compaction density of the positive electrode active material 100 on the positive electrode sheet 200 is calculated.

[0138] Please see Figure 6 Optionally, the negative electrode 330 includes a negative electrode current collector 331 and a negative electrode active layer 332. The negative electrode current collector 331 can 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, etc.

[0139] Optionally, the negative electrode active material can 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), acetylene black, carbon nanotubes, carbon fibers, graphene, etc.

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

[0142] Optionally, the negative electrode thickener may be, but is not limited to, at least one of polyacrylamide (PAM) and polymethyl methacrylate (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 forming a closed receiving cavity (not shown) for housing the electrolyte, the positive electrode 200, the separator 320, and the negative electrode 330. Understandably, the end cap assembly 350 electrically connects the positive electrode 200 and the negative electrode 330, respectively, and leads out the positive electrode 200 and the negative electrode 330 for electrical connection to external devices or other sodium batteries 300.

[0144] The positive electrode active material 100 of this application will be further described below through specific embodiments.

[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) 178g sodium pyrophosphate (sodium source), 150g ammonium dihydrogen phosphate (phosphorus source), 340g ferrous oxalate dihydrate (iron source), 32g glucose (carbon source) and 2.5kg water (solvent) were stirred evenly 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) The slurry was spray-dried at 105°C to obtain precursor powder;

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

[0150] Table 1 Performance parameters of sodium, phosphorus, and iron sources in 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 positive electrode active material 100: The two-probe method was used for measurement. Specifically, 2g to 3g of positive electrode active material 100 powder was added to the mold of a powder resistivity instrument with a diameter of 13mm, and after being pressurized to 200MPa, the powder resistivity value was measured.

[0154] (2) Specific capacity test of positive electrode active material 100: Positive electrode active material 100 was mixed with conductive carbon black (SP) and polyvinylidene fluoride (PVDF) at a mass ratio of 8:1:1, and a positive electrode slurry was prepared using a defoamer. PVDF was dissolved in N-methylpyrrolidone (NMP) at a mass fraction of 5%. The prepared positive electrode slurry was uniformly coated onto an aluminum foil current collector (positive electrode current collector 210) using a scraper, and then vacuum dried and cut to obtain the positive electrode sheet 200. A sodium metal sheet was used as the counter electrode, and a glass fiber membrane 320 was used as the separator. The positive electrode sheet 200, separator 320, sodium sheet, electrolyte, and casing were then assembled into a coin cell sodium battery 300. Subsequently, the specific capacity of the sodium battery 300 was measured using a constant current charge-discharge tester. The current density was 12 mA / g (the actual current value is the current density multiplied by the mass of the positive active material 100 of the positive electrode 200). The upper and lower limits of the test voltage were 1.5V-3.5V. The measured discharge specific capacity is the specific capacity of the positive active material 100.

[0155] (3) Resistivity M of positive electrode 200: The positive electrode 200 coated with the positive electrode paste in (2) on both sides is cut into five small square positive electrode 200s of size 5cm×5cm. The thickness of the positive electrode 200 is measured with a micrometer (the average value of the five measurements is taken). Then, the film resistance of each small square positive electrode 200 is measured with a resistance measuring instrument (the thickness value is entered and the measurement pressure is adjusted to be kept at 0.4 tons). Finally, the average value of the resistivity measurements of the five square positive electrode 200s is taken as the resistivity M of the positive electrode 200.

[0156] (4) Test of impurity factor P: The X-ray diffractometer was used for measurement.

[0157] (5) Content of impurity elements (potassium, boron, sulfur) in positive electrode active material 100: The content was measured using inductively coupled plasma optical emission spectrometry (ICP-OES). Specifically, 0.2g to 0.3g of positive electrode active material 100 powder was dissolved in a strong acid, and then the dissolved solution was added to the ICP-OES instrument for testing to obtain the content of each impurity element.

[0158] The 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 material 100 in Examples 1 to 7 and Comparative Examples 1 to 3

[0160]

[0161] The test results from Examples 1 to 3 and Comparative Example 1 show that as the potassium content in the sodium, phosphorus, and iron sources decreases, the impurity factor P of the prepared positive electrode active material 100 gradually decreases. As the potassium content in the sodium, phosphorus, and iron sources decreases, the amount of sodium iron pyrophosphate and sodium iron phosphate impurities generated during the preparation of the positive electrode active material 100 gradually decreases, thus increasing the specific capacity of the positive electrode active material 100 as the potassium content in the sodium, phosphorus, and iron sources decreases. This indicates that controlling the potassium content in the sodium, phosphorus, and iron sources can effectively suppress the formation of sodium iron pyrophosphate and sodium iron phosphate impurities during the preparation of the positive electrode active material 100, thereby increasing its specific capacity. However, changes in the potassium content in the sodium, phosphorus, and iron sources have little effect on the powder resistivity of the positive electrode active material 100 and the resistivity of the positive electrode sheet 200.

[0162] The test results from Examples 2, 4, 5, and Comparative Example 2 show that as the boron content in the sodium, phosphorus, and iron sources decreases, the impurity factor P of the prepared positive electrode active material 100 gradually decreases. Similarly, as the boron content in the sodium, phosphorus, and iron sources decreases, the amount of sodium iron pyrophosphate and sodium iron phosphate impurities generated during the preparation of the positive electrode active material 100 gradually decreases, thus increasing the specific capacity of the positive electrode active material 100. This indicates that controlling the amount of boron in the sodium, phosphorus, and iron sources can effectively suppress the formation of sodium iron pyrophosphate and sodium iron phosphate impurities during the preparation of the positive electrode active material 100, thereby improving its specific capacity. However, changes in the boron content in the sodium, phosphorus, and iron sources have little effect on the powder resistivity of the positive electrode active material 100 and the resistivity of the positive electrode sheet 200.

[0163] The test results from Examples 2, 6, 7, and Comparative Example 3 show that as the sulfur content in the sodium, phosphorus, and iron sources decreases, the impurity factor P of the prepared positive electrode active material 100 gradually decreases. Similarly, as the sulfur content in the sodium, phosphorus, and iron sources decreases, the amount of sodium iron pyrophosphate and sodium iron phosphate impurities generated during the preparation of the positive electrode active material 100 gradually decreases, thus increasing the specific capacity of the positive electrode active material 100. This indicates that controlling the sulfur content in the sodium, phosphorus, and iron sources can effectively suppress the formation of sodium iron pyrophosphate and sodium iron phosphate impurities during the preparation of the positive electrode active material 100, thereby improving its specific capacity. However, changes in the sulfur content in the sodium, phosphorus, and iron sources have 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 impurities generated during the preparation of positive electrode active material 100, thus reducing the specific capacity of positive electrode active material 100.

[0165] The test results of Comparative Example 5 show that the boron content in the sodium source and the boron content in the phosphorus source in Comparative Example 5 is too high. This results in a significant increase in the amount of sodium iron pyrophosphate and sodium iron phosphate impurities generated during the preparation of the positive electrode active material 100. Consequently, the boron content in the prepared positive electrode active material 100 is also greatly increased, reducing the specific capacity of the positive electrode active material 100.

[0166] The test results of Comparative Example 6 show that the potassium content in the sodium source in Comparative Example 6 is too high, which greatly increases the amount of sodium iron pyrophosphate and sodium iron phosphate impurities generated during the preparation of positive electrode active material 100. The potassium content in the prepared positive electrode active material 100 is also greatly increased, which reduces the specific capacity of 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) 178g sodium pyrophosphate (sodium source), 150g ammonium dihydrogen phosphate (phosphorus source), 340g ferrous oxalate dihydrate (iron source), 32g glucose (carbon source) and 2.5kg water (solvent) are stirred evenly to obtain a slurry, wherein the solid content of the slurry is 35%; the potassium content in the sodium source of each embodiment and comparative example is less than or equal to 15ppm, the potassium content in the phosphorus source is less than or equal to 15ppm, the potassium content in the iron source is less than or equal to 50ppm, the boron content in the sodium source is less than or equal to 15ppm, the boron content in the phosphorus source is less than or equal to 15ppm, the boron content in the iron source is less than or equal to 50ppm, the sulfur content in the sodium source is less than or equal to 30ppm, the sulfur content in the phosphorus source is less than or equal to 30ppm, and the sulfur content in the iron source is less than or equal to 100ppm.

[0170] (2) The slurry was spray-dried at 105°C to obtain precursor powder;

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

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

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

[0174]

[0175] The test results from Examples 8 to 12, and Comparative Examples 7 and 8 show that when the contents of potassium, boron, and sulfur in the sodium, phosphorus, and iron sources remain the same, the powder resistivity of the prepared positive electrode active material 100 gradually decreases with increasing sintering temperature, and the film resistivity of the positive electrode sheet 200 also gradually decreases. With increasing sintering temperature, the impurity factor P gradually increases, the contents of sodium iron pyrophosphate and sodium iron phosphate impurities in the positive electrode active material 100 gradually increase, and the specific capacity of the positive electrode active material 100 gradually decreases.

[0176] As can be seen from the test results of the above embodiments and comparative examples, in the preparation process of the positive electrode active material 100 of this application embodiment, by reducing the content of potassium, boron and sulfur elements in the sodium source, phosphorus source and iron source, the decomposition of sodium iron pyrophosphate during sintering can be suppressed, and the formation of sodium iron pyrophosphate impurity phase and sodium iron phosphate impurity phase can be suppressed. Thus, while ensuring that the positive electrode active material 100 has a high specific 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] Please see Figure 7 This application embodiment also provides an energy storage device 400, which includes a housing 410 and a sodium battery 300 as described in this application embodiment, wherein the sodium battery 300 is housed within the housing 410.

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

[0179] Optionally, the energy storage device 400 may include, but is not limited to, sodium battery modules, sodium battery packs, sodium battery systems, energy storage boxes, energy storage cabinets, and energy storage containers. The actual application form of the energy storage device 400 provided in this application embodiment may be, but is not limited to, the listed products, and may also be other application forms. This application embodiment does not strictly limit the application form of the energy storage device 400. The accompanying drawings of this application embodiment are only illustrative of the energy storage device 400 including multiple sodium batteries 300, and should not be construed as limiting the energy storage device 400 of this application embodiment.

[0180] Optionally, the number of sodium batteries 300 can be, but is not limited to, one or more. When there are multiple sodium batteries 300, they are stacked within the housing 410. It is understood that the stacked arrangement of the multiple sodium batteries 300 can be either arranged sequentially abutting each other, or arranged sequentially with intervals between them. Furthermore, the multiple sodium batteries 300 can be stacked laterally (e.g., horizontally) or longitudinally (e.g., along the direction of gravity). The stacking method and direction of the multiple sodium batteries 300 can be designed according to actual conditions, and this application does not impose specific limitations.

[0181] The term "multiple" refers to two or more.

[0182] Understandably, the multiple sodium batteries 300 of the energy storage device 400 can be connected in parallel, or in series, 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] Understandably, the housing 410 has a receiving cavity in which one or more sodium batteries 300 are received. In some embodiments, each receiving cavity receives one sodium battery 300. In other embodiments, each receiving cavity receives multiple sodium batteries 300.

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

[0185] It should be noted that energy storage (i.e., energy storage) has a wide range of applications, including generation-side energy storage, grid-side energy storage, and consumption-side energy storage. This application's embodiment of the energy storage system 500 uses generation-side energy storage as an example for detailed description, and should not be construed as limiting the energy storage system 500, nor as limiting the energy storage device 400, sodium battery 300, and positive electrode active material 100, etc., of this 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. The electrical energy stored in the energy storage device 400 can be used to supply electrical loads such as streetlights and household appliances during peak electricity prices, or to supply power when the power grid experiences a power outage. The electrical energy generated by the power conversion device 510 can also be supplied to the power grid through high-voltage cables to alleviate the power supply pressure on the power grid during peak periods.

[0187] Optionally, the power 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 electrical energy.

[0188] Optionally, the number of power conversion devices 510 can be one or more. When there are multiple power conversion devices 510, the multiple power conversion devices 510 can be connected in series, in parallel or in a mixed manner. This application does not make specific limitations.

[0189] Optionally, the power conversion device 510 can be, but is not limited to, at least one of photovoltaic panels, wind power generation devices, hydropower generation devices, etc.

[0190] Optionally, the number of energy storage devices 400 can be one or more. When there are multiple energy storage devices 400, the multiple energy storage devices 400 can be connected in series or in parallel. This application does not make specific limitations.

[0191] In this application, the terms "embodiment" and "implementation" mean that a specific feature, structure, or characteristic described in connection with an embodiment can be included in at least one embodiment of this application. The appearance of these phrases in various locations throughout the specification does not necessarily refer to the same embodiment, nor are they independent or alternative embodiments mutually exclusive with other embodiments. Those skilled in the art will understand, explicitly and implicitly, that the embodiments described in this application can be combined with other embodiments. Furthermore, it should be understood that the features, structures, or characteristics described in the various embodiments of this application can be arbitrarily combined to form yet another embodiment that does not depart from the spirit and scope of the technical solution of this application, provided there is no contradiction between them.

[0192] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of this application and are not intended to limit it. Although this application has been described in detail with reference to the above preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of this application should not depart from the spirit and scope of the technical solutions of this application.

Claims

1. A positive electrode active material, characterized in that, The positive electrode active material includes sodium iron pyrophosphate particles and a carbon coating layer, wherein the carbon coating layer is wrapped around the surface of the sodium iron pyrophosphate particles; the content of potassium in the positive electrode active material is less than or equal to 100 ppm; the content of boron in the positive electrode active material is less than or equal to 100 ppm; and the content of sulfur in the positive electrode active material is less than or equal to 200 ppm.

2. 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. Therefore, the positive electrode active material satisfies the relationship: P = (I1 + I2) / I3 ≤ 0.

1.

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

4. A method for preparing a positive electrode active material, characterized in that, include: It provides sodium, phosphorus, iron, and carbon sources; The sodium source, the phosphorus source, the iron source and the carbon source are mixed in a solvent to obtain a slurry; The slurry was spray-dried to obtain precursor powder; and The precursor powder is sintered to obtain the positive electrode active material; the positive electrode active material includes sodium iron pyrophosphate particles and a carbon coating layer, wherein the carbon coating layer is coated on the surface of the sodium iron pyrophosphate particles; in the positive electrode active material, the potassium content is less than or equal to 100 ppm, wherein 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, the potassium content in the iron source is less than or equal to 60 ppm, 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, the boron content in the iron source is less than or equal to 60 ppm, 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.

5. The method for preparing the positive electrode active material according to claim 4, characterized in that, The step of sintering the precursor powder to obtain the positive electrode active material includes: The precursor powder is sintered at a temperature of 580°C to 630°C to obtain the positive electrode active material.

6. A positive electrode plate, characterized in that, The positive electrode sheet includes: Positive current collector; and The positive electrode active layer comprises the positive electrode active material according to any one of claims 1-3 or the positive electrode active material prepared by the preparation method of the positive electrode active material according to any one of claims 4-5.

7. A sodium battery, characterized in that, include: Electrolyte, positive electrode, separator and negative electrode as described in claim 6.

8. An energy storage device, characterized in that, include: Box; as well as The sodium battery of claim 7, wherein the sodium battery is housed within the casing.

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