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

Abstract

The invention provides a positive electrode particle and a preparation method thereof, a positive electrode plate, a sodium battery and an energy storage device. The positive electrode particles comprise ferric sodium phosphate pyrophosphate particles and a carbon coating layer, the carbon coating layer wraps the surfaces of the ferric sodium phosphate pyrophosphate particles, the positive electrode particles comprise gray particles and golden particles, and in the positive electrode particles, the number ratio Z of the gray particles is larger than or equal to 0.8 and smaller than or equal to 1.

Description

Technical Field

[0001] This application relates to the field of energy storage, and specifically relates to a positive electrode particle, a preparation method thereof, a positive electrode sheet, a sodium battery, and an energy storage device. Background Art

[0002] Sodium iron pyrophosphate Na4Fe3(PO4)2P2O7 with a three-dimensional sodium ion diffusion channel and a sodium superionic conductor structure has characteristics such as a relatively high voltage plateau, a relatively high capacity, excellent rate and cycle stability, and has great potential to become a positive electrode material for large-scale production in sodium batteries. However, Na4Fe3(PO4)2P2O7 has a high powder resistivity and poor conductivity. When applied to a sodium battery, the internal resistance of the sodium battery is too large, reducing the cycle life of the sodium battery. Summary of the Invention

[0003] An embodiment of this application provides a positive electrode particle, which has a relatively low powder resistivity.

[0004] In a first aspect, an embodiment of this application provides a positive electrode particle, the positive electrode particle includes sodium iron pyrophosphate particles and a carbon coating layer, the carbon coating layer wraps the surface of the sodium iron pyrophosphate particles, the positive electrode particle includes gray particles and golden particles, and in the positive electrode particle, the proportion Z of the number of gray particles ranges from 0.8 ≤ Z ≤ 1.

[0005] Further, the positive electrode particle satisfies the relational expression:

[0006] I P / I G ≤ 0.01;

[0007] Wherein, I P is the intensity of the peak at 960 cm -1 to 1030 cm -1 in the Raman spectrum of the positive electrode particle; I G is the intensity of the peak at 1570 cm -1 to 1620 cm -1 in the Raman spectrum of the positive electrode particle.

[0008] Further, the positive electrode particle also satisfies the relational expression:

[0009] I D / I G ≤ 1.2;

[0010] Wherein, I D is the intensity of the peak at 1330 cm -1 to 1370 cm -1 in the Raman spectrum of the positive electrode particle; I GFor the Raman spectrum of the positive electrode particles, the intensity of the peak at 1570 cm -1 to 1620 cm -1 .

[0011] Furthermore, in the sodium iron pyrophosphate phosphate particles, the mass fraction of sodium iron phosphate is less than or equal to 5%, and the mass fraction of sodium iron pyrophosphate is less than or equal to 5%.

[0012] Furthermore, in the positive electrode particles, the range of the mass fraction A of the carbon coating layer is: 1% ≤ A ≤ 10%.

[0013] Furthermore, the range of the powder resistivity R of the positive electrode particles is 10 Ω·cm ≤ R ≤ 9999 Ω·cm.

[0014] In a second aspect, an embodiment of the present application further provides a method for preparing positive electrode particles, which includes:

[0015] Mixing a sodium source, a phosphorus source, an iron source, and a carbon source in a solvent to obtain a slurry; spray-drying the slurry to obtain a precursor powder; and

[0016] Performing segmented sintering on the precursor powder to obtain the positive electrode particles, wherein the positive electrode particles include sodium iron pyrophosphate phosphate particles and a carbon coating layer, the carbon coating layer wraps the surface of the sodium iron pyrophosphate phosphate particles, the positive electrode particles include gray particles and golden particles, and in the positive electrode particles, the range of the proportion Z of the number of gray particles is: 0.8 ≤ Z ≤ 1.

[0017] Furthermore, the performing segmented sintering on the precursor powder includes:

[0018] Performing the first-stage sintering at a first temperature T1 of 200°C ≤ T1 ≤ 300°C;

[0019] Performing the second-stage sintering at a second temperature T2 of 300°C ≤ T4 ≤ 400°C; and

[0020] Performing the third-stage sintering at a third temperature T3 of 450°C ≤ T3 ≤ 600°C.

[0021] In a third aspect, an embodiment of the present application further provides a positive electrode sheet, which includes:

[0022] A positive electrode current collector; and

[0023] A positive electrode active layer, the positive electrode active layer includes the positive electrode particles described in the first aspect of the present application or the positive electrode particles prepared by the method for preparing positive electrode particles described in the second aspect of the present application.

[0024] Furthermore, the range of the film resistivity M of the positive electrode sheet is 0.1 Ω·m ≤ M ≤ 5 Ω·m.

[0025] In a fourth aspect, an embodiment of the present application further provides a sodium battery, which includes: an electrolyte, the positive electrode sheet described in the third aspect of the present application, a separator, and a negative electrode sheet.

[0026] In a fifth aspect, an embodiment of the present application further provides an energy storage device, which includes:

[0027] a box body; and

[0028] the sodium battery described in the fourth aspect of the present application, and the sodium battery is housed in the box body.

[0029] The positive electrode particles of the present application include sodium iron pyrophosphate phosphate particles and a carbon coating layer. The carbon coating layer wraps the surface of the sodium iron pyrophosphate phosphate particles. The positive electrode particles include gray particles and golden particles. In the positive electrode particles, the range of the proportion Z of the number of gray particles is: 0.8 ≤ Z ≤ 1. The high proportion of gray particles in the positive electrode particles of the present application indicates that the positive electrode particles have a high degree of carbon coating, lower powder resistivity, and higher conductivity. When applied to a sodium battery, the sodium battery can have a higher cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings required for use in the embodiments. Obviously, the drawings in the following description are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can also be obtained based on these drawings.

[0031] Figure 1 is a schematic structural diagram of positive electrode particles according to an embodiment of the present application.

[0032] Figure 2 is a schematic flowchart of a method for preparing positive electrode particles according to an embodiment of the present application.

[0033] Figure 3 is a schematic flowchart of the preparation process of a precursor powder according to an embodiment of the present application. [[ID=3②]]

[0034] Figure 4 is a schematic flowchart of the segmented sintering of a precursor powder according to an embodiment of the present application.

[0035] Figure 5 is a schematic structural diagram of a positive electrode sheet according to an embodiment of the present application.

[0036] Figure 6 is a schematic structural diagram of a sodium battery according to an embodiment of the present application.

[0037] Figure 7 is a schematic diagram of a sodium battery according to an embodiment of the present application alongFigure 6 Schematic cross-sectional structure diagram in the A-A direction.

[0038] Figure 8 It is a schematic structural diagram of a negative electrode plate according to an embodiment of the present application.

[0039] Figure 9 It is a scanning electron microscope image of the positive electrode particles prepared in Example 1 of the present application.

[0040] Figure 10 It is an optical microscope image of the positive electrode particles prepared in Example 1 of the present application taken under a Raman spectrometer.

[0041] Figure 11 It is Figure 10 The Raman spectrum of the golden particles in

[0042] Figure 12 It is Figure 10 The Raman spectrum of the gray particles in

[0043] Figure 13 It is a schematic structural diagram of an energy storage device according to an embodiment of the present application.

[0044] Figure 14 It is a structural block diagram of an energy storage system according to an embodiment of the present application.

[0045] Figure 15 It is an application scenario diagram of an energy storage system according to an embodiment of the present application.

[0046] Explanation of reference numerals:

[0047] 100 - Positive electrode particles, 10 - Sodium iron pyrophosphate particles, 20 - Carbon coating layer, 200 - Positive electrode plate, 210 - Positive current collector, 220 - Positive active layer, 300 - Sodium battery, 320 - Separator, 330 - Negative electrode plate, 331 - Negative current collector, 332 - Negative active layer, 340 - Housing, 350 - End cap assembly, 400 - Energy storage device, 410 - Box body, 500 - Energy storage system, 510 - Electric energy conversion device. Detailed implementation manners

[0048] In order to enable those skilled in the art to better understand the solution of 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 in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without making creative efforts belong to the scope of protection of the present application.

[0049] The terms "first", "second", etc. in the description, claims and the above-mentioned drawings of this application are used to distinguish different objects, rather than to describe a specific order. In addition, the terms "include" and "have" and any variations thereof are intended to cover non-exclusive inclusion. For example, a process, method, system, product or device that includes a series of steps or units is not limited to the listed steps or units, but may optionally further include steps or units not listed, or may optionally further include other steps or units inherent to these processes, methods, products or devices.

[0050] The technical solutions in the embodiments of this application will be described below in conjunction with the drawings.

[0051] It should be noted that for ease of description, in the embodiments of this application, the same reference numerals represent the same components, and for the sake of brevity, the detailed description of the same components is omitted in different embodiments.

[0052] Currently, the generation of green electric energy generally depends on photovoltaic, wind power, water potential, etc. However, wind energy and solar energy generally have problems of strong intermittency and large volatility, which will cause instability of the power grid. During peak electricity consumption, there is not enough electricity, and during low electricity consumption, there is too much electricity. The unstable voltage will also damage the electricity. Therefore, due to insufficient electricity demand or insufficient grid acceptance capacity, the problem of "abandoning wind and light" may be triggered. To solve these problems, energy storage is required. That is, electric energy is converted into other forms of energy and stored through physical or chemical means, and the energy is converted back into electric energy and released when needed. Simply put, energy storage is similar to a large "portable power bank". When photovoltaic and wind energy are sufficient, the electric energy is stored, and the stored energy is released when needed.

[0053] The battery is the smallest energy storage unit of the energy storage device and the energy storage system, and the performance of the battery directly affects the performance and application of the energy storage device and the energy storage system. The battery includes lithium batteries and sodium batteries.

[0054] Sodium iron pyrophosphate phosphate Na4Fe3(PO4)2P2O7 with a three-dimensional sodium ion diffusion channel and a sodium superionic conductor structure has the characteristics of a relatively high voltage platform, a relatively high capacity, excellent rate and cycle stability, and has great potential to become a cathode material for large-scale production in sodium batteries. However, Na4Fe3(PO4)2P2O7 has a high powder resistivity and poor conductivity. When applied to sodium batteries, the internal resistance of the sodium battery is too large, reducing the cycle life of the sodium battery. In related technologies, carbon coating is used to reduce the powder resistivity of sodium iron pyrophosphate phosphate and improve the electron transport efficiency of sodium iron pyrophosphate phosphate. However, the synthesis temperature of sodium iron pyrophosphate phosphate is relatively narrow. In order to better improve the graphitization degree of the carbon coating layer, sintering is usually carried out at a relatively high temperature, which makes sodium iron pyrophosphate phosphate prone to decomposition and reduces the specific capacity of sodium iron pyrophosphate phosphate. In view of this, the following embodiments of the present application provide a cathode particle and a preparation method thereof. The cathode particle of the present application has a relatively low powder resistivity and a relatively high specific capacity.

[0055] Please refer to Figure 1 , embodiments of the present application provide a cathode particle 100. The cathode particle 100 includes sodium iron pyrophosphate phosphate particles 10 (also known as composite sodium phosphate iron, with the chemical formula Na4Fe3(PO4)2P2O7, abbreviated as NFPP) and a carbon coating layer 20. The carbon coating layer 20 wraps around the surface of the sodium iron pyrophosphate phosphate particles 10. The cathode particle 100 includes gray particles and golden particles. In the cathode particle 100, the range of the proportion Z of the number of gray particles is: 0.8 ≤ Z ≤ 1.

[0056] The cathode particle 100 of the embodiment of the present application can be applied to a sodium battery (such as a sodium ion battery) as the cathode active material of the cathode electrode sheet of the sodium battery.

[0057] It should be noted that the "gray particles" refer to the particles in the cathode particle 100 that present a gray color. The "golden particles" refer to the particles in the cathode particle 100 that present a golden color.

[0058] It should be noted that the gray particles and the golden particles can be observed through an optical microscope. The proportion of the number of gray particles in the cathode particle 100 can be calculated by selecting a region under the optical microscope, selecting a plurality of particles (such as 100, 200, 300, 400, etc.) in this region for statistics, calculating the number of golden particles and the number of gray particles respectively, and then calculating the proportion Z of the number of gray particles, where Z = the number of gray particles / (the number of golden particles + the number of gray particles).

[0059] It should be noted that the observation of the gray particles and the gold particles can be carried out under the optical microscope of a Raman spectrometer. When preparing the sample, a small amount of the positive electrode particles 100 can be taken and placed in the center of the glass slide, and the aluminum foil is covered on the positive electrode particles 100 with the smooth side facing down. Another cover glass is picked up and covered on the aluminum foil, and gently squeezed evenly for 5 seconds, and then the upper cover glass and aluminum foil are removed to obtain a flat surface of the positive electrode particle 100 sample. During the test, the laser power of the Raman spectrometer is set to 1 mW to 10 mW, and the scanning range is 700 cm -1 to 3500 cm -1 , and the laser wavelength is 532 nm.

[0060] Specifically, the proportion Z of the number of the gray particles in the positive electrode particles 100 can be, but is not limited to, 0.8, 0.81, 0.82, 0.83, 0.84, 0.85, 0.86, 0.87, 0.88, 0.9, 0.91, 0.92, 0.93, 0.94, 0.95, 0.96, 0.97, 0.98, 1.0, etc. If the proportion Z of the number of the gray particles in the positive electrode particles 100 is too low, the proportion of the number of the gold particles in the positive electrode particles 100 is too high, indicating that the carbon coating degree of the positive electrode particles 100 is insufficient, resulting in too low powder resistivity of the positive electrode particles 100. When applied to a sodium battery, the internal resistance of the sodium battery is too large, which is not conducive to the exertion of the capacity of the positive electrode particles 100 and reduces the cycle life of the sodium battery.

[0061] It can be understood that when Z = 1, all the positive electrode particles 100 are gray particles, that is, at this time, the positive electrode particles 100 do not include gold particles.

[0062] The positive electrode particles 100 of the present application include sodium iron pyrophosphate phosphate particles 10 and a carbon coating layer 20. The carbon coating layer 20 wraps the surface of the sodium iron pyrophosphate phosphate particles 10. The positive electrode particles 100 include gray particles and gold particles. In the positive electrode particles 100, the range of the proportion Z of the number of the gray particles is: 0.8 ≤ Z ≤ 1. The positive electrode particles 100 of the present application have a relatively high proportion of gray particles, indicating that the positive electrode particles 100 have a relatively high carbon coating degree, lower powder resistivity, and higher conductivity. When applied to a sodium battery, the sodium battery can have a higher cycle life.

[0063] In some embodiments, the positive electrode particles 100 satisfy the relationship:

[0064] I P / I G ≤ 0.01;

[0065] wherein, I P is in the Raman spectrum of the positive electrode particles 100, at 960 cm -1From 960 cm to 1030 cm -1 The intensity of the peak at; I G In the Raman spectrum of the positive electrode particles 100, 1570 cm -1 To 1620 cm -1 The intensity of the peak at.

[0066] Understandably, in the Raman spectrum of the positive electrode particles 100, 960 cm -1 To 1030 cm -1 The intensity I of the peak at P And in the Raman spectrum of the positive electrode particles 100, 1570 cm -1 To 1620 cm -1 The intensity I of the peak at G The ratio is less than or equal to 0.01.

[0067] It should be noted that the Raman spectrometer can scan a single particle of the positive electrode particles 100 to obtain a Raman spectrum, or can perform a surface scan on the positive electrode particles 100, that is, scan a certain area. The I of the present application P / I G Is the average value of I P / I G Of the Raman spectra of multiple (such as 100, 300, 500, 1000, etc.) positive electrode particles 100.

[0068] It should be noted that in the Raman spectrum, 960 cm -1 To 1030 cm -1 The peak at is the peak of the P-O bond in PO4 in the sodium iron pyrophosphate phosphate structure. When the intensity of the peak at 960 cm 3- To 1030 cm -1 Is weaker, it indicates that the content of PO4 in the positive electrode particles 100 is less, the sodium iron pyrophosphate phosphate particles 10 are more completely and uniformly coated by the carbon coating layer 20, and the powder resistivity of the positive electrode particles 100 is lower; conversely, the intensity of the peak at 960 cm -1 To 1030 cm 3- Is stronger, it indicates that the content of PO4 in the positive electrode particles 100 is more, the sodium iron pyrophosphate phosphate particles 10 are less completely coated by the carbon coating layer 20, and the powder resistivity of the positive electrode particles 100 is higher. In the Raman spectrum, 1570 cm -1 To 1620 cm -1 The peak at is the peak of the G bond in graphitized carbon. Therefore, 1570 cm 3- To 1620 cm -1 To 1620 cm -1 The peak at is the peak of the G bond in graphitized carbon. Therefore, 1570 cm -1 To 1620 cm -1The stronger the peak at the position, the higher the graphitization degree of the carbon coating layer 20 of the positive electrode particles 100, and the higher the electron transfer rate of the positive electrode particles 100.

[0069] Specifically, I P / I G can be, but is not limited to, 0, 0.0001, 0.0005, 0.0008, 0.001, 0.002, 0.003, 0.004, 0.005, 0.006, 0.007, 0.008, 0.009, 0.01, etc. I P / I G The smaller it is, the more completely the sodium iron pyrophosphate particles 10 in the positive electrode particles 100 are wrapped by the carbon coating layer 20, the higher the graphitization degree of the carbon coating layer 20 of the positive electrode particles 100, and the higher the electron transfer rate of the positive electrode particles 100; I P / I G When it is too high, the sodium iron pyrophosphate particles 10 of the positive electrode particles 100 are too little wrapped by the carbon coating layer 20, the graphitization degree of the carbon coating layer 20 of the positive electrode particles 100 is too low, reducing the electron transfer rate of the positive electrode particles 100 and increasing the powder resistivity of the positive electrode particles 100.

[0070] In this embodiment, in the Raman spectrum of the positive electrode particles 100, in the Raman spectrum of the positive electrode particles 100, 960 cm -1 to 1030 cm -1 The intensity I of the peak at the position P and the intensity I of the peak at 1570 cm -1 to 1620 cm -1 at the position in the Raman spectrum of the positive electrode particles 100 G The ratio is less than or equal to 0.01. This shows that in the positive electrode particles 100 of the embodiment of the present application, the proportion of the sodium iron pyrophosphate particles 10 exposed outside is very small and is evenly wrapped by the carbon coating layer 20; in addition, the carbon coating layer 20 of the positive electrode particles 100 has a high graphitization degree, so that the positive electrode particles 100 have a high electron transfer rate and a low powder resistivity. When applied to a sodium battery, the sodium battery has a longer service life.

[0071] In some embodiments, the positive electrode particles 100 further satisfy the relational expression:

[0072] I D / I G ≤1.2;

[0073] wherein, I D is in the Raman spectrum of the positive electrode particles 100, 1330 cm -1 to 1370 cm -1The intensity of the peak at (also known as the defect peak, D-Band, D peak); I G In the Raman spectrum of the positive electrode particle 100, at 1570 cm -1 to 1620 cm -1 The intensity of the peak at (also known as the graphite peak, G-Band, G peak).

[0074] Understandably, in the Raman spectrum of the positive electrode particle 100, at 1330 cm -1 to 1370 cm -1 The intensity I of the peak at D In the Raman spectrum of the positive electrode particle 100, at 1570 cm -1 to 1620 cm -1 The intensity I of the peak at G The ratio is less than or equal to 0.01.

[0075] It should be noted that in the Raman spectrum, at 1330 cm -1 to 1370 cm -1 The peak at is the peak of the D bond of amorphous carbon.

[0076] It should be noted that the I D / I G Of the Raman spectra of multiple (such as 100, 300, 500, 1000, etc.) positive electrode particles 100, the I D / I G The average value of.

[0077] Specifically, I D / I G Can be but is not limited to 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.1, 1.2, etc.

[0078] In this embodiment, the smaller the I D / I G The less the content of amorphous carbon in the carbon coating layer 20 of the positive electrode particle 100, the higher the content of graphitized carbon, the higher the graphitization degree of the carbon coating layer 20, and the higher the electron transport rate; I D / I G When it is too large, in the carbon coating layer 20 of the positive electrode particle 100, the content of amorphous carbon is too high and the content of graphitized carbon is too low, which is not conducive to improving the electron transport rate of the positive electrode particle 100 and is not conducive to reducing the powder resistivity of the positive electrode particle 100.

[0079] It should be noted that when performing Raman spectrum testing on the golden particles in the positive electrode particle 100 of the present application, in addition to having 1330 cm -1 to 1370 cm-1 The peak at (i.e., the D peak) and 1570 cm -1 to 1620 cm -1 In addition to the peak at (i.e., the G peak), there is also a peak belonging to sodium iron pyrophosphate at 960 cm -1 to 1030 cm -1 This indicates that the degree of carbon coating of the golden particles is relatively low; while in the Raman spectrum of the gray particles, there is only a peak at 1330 cm -1 to 1370 cm -1 The peak at (i.e., the D peak) and 1570 cm -1 to 1620 cm -1 The peak at (i.e., the G peak), and there is no peak at 960 cm -1 to 1030 cm -1 This indicates that the degree of carbon coating of the gray particles is relatively high.

[0080] In some embodiments, in the sodium iron pyrophosphate particles 10, the mass fraction of sodium iron phosphate is less than or equal to 5%, and the mass fraction of sodium iron pyrophosphate is less than or equal to 5%.

[0081] Specifically, the mass fraction of sodium iron phosphate in the sodium iron pyrophosphate particles 10 may be, but is not limited to, less than or equal to 5%, less than or equal to 4.5%, less than or equal to 4%, less than or equal to 3.5%, less than or equal to 3%, less than or equal to 2.5%, less than or equal to 2%, less than or equal to 1.5%, less than or equal to 1%, less than or equal to 0.5%, etc.

[0082] Specifically, the mass fraction of sodium iron pyrophosphate in the sodium iron pyrophosphate particles 10 may be, but is not limited to, less than or equal to 5%, less than or equal to 4.5%, less than or equal to 4%, less than or equal to 3.5%, less than or equal to 3%, less than or equal to 2.5%, less than or equal to 2%, less than or equal to 1.5%, less than or equal to 1%, less than or equal to 0.8%, less than or equal to 0.5%, etc.

[0083] When sodium iron pyrophosphate is carbon-coated to form the carbon coating layer 20, it needs to be carried out at a relatively high temperature, and it is inevitable that part of the sodium iron pyrophosphate will decompose to generate sodium iron phosphate and sodium pyrophosphate. In the sodium iron pyrophosphate particles 10, if the mass fraction of at least one of sodium iron phosphate and sodium pyrophosphate is too low, it will increase the difficulty of preparing the positive electrode particles 100, or reduce the amount of carbon coating of the positive electrode particles 100, and reduce the electron transfer rate of the positive electrode particles 100; when the mass fraction of at least one of sodium iron phosphate and sodium pyrophosphate in the sodium iron pyrophosphate particles 10 is too high, it will reduce the specific capacity of the positive electrode particles 100, and when the positive electrode particles 100 are applied to sodium batteries, the energy density of the sodium batteries will be reduced. In the sodium iron pyrophosphate particles 10 of the positive electrode particles 100 of the present application, the mass fraction of sodium iron phosphate is less than or equal to 5%, and the mass fraction of sodium pyrophosphate is less than or equal to 5%. This shows that the content of sodium iron pyrophosphate in the sodium iron pyrophosphate particles 10 is relatively high, so that the positive electrode particles 100 have a relatively high specific capacity, and when applied to sodium batteries, the sodium batteries have a higher energy density.

[0084] Further, in the sodium iron pyrophosphate particles 10, the mass fraction of sodium iron phosphate is less than or equal to 3%, and the mass fraction of sodium pyrophosphate is less than or equal to 4%. In this way, the positive electrode particles 100 can have a lower powder resistivity and a relatively high specific capacity.

[0085] Still further, in the sodium iron pyrophosphate particles 10, the mass fraction of sodium iron phosphate is less than or equal to 1%, and the mass fraction of sodium pyrophosphate is less than or equal to 2.5%. In this way, the positive electrode particles 100 can have a lower powder resistivity and a relatively high specific capacity.

[0086] In some embodiments, in the positive electrode particles 100, the range of the mass fraction A of the carbon coating layer 20 is: 1% ≤ A ≤ 10%.

[0087] Specifically, in the positive electrode particles 100, the mass fraction A of the carbon coating layer 20 can be, but is not limited to, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, etc.

[0088] The mass fraction of the carbon coating layer 20 of the positive electrode particles 100 of the present application can be measured by a high-frequency infrared carbon-sulfur analyzer. Optionally, the test conditions of the high-frequency infrared carbon-sulfur analyzer can be, but are not limited to, 18 MHz and 2.7 kw.

[0089] In this embodiment, if the mass fraction A of the carbon coating layer 20 in the positive electrode particles 100 is too low, the electron transfer efficiency of the positive electrode particles 100 is reduced, which is not conducive to reducing the powder resistivity of the positive electrode particles 100 and is not conducive to improving the cycle life of the sodium battery using the positive electrode particles 100; if the mass fraction A of the carbon coating layer 20 in the positive electrode particles 100 is too high, although the positive electrode particles 100 can have a lower resistivity, the specific capacity per gram and the powder compaction density of the positive electrode particles 100 are reduced. When the positive electrode particles 100 are applied to the positive electrode sheet, the compaction density and energy density of the positive electrode sheet are reduced.

[0090] In some embodiments, the powder resistivity R of the positive electrode particles 100 ranges from 10 Ω·cm ≤ R ≤ 9999 Ω·cm.

[0091] Specifically, the powder resistivity R of the positive electrode particles 100 can be, but is not limited to, 10 Ω·cm, 20 Ω·cm, 40 Ω·cm, 60 Ω·cm, 80 Ω·cm, 100 Ω·cm, 200 Ω·cm, 400 Ω·cm, 600 Ω·cm, 800 Ω·cm, 10 3 Ω·cm, 3×10 3 Ω·cm, 5×10 3 Ω·cm, 6×10 3 Ω·cm, 7×10 3 Ω·cm, 8×10 3 Ω·cm, 9×10 3 Ω·cm, 9999 Ω·cm, etc.

[0092] In this embodiment, if the powder resistivity R of the positive electrode particles 100 is too low, the content and graphitization degree of the carbon coating layer 20 in the positive electrode particles 100 need to be increased, which increases the preparation difficulty of the positive electrode particles 100 and reduces the specific capacity per gram of the positive electrode particles 100; if the powder resistivity R of the positive electrode particles 100 is too high, when the positive electrode particles 100 are applied to the sodium battery, the internal resistance of the sodium battery is too large, reducing the cycle life of the sodium battery. The positive electrode particles 100 of this embodiment have a lower powder resistivity. When applied to the sodium battery, the sodium battery can have a lower resistance, which is conducive to the exertion of the specific capacity per gram of the positive electrode particles 100 and improves the cycle life of the sodium battery.

[0093] Optionally, the range of D10 of the positive electrode particles 100 is: 1 μm ≤ D10 ≤ 5 μm, where D10 is the particle size corresponding to when the cumulative particle size distribution volume percentage reaches 10%. Specifically, D10 of the positive electrode particles 100 can be, but is not limited to, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, etc. If D10 of the positive electrode particles 100 is too small, the preparation difficulty of the positive electrode particles 100 increases; if D10 of the positive electrode particles 100 is too large, the powder compaction density of the positive electrode particles 100 decreases.

[0094] Optionally, the range of D50 of the positive electrode particles 100 is: 5 μm ≤ D50 ≤ 15 μm, where D50 is the particle size corresponding to when the cumulative particle size distribution volume percentage reaches 50%. Specifically, D50 of the positive electrode particles 100 can be, but is not limited to, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 15 μm, etc. If D50 of the positive electrode particles 100 is too small, the powder compaction density of the positive electrode particles 100 decreases; if D50 of the positive electrode particles 100 is too large, the powder compaction density of the positive electrode particles 100 will also decrease.

[0095] Optionally, the range of D90 of the positive electrode particles 100 is: 15 μm ≤ D90 ≤ 35 μm, where D90 is the particle size corresponding to when the cumulative particle size distribution volume percentage reaches 90%. Specifically, D90 of the positive electrode particles 100 can be, but is not limited to, 15 μm, 16 μm, 17 μm, 18 μm, 19 μm, 20 μm, 21 μm, 22 μm, 23 μm, 24 μm, 25 μm, 26 μm, 27 μm, 28 μm, 29 μm, 30 μm, 31 μm, 32 μm, 33 μm, 34 μm, 35 μm, etc. If D90 of the positive electrode particles 100 is too small, the powder compaction density of the positive electrode particles 100 decreases; if D90 of the positive electrode particles 100 is too large, abnormal processing of the positive electrode sheet will easily occur.

[0096] The positive electrode particles 100 of the embodiments of the present application can be prepared by the methods described in the following embodiments of the present application. In addition, they can also be prepared by other methods. The preparation methods of the embodiments of the present application are only one or more preparation methods of the positive electrode particles 100 of the present application, and should not be construed as a limitation on the positive electrode particles 100 provided in the embodiments of the present application.

[0097] Please refer to Figure 2 , the embodiments of the present application also provide a preparation method of the positive electrode particles 100, which includes:

[0098] S201. Mix a sodium source, a phosphorus source, an iron source, and a carbon source in a solvent to obtain a slurry; spray-dry the slurry to obtain a precursor powder; and

[0099] Optionally, the sodium source may be, but is not limited to, at least one of sodium carbonate, sodium bicarbonate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium pyrophosphate, sodium acetate, sodium hydroxide, sodium nitrate, sodium peroxide, sodium citrate, sodium oxalate, sodium sulfate, etc.

[0100] Optionally, the iron source may be, but is not limited to, at least one of iron phosphate, ferrous oxalate, iron nitrate, metallic iron, iron oxide, ferrous oxide, iron sulfate, ferrous sulfate, ferrous acetate, magnetite, etc.

[0101] Optionally, the phosphorus source may be, but is not limited to, at least one of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, phosphoric acid, phosphorus pentoxide, etc.

[0102] It can be understood that the same compound can serve as both a sodium source and a phosphorus source, such as sodium dihydrogen phosphate, sodium pyrophosphate, etc.

[0103] Optionally, the carbon source may be, but is not limited to, at least one of glucose, citric acid, ascorbic acid, polyethylene glycol, acetylene black, graphene, carbon nanotubes, sucrose, starch, etc.

[0104] S202. Sinter the precursor powder in segments to obtain the positive electrode particles 100, wherein the positive electrode particles 100 include sodium iron pyrophosphate phosphate particles 10 and a carbon coating layer 20, the carbon coating layer 20 wraps around the surface of the sodium iron pyrophosphate phosphate particles 10, the positive electrode particles 100 include gray particles and golden particles, and in the positive electrode particles 100, the range of the proportion Z of the number of gray particles is: 0.8 ≤ Z ≤ 1.

[0105] In the preparation method of the positive electrode particles 100 according to the embodiments of the present application, a carbon source is added during the preparation process. In addition, the preparation is carried out by means of segmented sintering, so that the prepared sodium iron pyrophosphate phosphate particles 10 can be more completely coated by the carbon coating layer 20, and the graphitization degree of the carbon coating layer 20 is higher, thereby enabling the positive electrode particles 100 to have a lower powder resistivity. In addition, the segmented sintering method can better avoid the decomposition of the sodium iron pyrophosphate phosphate particles 10 in the positive electrode particles 100, thereby better improving the specific capacity of the positive electrode particles 100. The positive electrode particles 100 prepared in this application include gray particles and golden particles. In the positive electrode particles 100, the proportion Z of the number of the gray particles ranges from 0.8 ≤ Z ≤ 1. The positive electrode particles 100 of this application have a relatively high proportion of gray particles, which indicates that the positive electrode particles 100 have a relatively high carbon coating degree, a lower powder resistivity, and a higher conductivity. When applied to sodium batteries, the sodium batteries can have a higher cycle life.

[0106] Please refer to Figure 3 , in some embodiments, in S201, a sodium source, a phosphorus source, an iron source, and a carbon source are mixed in a solvent to obtain a slurry; the slurry is spray-dried to obtain a precursor powder, including:

[0107] S2011, weigh a sodium source, an iron source, a phosphorus source, and a carbon source in a preset ratio, mix them in a solvent, and perform sand milling to obtain a slurry; and

[0108] Optionally, among the sodium source, the iron source, and the phosphorus source, the molar ratio of sodium, iron, and phosphorus is 4:3:4.

[0109] Optionally, the sodium source, the phosphorus source, the iron source, and the carbon source are mixed in water to obtain a slurry, and sand milling treatment is performed at a rotation speed of 1000 rpm to 4000 rpm until the average particle size D50' of the particulate matter in the slurry ≤ 1 μm, and the time for the sand milling treatment is 0.5 h to 4 h. In this embodiment, sand milling can reduce the particle size of the particulate matter in the slurry, providing a prerequisite for the formation of sodium iron pyrophosphate phosphate. In addition, the sand milling treatment can fully mix and homogenize the soluble substances and insoluble substances in the slurry, avoiding uneven mixing of the raw materials when there are multiple insoluble raw materials in the slurry and increasing the probability of generating heterophases in the finally formed sodium iron pyrophosphate phosphate.

[0110] In the embodiments of the present application, when it comes to the numerical range from a to b, if not specifically specified, it means that the numerical value can be any numerical value between a and b, including the endpoint value a and the endpoint value b.

[0111] Specifically, the time for the sanding treatment can be, but is not limited to, 0.5 h, 1 h, 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, etc. If the time for the sanding treatment is too short, the particle size of the insoluble raw materials in the sodium source, phosphorus source, iron source, and carbon source will be too large, and the mixing between the components will be uneven, resulting in a higher content of impurity phases during the sintering process. If the time for the sanding treatment is too long, the production efficiency will be reduced.

[0112] Specifically, the rotation speed of the sanding can be, but is not limited to, 1000 rpm, 1500 rpm, 2000 rpm, 2500 rpm, 3000 rpm, 3500 rpm, 4000 rpm, etc. If the rotation speed of the sanding is too low, the particle size of the insoluble raw materials in the sodium source, phosphorus source, iron source, and carbon source will be too large, and the mixing between the components will be uneven, resulting in a higher content of impurity phases during the sintering process. If the rotation speed of the sanding is too high, the slurry is likely to splash.

[0113] S2012, spray-dry the slurry at a drying temperature of 90°C to 120°C to obtain a precursor powder.

[0114] Specifically, the drying temperature can be, but is not limited to, 90°C, 92°C, 94°C, 97°C, 98°C, 100°C, 101°C, 102°C, 103°C, 104°C, 105°C, 106°C, 107°C, 108°C, 110°C, 112°C, 114°C, 116°C, 118°C, 120°C, etc.

[0115] In this embodiment, if the temperature for spray-drying the slurry is too low, the evaporation rate of the solvent (such as water) is too slow, so that the speed of forming spherical shapes of the precursor powder is too slow, and it is easy to form a hollow spherical morphology or a collapsed spherical morphology, affecting the processing performance and tap density of the finally prepared cathode particles 100. If the temperature for spray-drying the slurry is too high, the evaporation rate of the solvent (such as water) is too fast, so that the speed of forming spherical shapes of the precursor powder is too fast, and the sphericity of the formed precursor powder is reduced.

[0116] Optionally, the segmented sintering includes a first-stage sintering, a second-stage sintering, and a third-stage sintering. The temperature of the first-stage sintering is lower than the temperature of the second-stage sintering, and the temperature of the second-stage sintering is lower than the temperature of the third-stage sintering. In this embodiment, through the three-stage sintering, the prepared sodium iron pyrophosphate particles 10 can be more completely coated by the carbon coating layer 20, and the graphitization degree of the carbon coating layer 20 is higher, so that the cathode particles 100 have a lower powder resistivity. In addition, the segmented sintering method can better improve the uniformity between the components, so as to better improve the specific capacity of the cathode particles 100.

[0117] Please refer to Figure 4In some embodiments, in S202, sintering the precursor powder in stages includes:

[0118] S2021, performing a first stage sintering at a first temperature T1 of 200° C. ≤ T1 ≤ 300° C.;

[0119] Optionally, the first sintering stage is performed under the protection of an inert atmosphere. Optionally, the inert atmosphere includes at least one of helium, nitrogen, and argon.

[0120] Specifically, the temperature T1 (i.e., the first temperature) of the first sintering stage can be, but is not limited to, 200° C., 210° C., 220° C., 230° C., 240° C., 250° C., 260° C., 270° C., 280° C., 290° C., 300° C., etc. The first sintering stage can allow the components in the precursor powder to be more fully melt-mixed, thereby better avoiding the generation of impurities in the sodium iron phosphate pyrophosphate particles 10 of the prepared positive electrode particles 100, thereby increasing the gram capacity of the positive electrode particles 100. If the temperature T1 of the first sintering stage is too low, some components may not be fully melted, resulting in uneven mixing of the raw materials in the precursor powder, increasing the probability of the generation of impurities in the sodium iron phosphate pyrophosphate particles 10 of the prepared positive electrode particles 100, and reducing the gram capacity of the positive electrode particles 100; if the temperature T1 of the first sintering stage is too high, the precursor powder will produce gas in advance, thereby making the components in the precursor powder uneven. During the subsequent second and third sintering stages, impurities may be easily generated in the sodium iron phosphate pyrophosphate particles 10 of the positive electrode particles 100, thereby reducing the gram capacity of the positive electrode particles 100.

[0121] Optionally, the holding time of the first stage sintering ranges from 5h to 12h. Specifically, the holding time of the first stage sintering can be, but is not limited to, 5h, 6h, 8h, 10h, 12h, etc. If the holding time of the first stage sintering is too short, it is easy for some components to not be fully melted, and the raw materials in the precursor powder are mixed unevenly, which increases the probability of the generation of impurities in the sodium iron phosphate pyrophosphate particles 10 of the prepared positive electrode particles 100, and reduces the gram capacity of the positive electrode particles 100; if the holding time of the first stage sintering is too long, the production efficiency of the positive electrode particles 100 is reduced, and the preparation cost of the positive electrode particles 100 is increased.

[0122] S2022, performing a second sintering at a second temperature T2 of 300° C. ≤ T4 ≤ 400° C.; and

[0123] Optionally, the second sintering is performed under the protection of an inert atmosphere. Optionally, the inert atmosphere includes at least one of helium, nitrogen, and argon.

[0124] Specifically, the temperature T2 (i.e., the second temperature) of the second sintering stage can be, but is not limited to, 300°C, 310°C, 320°C, 330°C, 340°C, 350°C, 360°C, 370°C, 380°C, 390°C, 400°C, etc. If the temperature T2 of the second sintering stage is too low, it is easy for the precursor to produce incomplete gas, increasing the probability of the generation of impurities in the sodium iron phosphate pyrophosphate particles 10 of the positive electrode particles 100, and reducing the gram capacity of the positive electrode particles 100; in addition, it will also cause the pores inside the positive electrode particles 100 to be too large, reducing the compacted density and gram capacity of the positive electrode particles 100. If the temperature T2 of the second sintering stage is too high, impurities are likely to be generated during the gas generation process, reducing the gram capacity of the positive electrode particles 100.

[0125] Optionally, the holding time of the second stage sintering ranges from 8h to 20h. Specifically, the holding time of the second stage sintering can be, but is not limited to, 8h, 10h, 12h, 14h, 16h, 18h, 20h, etc. If the holding time of the second stage sintering is too short, it is easy for the precursor to produce incomplete gas, increase the probability of the generation of impurities in the sodium iron phosphate pyrophosphate particles 10 of the positive electrode particles 100, and reduce the gram capacity of the positive electrode particles 100; in addition, it will also make the pores inside the positive electrode particles 100 obtained too large, reducing the compaction density and gram capacity of the positive electrode particles 100. If the holding time of the second stage sintering is too long, the production efficiency of the positive electrode particles 100 is reduced and the preparation cost of the positive electrode particles 100 is increased.

[0126] S2023, performing a third stage of sintering at a third temperature T3 of 450°C ≤ T3 ≤ 600°C.

[0127] Optionally, the third sintering stage is performed under the protection of an inert atmosphere. Optionally, the inert atmosphere includes at least one of helium, nitrogen, and argon.

[0128] Specifically, the temperature T3 of the third sintering stage (i.e., the third temperature) can be, but is not limited to, 450°C, 460°C, 470°C, 480°C, 490°C, 500°C, 510°C, 520°C, 530°C, 540°C, 550°C, 560°C, 570°C, 580°C, 590°C, 600°C, etc. If the temperature T3 of the third sintering stage is too low, the temperature of the sodium ferric pyrophosphate phosphate will not be reached, thereby reducing the content of the sodium ferric pyrophosphate in the positive electrode particles 100 and reducing the gram capacity of the positive electrode particles 100. If the temperature T3 of the third sintering stage is too high, the generated sodium ferric pyrophosphate particles 10 are easily decomposed to produce sodium ferric phosphate and sodium ferric pyrophosphate impurities, thereby reducing the gram capacity of the positive electrode particles 100.

[0129] Optionally, the holding time of the third-stage sintering ranges from 8 h to 20 h. Specifically, the holding time of the third-stage sintering can be, but is not limited to, 8 h, 10 h, 12 h, 14 h, 16 h, 18 h, 20 h, etc. If the holding time of the third-stage sintering is too short, the formation of the sodium iron pyrophosphate phosphate particles 10 phase is incomplete or too little, reducing the specific capacity per gram of the positive electrode particles 100. If the holding time of the third-stage sintering is too long, the production efficiency of the positive electrode particles 100 is reduced, and the preparation cost of the positive electrode particles 100 is increased.

[0130] Please refer to Figure 5 , the embodiment of the present application further provides a positive electrode plate 200, the positive electrode plate 200 includes a positive electrode current collector 210 and a positive electrode active layer 220, and the positive electrode active layer 220 includes the positive electrode particles 100 of the embodiment of the present application.

[0131] For a detailed description of other aspects of the positive electrode particles 100, please refer to the description of the corresponding part of the above embodiment, which will not be repeated here.

[0132] Optionally, the positive electrode current collector 210 can be, but is not limited to, carbon-coated aluminum foil, pure aluminum foil, and aluminum sheet.

[0133] Optionally, the positive electrode active layer 220 further includes a positive electrode conductive agent, a positive electrode binder, a positive electrode thickening agent, etc.

[0134] Optionally, the positive electrode conductive agent can be, but is not limited to, at least one of conductive carbon black, acetylene black, carbon nanotubes, carbon fibers, graphene, etc.

[0135] Optionally, the positive electrode binder can be, but is not limited to, at least one of polyvinylidene fluoride (abbreviated as PVDF), polyamide (abbreviated as PA), polyacrylonitrile (abbreviated as PAN), polyacrylate, polyvinylether, polymethylmethacrylate (abbreviated as PMMA), polyhexafluoropropylene, polymerized styrene butadiene rubber (abbreviated as SBR), etc.

[0136] In some embodiments, the sheet resistivity M of the positive electrode plate 200 ranges from 0.1 Ω·m ≤ M ≤ 5 Ω·m.

[0137] Specifically, the sheet resistivity M of the positive electrode sheet 200 can be, but is not limited to, 0.1 Ω·m, 0.2 Ω·m, 0.4 Ω·m, 0.6 Ω·m, 0.8 Ω·m, 1 Ω·m, 1.5 Ω·m, 2 Ω·m, 2.5 Ω·m, 3 Ω·m, 3.5 Ω·m, 4 Ω·m, 4.5 Ω·m, 5 Ω·m, etc. If the sheet resistivity M of the positive electrode sheet 200 is too low, the content of the conductive agent in the positive electrode sheet 200 needs to be increased, which increases the preparation difficulty of the positive electrode sheet 200 and reduces the capacity of the positive electrode sheet 200; if the sheet resistivity M of the positive electrode sheet 200 is too high, when the positive electrode sheet 200 is applied to a sodium battery, the internal resistance of the sodium battery is too large, reducing the cycle life of the sodium battery. The positive electrode sheet 200 of this embodiment has a relatively low sheet resistivity. When applied to a sodium battery, it can make the sodium battery have a lower resistance, which is beneficial to the exertion of the capacity of the sodium battery and improves the cycle life of the sodium battery.

[0138] Please refer to Figure 6 and Figure 7 , this embodiment of the present application also provides a sodium battery 300, which includes an electrolyte, the positive electrode sheet 200 described in this embodiment of the present application, a separator 320, and a negative electrode sheet 330.

[0139] For a detailed description of other aspects of the positive electrode sheet 200, please refer to the description of the corresponding part of the above embodiment, which will not be repeated here.

[0140] The sodium battery 300 of this embodiment of the present application can be at least one of, but is not limited to, a sodium ion battery, a sodium metal battery, a lithium-sodium hybrid battery, etc.

[0141] It can be understood that the positive electrode sheet 200 and the negative electrode sheet 330 are respectively located on opposite sides of the separator 320, that is, the separator 320 is located between the positive electrode sheet 200 and the negative electrode sheet 330, separating the positive electrode sheet 200 and the negative electrode sheet 330.

[0142] It should be noted that the positive electrode sheet 200, the separator 320, and the negative electrode sheet 330 are all at least partially immersed in the electrolyte.

[0143] Optionally, the electrolyte includes an electrolyte salt, an organic solvent, and a film-forming additive.

[0144] Optionally, the electrolyte salt can include, but is not limited to, sodium salts. Optionally, the sodium salt can be at least one of, but is not limited to, sodium hexafluorophosphate (NaPF6), sodium tetrafluoroborate (NaBF4), sodium bis(fluorosulfonyl)imide, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(oxalato)borate, sodium difluorodioxalate phosphate, sodium difluorooxalate borate, sodium difluorophosphate (NaPO2F2), sodium trifluoromethanesulfonate (CF3SO3Na), etc.

[0145] 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 (abbreviated as EC), propylene carbonate (abbreviated as PC), etc. The dielectric constant of ethylene carbonate is much larger than that of propylene carbonate, and ethylene carbonate can better promote the formation of a solid electrolyte interface membrane (abbreviated as SEI). Optionally, the chain carbonate may include, but is not limited to, at least one of dimethyl carbonate (abbreviated as DMC), diethyl carbonate (abbreviated as DEC), ethyl methyl carbonate (abbreviated as EMC), etc. Optionally, the organic solvent further includes at least one of ethyl acetate, propyl acetate, ethyl propionate, propyl propionate, γ-butyrolactone, 2,2-difluoroethyl acetate.

[0146] Optionally, the film-forming additive may include, but is not limited to, at least one of propargylbenzenesulfonic acid, vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfite (DTD), methylene methanedisulfonate (MMDS), butylsulfonic acid lactone (BS), 1,3-propylene sulfonic acid lactone (PST), etc.

[0147] Optionally, the separator 320 may be, but is not limited to, at least one of a polypropylene film (abbreviated as PP film), a polyethylene film (abbreviated as PE film), a ceramic film, a glass fiber film (abbreviated as glass fiber film), etc.

[0148] Optionally, the thickness of the separator 320 is 10 μm to 18 μm. Specifically, the thickness of the separator 320 may be, but is not limited to, 10 μm, 11 μm, 12 μm, 13 μm, 14 μm, 14.5 μm, 15 μm, 15.5 μm, 16 μm, 16.5 μm, 17 μm, 17.5 μm, and 18 μm, etc.

[0149] Please refer to Figure 8 , optionally, the negative electrode plate 330 includes a negative electrode current collector 331 and a negative electrode active layer 332.

[0150] Optionally, the negative electrode current collector 331 may be, but is not limited to, copper foil, copper sheet, aluminum foil, and carbon-coated aluminum foil.

[0151] Optionally, the negative electrode active layer 332 includes a negative electrode active material, a negative electrode conductive agent, a negative electrode binder, a negative electrode thickener, etc.

[0152] Optionally, the negative electrode active material may be, but is not limited to, hard carbon.

[0153] Optionally, the negative electrode conductive agent may be, but is not limited to, at least one of conductive carbon black (SP for short), acetylene black, carbon nanotubes, carbon fibers, graphene, and the like.

[0154] Optionally, the negative electrode binder may be, but is not limited to, at least one of polyvinylidene fluoride, polyamide, polyacrylonitrile, polyacrylate, polyvinyl ether, polymethyl methacrylate, polyhexafluoropropylene, styrene-butadiene rubber, and the like.

[0155] Optionally, the negative electrode thickener may be, but is not limited to, at least one of sodium carboxymethyl cellulose (CMC), polyacrylamide (PAM), and polymethacrylate (PMA).

[0156] Optionally, the sodium battery 300 further includes a housing 340 and an end cap assembly 350. The housing 340 and the end cap assembly 350 enclose a closed receiving chamber (not shown) for accommodating the electrolyte, the positive electrode sheet 200, the separator 320, and the negative electrode sheet 330. It is understood that the end cap assembly 350 electrically connects the positive electrode sheet 200 and the negative electrode sheet 330, respectively, and leads the positive electrode sheet 200 and the negative electrode sheet 330 out for electrical connection to external devices or other sodium batteries 300.

[0157] The following further describes the positive electrode particle 100 according to the embodiment of the present application through specific examples.

[0158] Example 1

[0159] The positive electrode particles 100 of this embodiment are prepared by the following steps:

[0160] (1) Weighing sodium pyrophosphate, ammonium dihydrogen phosphate, and ferrous oxalate dihydrate in a preset ratio so that the molar ratio of sodium, iron, and phosphorus is 4:3:4, and weighing glucose as a carbon source, wherein the mass fraction of glucose in the raw materials (the total mass of sodium pyrophosphate, ammonium dihydrogen phosphate, ferrous oxalate dihydrate, and glucose) is 7%. Sodium pyrophosphate, ammonium dihydrogen phosphate, ferrous oxalate dihydrate, and glucose are uniformly stirred in water, and sand-milled to a particle size of D50'≤1 μm to obtain a slurry;

[0161] (2) spray drying the slurry at 110° C. to obtain a precursor powder;

[0162] (3) The precursor powder was sintered in a sintering furnace under nitrogen protection at a temperature of 250°C for the first stage, and the sintering time of the first stage was 6 hours;

[0163] (4) performing a second sintering step in a sintering furnace at a temperature of 350° C. under nitrogen protection for 10 hours; and

[0164] (5) In a sintering furnace, under nitrogen protection, the third-stage sintering is carried out at a temperature of 550 °C for 10 h to obtain the positive electrode particles 100.

[0165] Comparative Example 1

[0166] The positive electrode particles 100 of this comparative example are prepared by the following steps:

[0167] (1) Weigh a preset ratio of sodium pyrophosphate, ammonium dihydrogen phosphate, and ferrous oxalate dihydrate so that the molar ratio of sodium, iron, and phosphorus is 4:3:4. Weigh glucose as a carbon source. Among them, the mass fraction of glucose in the raw materials (the total mass of sodium pyrophosphate, ammonium dihydrogen phosphate, ferrous oxalate dihydrate, and glucose) is 7%. Stir sodium pyrophosphate, ammonium dihydrogen phosphate, ferrous oxalate dihydrate, and glucose evenly in water and grind them until the D50' of the particle size is ≤ 1 μm to obtain a slurry.

[0168] (2) Spray-dry the slurry at 110 °C to obtain a precursor powder.

[0169] (3) Put the precursor powder into a sintering furnace, under nitrogen protection, sinter at a temperature of 550 °C for 10 h to obtain the positive electrode particles 100.

[0170] Comparative Example 2

[0171] The positive electrode particles 100 of this comparative example are prepared by the following steps:

[0172] (1) Weigh a preset ratio of sodium pyrophosphate, ammonium dihydrogen phosphate, and ferrous oxalate dihydrate so that the molar ratio of sodium, iron, and phosphorus is 4:3:4. Weigh glucose as a carbon source. Among them, the mass fraction of glucose in the raw materials (the total mass of sodium pyrophosphate, ammonium dihydrogen phosphate, ferrous oxalate dihydrate, and glucose) is 7%. Stir sodium pyrophosphate, ammonium dihydrogen phosphate, ferrous oxalate dihydrate, and glucose evenly in water and grind them until the D50' of the particle size is ≤ 1 μm to obtain a slurry.

[0173] (2) Spray-dry the slurry at 110 °C to obtain a precursor powder.

[0174] (3) Put the precursor powder into a sintering furnace, under nitrogen protection, sinter at a temperature of 250 °C for 6 h;

[0175] (4) In a sintering furnace, under nitrogen protection, sinter at a temperature of 550 °C for 10 h to obtain the positive electrode particles 100.

[0176] Comparative Example 3

[0177] The positive electrode particles 100 of this comparative example are prepared by the following steps:

[0178] (1) Weigh sodium pyrophosphate, ammonium dihydrogen phosphate, and ferrous oxalate dihydrate in a preset ratio so that the molar ratio of sodium, iron, and phosphorus is 4:3:4. Weigh glucose as the carbon source, where the mass fraction of glucose in the raw materials (the total mass of sodium pyrophosphate, ammonium dihydrogen phosphate, ferrous oxalate dihydrate, and glucose) is 7%. Stir sodium pyrophosphate, ammonium dihydrogen phosphate, ferrous oxalate dihydrate, and glucose evenly in water and grind them until the D50’ of the particle size ≤ 1 μm to obtain a slurry.

[0179] (2) Spray-dry the slurry at 110 °C to obtain precursor powder.

[0180] (3) Place the precursor powder in a sintering furnace and sinter it at 350 °C for 10 h under nitrogen protection; and

[0181] (5) Place it in a sintering furnace and sinter it at 550 °C for 10 h under nitrogen protection to obtain the positive electrode particles 100.

[0182] Perform the following performance tests on the positive electrode particles 100 prepared in Example 1 and Comparative Examples 1 to 3:

[0183] (1) Carbon content test: Measure it using a high-frequency infrared carbon-sulfur analyzer, and the test conditions are 18 MHz and 2.7 kw.

[0184] (2) Proportion test of the number of gray particles: Under an optical microscope, select an area, and select multiple particles (such as 100, 200, 300, 400, etc.) in this area for statistics. Calculate the number of golden particles and gray particles respectively, and then calculate the proportion Z of the number of gray particles, where Z = the number of gray particles / (the number of golden particles + the number of gray particles).

[0185] (3) Powder resistivity test: Measure it using the two-probe method, and the test parameters are: the applied pressure range is 0 to 200 MPa.

[0186] (4) Discharge specific capacity test: Prepare a positive electrode slurry according to the mass ratio of the positive electrode particles 100, polyvinylidene fluoride (PVDF), and conductive carbon black (SP) of 8:1:1. Then coat the positive electrode slurry on an aluminum foil (positive electrode current collector 210), and after drying, roll and cut it into small round pieces as the positive electrode plate 200. Use a composite sodium sheet as the negative electrode, a glass fiber membrane as the separator 320, and 1 mol / L sodium hexafluorophosphate as the electrolyte to assemble a button sodium battery 300. Hang the assembled button sodium battery 300 on the test channel of Blue Power or Neware, and perform charge and discharge cycling in the voltage range of 1.5 V to 3.5 V at a rate of 0.1C. Obtain the specific capacity of the positive electrode particles 100 according to the charge and discharge cycling test data.

[0187] (5) Diaphragm resistivity test: Prepare a positive electrode paste with the positive electrode particles 100, polyvinylidene fluoride (PVDF), and conductive carbon black (SP) in a mass ratio of 8:1:1. Then coat the positive electrode paste on the aluminum foil (positive electrode current collector 210). After drying, roll it into a positive electrode plate 200. Cut the rolled positive electrode plate 200 into a rectangular size of 5 cm * 10 cm. Use a micrometer to randomly measure the thickness of the positive electrode plate 200 at 10 positions, and calculate the average value of the thickness of the positive electrode plate 200. Then place the positive electrode plate 200 between the two electrodes of the diaphragm resistance meter, set the test pressure and pressure holding time parameters on the MRMS software, and start the test. The instrument automatically calculates the diaphragm resistivity of the positive electrode plate 200. Randomly select 10 positions for each positive electrode plate 200 to conduct the test, and finally calculate the average value of the diaphragm resistivity of the positive electrode plate 200.

[0188] The test results of the positive electrode particles 100 in Example 1 and Comparative Examples 1 to 3 are shown in Table 1 below.

[0189] Table 1 Performance parameters of the positive electrode particles 100 in Example 1 and Comparative Examples 1 to 3

[0190]

[0191] From the test results of Example 1 and Comparative Examples 1 to 3 in Table 1, it can be seen that compared with single-stage sintering (i.e., Comparative Example 1) or two-stage sintering (i.e., Comparative Examples 2 and 3), when the carbon content of the positive electrode particles 100 is the same, the positive electrode particles 100 prepared by three-stage sintering in Example 1 of the present application have a higher proportion of gray particles, and the powder resistivity of the positive electrode particles 100 is greatly reduced. Compared with Comparative Examples 1 and 3, the powder resistivity is 2 to 5 orders of magnitude lower. In addition, the positive electrode plate 200 prepared from the positive electrode particles 100 in Example 1 of the present application has a lower diaphragm resistivity. Compared with the positive electrode plates 200 prepared from the positive electrode particles 100 in Comparative Examples 1 to 3, the diaphragm resistivity of the positive electrode plate 200 in Example 1 is reduced to 1 / 7.33 to 1 / 5 of the original. Moreover, the positive electrode particles 100 prepared by three-stage sintering in Example 1 of the present application also have a higher discharge capacity per gram.

[0192] The scanning electron microscope image (SEM image) of the positive electrode particles 100 prepared in Example 1 is as Figure 9 shown. From Figure 9 it can be seen that after three-stage sintering, spherical positive electrode particles 100 with different particle sizes are obtained.

[0193] Figure 10 is the optical microscope image of the positive electrode particles 100 in Example 1 taken under a Raman spectrometer. Figure 11 For Figure 10 the Raman spectrum of the gold particles inFigure 12 is Figure 10 the Raman spectrum of the gray particles in

[0194] It can be seen from Figure 10 that the positive electrode particles prepared in Example 1 include golden particles and gray particles. It can be seen from Figure 11 that in the Raman spectrum of the golden particles, there is a peak of the P-O bond in the phosphate ion (PO4 -1 to 1030 cm -1 ), and there is a peak of the D bond of amorphous carbon at 1330 cm 3- to 1370 cm -1 ; there is a peak of the G bond of graphitized carbon at 1570 cm -1 to 1620 cm -1 . It can be seen from -1 that in the Raman spectrum of the gray particles, there is no peak of the P-O bond in the phosphate ion (PO4 Figure 12 to 1030 cm -1 , and there is a peak of the D bond of amorphous carbon at 1330 cm -1 to 1370 cm 3- ; there is a peak of the G bond of graphitized carbon at 1570 cm -1 to 1620 cm -1 . -1 to 1620 cm -1

[0195] Example 2

[0196] The difference between this example and Example 1 is that the temperature of the third-stage sintering in this example is 450 °C.

[0197] Example 3

[0198] The difference between this example and Example 1 is that the temperature of the third-stage sintering in this example is 500 °C.

[0199] Example 4

[0200] The difference between this example and Example 1 is that the temperature of the third-stage sintering in this example is 600 °C.

[0201] Comparative Example 4

[0202] The difference between this comparative example and Example 1 is that the temperature of the third-stage sintering in this example is 400 °C.

[0203] Comparative Example 5

[0204] The difference between this comparative example and Example 1 is that the temperature of the third-stage sintering in this example is 650 °C.

[0205] The test results of the positive electrode particles 100 in Examples 2 to 4 and Comparative Examples 4 to 5 are shown in Table 2 below.

[0206] Table 2 Performance Parameters of Positive Electrode Particles 100 in Examples 2 to 4 and Comparative Examples 4 to 5

[0207]

[0208] From the test results in Table 1 and Table 2 for Examples 1 to 4, and Comparative Examples 4 and 5, it can be seen that when the temperature of the third-stage sintering is too low (such as in Comparative Example 4), the positive electrode particles 100 prepared have a high carbon content, a low proportion of the number of gray particles in the positive electrode particles 100, a high powder resistivity of the positive electrode particles 100, a high film resistivity of the positive electrode sheet 200 prepared from the positive electrode particles 100, and the discharge specific capacity of the positive electrode particles 100 is also greatly reduced. As the temperature of the third-stage sintering increases (Examples 1 to 4), the carbon content of the prepared positive electrode particles 100 gradually decreases, the proportion of the number of gray particles in the positive electrode particles 100 gradually increases, the powder resistivity of the positive electrode particles 100 gradually decreases, the film resistivity of the positive electrode sheet 200 prepared from the positive electrode particles 100 gradually decreases, and the discharge specific capacity of the positive electrode particles 100 gradually increases. When the temperature of the third-stage sintering continues to increase (such as in Comparative Example 5), the carbon content of the prepared positive electrode particles 100 continues to decrease, the proportion of the number of gray particles in the positive electrode particles 100 increases, the powder resistivity of the positive electrode particles 100 continues to decrease, the film resistivity of the positive electrode sheet 200 prepared from the positive electrode particles 100 also continues to decrease. However, the discharge specific capacity of the positive electrode particles 100 is greatly reduced, indicating that when the temperature of the third-stage sintering is too high, a large amount of sodium iron pyrophosphate phosphate in the positive electrode particles 100 decomposes, thus greatly reducing the discharge specific capacity of the positive electrode particles 100.

[0209] Example 5

[0210] The difference between this example and Example 1 is that the temperature of the first-stage sintering in this example is 200 °C.

[0211] Example 6

[0212] The difference between this example and Example 1 is that the temperature of the first-stage sintering in this example is 300 °C.

[0213] Example 7

[0214] The difference between this example and Example 1 is that the temperature of the second-stage sintering in this example is 300 °C.

[0215] Example 8

[0216] The difference between this embodiment and Embodiment 1 is that the temperature of the second-stage sintering in this embodiment is 400 °C.

[0217] Comparative Example 6

[0218] The difference between this comparative example and Embodiment 1 is that the temperature of the first-stage sintering in this embodiment is 150 °C.

[0219] Comparative Example 7

[0220] The difference between this comparative example and Embodiment 1 is that the temperature of the first-stage sintering in this embodiment is 320 °C.

[0221] Comparative Example 8

[0222] The difference between this comparative example and Embodiment 1 is that the temperature of the second-stage sintering in this embodiment is 270 °C.

[0223] Comparative Example 9

[0224] The difference between this comparative example and Embodiment 1 is that the temperature of the second-stage sintering in this embodiment is 430 °C.

[0225] The test results of the positive electrode particles 100 of Embodiments 5 to 8 and Comparative Examples 6 to 9 are shown in Table 3 below.

[0226] Table 3 Performance parameters of the positive electrode particles 100 of Embodiments 5 to 8 and Comparative Examples 6 to 9

[0227]

[0228] From the test results of Embodiment 1, Embodiment 5, Embodiment 6, Comparative Example 6, and Comparative Example 7, it can be seen that the temperature of the first-stage sintering has little effect on the carbon content of the prepared positive electrode particles 100, the proportion of the number of gray particles in the positive electrode particles 100, the powder resistance of the positive electrode particles 100, and the film resistivity of the positive electrode sheet 200 prepared from the positive electrode particles 100. However, with the increase in the temperature of the first-stage sintering, the discharge specific capacity of the prepared positive electrode particles 100 first gradually increases and then gradually decreases.

[0229] From Embodiment 1, Embodiment 7, Embodiment 8, Comparative Example 8, and Comparative Example 9, as the temperature of the second-stage sintering increases, the carbon content of the prepared positive electrode particles 100 gradually decreases, the proportion of the number of gray particles in the positive electrode particles 100 gradually increases, the powder resistivity of the positive electrode particles 100 gradually decreases, the film resistivity of the positive electrode sheet 200 prepared from the positive electrode particles 100 gradually decreases, and the discharge specific capacity of the positive electrode particles 100 first gradually increases and then gradually decreases.

[0230] From the test results of the above embodiments and comparative examples, it can be seen that the change in the sintering temperature of the third stage has the greatest impact on the performance of the cathode particles 100, such as the carbon content, powder resistivity, proportion of the number of gray particles, and diaphragm resistivity; the change in the sintering temperature of the second stage has the second greatest impact on the carbon content, powder resistivity, proportion of the number of gray particles, and diaphragm resistivity of the cathode particles 100; the change in the sintering temperature of the first stage has the smallest impact on the carbon content, powder resistivity, proportion of the number of gray particles, and diaphragm resistivity of the cathode particles 100 among the three stages.

[0231] Examples 9 to 14, Comparative Examples 10 to 11

[0232] The differences between Examples 9 to 14, Comparative Examples 10 to 11 and Example 1 lie in the different addition amounts of the carbon source, resulting in different carbon contents of the cathode particles 100.

[0233] The test results of the cathode particles 100 of Examples 9 to 14 and Comparative Examples 10 to 11 are shown in Table 4 below.

[0234] Table 4 Performance parameters of the cathode particles 100 of Examples 9 to 14 and Comparative Examples 10 to 11

[0235]

[0236] From the test results of Example 1, Examples 9 to 14, and Comparative Examples 10 to 11, it can be seen that when other conditions remain unchanged, by changing the carbon content of the cathode particles 100, as the carbon content of the cathode particles 100 increases, the powder resistivity of the cathode particles 100 gradually decreases, the diaphragm resistivity of the positive electrode sheet 200 prepared from the cathode particles 100 gradually decreases, and the discharge specific capacity of the cathode particles 100 also gradually decreases.

[0237] Please refer to Figure 13 , the embodiment of the present application also provides an energy storage device 400, which includes a box body 410 and the sodium battery 300 described in the embodiment of the present application, and the sodium battery 300 is received in the box body 410.

[0238] The energy storage device 400 of the present application can be applied to, but not limited to, power generation side energy storage, grid side energy storage, and user side energy storage, etc.

[0239] Optionally, the number of sodium batteries 300 may be, but is not limited to, one or more. When there are multiple sodium batteries 300, the multiple sodium batteries 300 are stacked and arranged in the box body 410. It can be understood that the multiple sodium batteries 300 being stacked and arranged may mean that the multiple sodium batteries 300 are arranged in abutting sequence, or the multiple sodium batteries 300 are arranged in sequence with intervals. In addition, the multiple sodium batteries 300 may be stacked horizontally (such as in the horizontal direction) or vertically (such as in the direction of gravity). The stacking method and stacking direction of the multiple sodium batteries 300 can be designed according to actual situations, and the present application does not make specific limitations.

[0240] The term "multiple" means greater than or equal to two.

[0241] It can be understood that the multiple sodium batteries 300 of the energy storage device 400 may be connected in parallel with each other; or connected in series with each other; or partially in parallel and partially in series (in other words, in a hybrid connection). For the connection method of the multiple sodium batteries 300 of the same energy storage device 400, the present application does not make specific limitations.

[0242] Optionally, the energy storage device 400 may include, but is not limited to, a sodium battery 300 module, a sodium battery 300 pack, a sodium battery 300 system, an energy storage box, an energy storage cabinet, an energy storage container, etc. The actual application forms of the energy storage device 400 provided by the embodiments of the present application may be, but are not limited to, the listed products, and may also be other application forms. The embodiments of the present application do not strictly limit the application forms of the energy storage device 400. In the drawings of the embodiments of the present application, only the case where the energy storage device 400 includes multiple sodium batteries 300 is used for illustration, and should not be construed as a limitation on the energy storage device 400 of the embodiments of the present application.

[0243] It can be understood that the box body 410 has a receiving cavity, and one or more sodium batteries 300 are received in the receiving cavity. In some embodiments, each receiving cavity receives one sodium battery 300. In other embodiments, each receiving cavity receives multiple sodium batteries 300.

[0244] Please refer to Figure 14 and Figure 15 , the embodiments of the present application also provide an energy storage system 500, which includes the energy storage device 400 described in the embodiments of the present application; and a power conversion device 510, the power conversion device 510 is electrically connected to the energy storage device 400, the power conversion device 510 is used to convert other forms of energy into electric energy, and the energy storage device 400 is used to store the electric energy.

[0245] It should be noted that energy storage (i.e., energy storage) has a wide range of application scenarios, including energy storage on the power generation side, energy storage on the grid side, and energy storage on the user side, etc. The energy storage system 500 of the embodiments of the present application takes energy storage on the power generation side as an example to introduce the energy storage system 500 of the embodiments of the present application in detail, which should not be construed as a limitation on the energy storage system 500 of the embodiments of the present application, nor should it be construed as a limitation on the energy storage device 400 and the sodium battery 300 of the embodiments of the present application.

[0246] During operation, the electric energy conversion device 510 is used to convert other forms of energy into electric energy and store it in the energy storage device 400. The electric energy stored in the energy storage device 400 can be supplied to electrical loads such as street lights and household appliances for use during peak electricity prices, or for power supply when the power grid is powered off / out of power. The electric energy generated by the electric energy conversion device 510 can also be supplied to the power grid through high-voltage cables to relieve the power supply pressure during peak power grid periods.

[0247] Optionally, the electric energy conversion device 510 can convert at least one of other forms of energy such as solar energy, light energy, wind energy, thermal energy, tidal energy, biomass energy, and mechanical energy into electric energy.

[0248] Optionally, the number of the electric energy conversion devices 510 can be one or more. When there are multiple electric energy conversion devices 510, the multiple electric energy conversion devices 510 can be connected in series, in parallel, or in a mixed connection. The present application does not make specific limitations.

[0249] Optionally, the electric energy conversion device 510 can be, but is not limited to, at least one of a photovoltaic panel, a wind power generation device, a water power generation device, etc.

[0250] Optionally, the number of the energy storage devices 400 can be one or more. When the number of the energy storage devices 400 is multiple, the multiple energy storage devices 400 are connected in series or in parallel with each other. The present application does not make specific limitations.

[0251] When "embodiment" or "implementation manner" is mentioned in the present application, it means that the specific features, structures, or characteristics described in combination with the embodiment can be included in at least one embodiment of the present application. The phrase appears at various positions in the specification does not necessarily refer to the same embodiment, nor is it an independent or alternative embodiment mutually exclusive with other embodiments. Those skilled in the art explicitly and implicitly understand that the embodiments described in the present application can be combined with other embodiments. In addition, it should also be understood that the features, structures, or characteristics described in each embodiment of the present application can be combined arbitrarily without contradiction to form another embodiment that does not deviate from the spirit and scope of the technical solution of the present application.

[0252] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present application and not to limit them. Although the present application has been described in detail with reference to the above preferred embodiments, those of ordinary skill in the art should understand that modifications or equivalent replacements can be made to the technical solutions of the present application without departing from the spirit and scope of the technical solutions of the present application.

Claims

1. A positive electrode particle, characterized in that, The positive electrode particles include sodium iron pyrophosphate phosphate particles and a carbon coating layer. The carbon coating layer wraps around the surface of the sodium iron pyrophosphate phosphate particles. The positive electrode particles include gray particles and golden particles. Among the positive electrode particles, the range of the proportion Z of the number of gray particles is: 0.8 ≤ Z ≤ 1.

2. The positive electrode particle according to claim 1, characterized in that, The positive electrode particles satisfy the relational expression: I P / I G ≤0.01; Among them, I P is the intensity of the peak at 960 cm -1 to 1030 cm -1 in the Raman spectrum of the positive electrode particles; I G is the intensity of the peak at 1570 cm -1 to 1620 cm -1 in the Raman spectrum of the positive electrode particles.

3. The positive electrode particle according to claim 2, characterized in that, The positive electrode particles also satisfy the relational expression: I D / I G ≤1.2; Wherein, I D is the intensity of the peak at 1330 cm -1 to 1370 cm -1 in the Raman spectrum of the positive electrode particles; I G is the intensity of the peak at 1570 cm -1 to 1620 cm -1 in the Raman spectrum of the positive electrode particles.

4. The positive electrode particle according to claim 1, characterized in that, In the sodium iron pyrophosphate phosphate particles, the mass fraction of sodium iron phosphate is less than or equal to 5%, and the mass fraction of sodium pyrophosphate iron is less than or equal to 5%.

5. The positive electrode particle according to claim 1, characterized in that, Among the positive electrode particles, the range of the mass fraction A of the carbon coating layer is: 1% ≤ A ≤ 10%.

6. The positive electrode particle according to claim 1, characterized in that, The powder resistivity R of the positive electrode particles ranges from 10 Ω·cm ≤ R ≤ 9999 Ω·cm.

7. A method for preparing cathode particles, characterized in that, including: Mixing a sodium source, a phosphorus source, an iron source, and a carbon source in a solvent to obtain a slurry; spray-drying the slurry to obtain a precursor powder; and Performing segmented sintering on the precursor powder to obtain the positive electrode particles. Among them, the positive electrode particles include sodium iron pyrophosphate phosphate particles and a carbon coating layer. The carbon coating layer wraps around the surface of the sodium iron pyrophosphate phosphate particles. The positive electrode particles include gray particles and golden particles. Among the positive electrode particles, the range of the proportion Z of the number of gray particles is: 0.8 ≤ Z ≤ 1.

8. The method for preparing the positive electrode particles according to claim 7, wherein The performing segmented sintering on the precursor powder includes: Performing the first-stage sintering at a first temperature T1 where 200°C ≤ T1 ≤ 300°C; Performing the second-stage sintering at a second temperature T2 where 300°C ≤ T4 ≤ 400°C; and Performing the third-stage sintering at a third temperature T3 where 450°C ≤ T3 ≤ 600°C.

9. A positive electrode sheet, characterized in that, The positive electrode sheet includes: A positive electrode current collector; and A positive electrode active layer, where the positive electrode active layer includes the positive electrode particles according to any one of claims 1-6 or the positive electrode particles prepared by the preparation method of the positive electrode particles according to any one of claims 7-8.

10. The positive electrode sheet according to claim 9, wherein The sheet resistivity M of the positive electrode sheet ranges from 0.1 Ω·m ≤ M ≤ 5 Ω·m.

11. A sodium battery, characterized in that, including: An electrolyte, the positive electrode sheet according to claim 9 or 10, a separator, and a negative electrode sheet.

12. An energy storage device, characterized in that, including: A box body; and The sodium battery according to claim 11, where the sodium battery is housed in the box body.

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