Sodium ferric phosphate pyrophosphate secondary particle and preparation method thereof, positive pole piece, sodium battery and energy storage device

Abstract

The invention provides ferric sodium pyrophosphate secondary particles and a preparation method thereof, a positive pole piece, a sodium battery and an energy storage device. The embodiment of the invention provides a ferric sodium phosphate pyrophosphate secondary particle, the ferric sodium phosphate pyrophosphate secondary particle comprises a plurality of ferric sodium phosphate pyrophosphate primary particles, and the cross section of the ferric sodium phosphate pyrophosphate secondary particle comprises a plurality of holes, the average value of the distances between each hole and the n holes closest to the hole is a hole parameter I, the range of the average value of the hole parameters I of the multiple holes is 30 nm to 160 nm, n is larger than or equal to 2 and smaller than or equal to 6, and n is an integer.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of energy storage, in particular to a sodium iron phosphate pyrophosphate secondary particle, a preparation method thereof, a positive electrode sheet, a sodium battery and an energy storage device. BACKGROUND

[0002] The sodium iron phosphate pyrophosphate Na4Fe3(PO4)2P2O7 has a three-dimensional sodium ion diffusion channel and a sodium superionic conductor structure, and has the characteristics of high voltage platform, high capacity, excellent rate and cycle stability, and has great potential to become a positive electrode material for large-scale production in sodium batteries. However, in the related art, it is difficult for sodium iron phosphate pyrophosphate to have high tap density and high electrolyte wettability and rate performance. SUMMARY

[0003] The embodiments of the present application provide a sodium iron phosphate pyrophosphate secondary particle having high tap density and high rate performance.

[0004] In a first aspect, the embodiments of the present application provide a sodium iron phosphate pyrophosphate secondary particle, which includes a plurality of sodium iron phosphate pyrophosphate primary particles, and a cross section of the sodium iron phosphate pyrophosphate secondary particle includes a plurality of pores. An average value of a spacing between each pore and n most adjacent pores is a pore channel parameter I, an average value of the pore channel parameter I of the plurality of pores ranges from 30 nm to 160 nm, 2≤n≤6, and n is an integer.

[0005] Further, the pores include first pores, second pores and third pores, a pore size of the first pores is less than 30 nm, a pore size of the second pores is greater than or equal to 30 nm and less than or equal to 160 nm, and a pore size of the third pores is greater than 160 nm. A number ratio of the first pores in the pores of the sodium iron phosphate pyrophosphate secondary particle ranges from 25% to 47%, a number ratio of the second pores in the pores of the sodium iron phosphate pyrophosphate secondary particle ranges from 52% to 74.8%, and a number ratio of the third pores in the pores of the sodium iron phosphate pyrophosphate secondary particle ranges from 0.2% to 1%.

[0006] Further, a total area ratio of the plurality of pores on the cross section of the sodium iron phosphate pyrophosphate secondary particle ranges from 4% to 8%.

[0007] Further, a tap density of the sodium iron phosphate pyrophosphate secondary particle ranges from 2.16 g / cm 3 to 2.38 g / cm 3 .

[0008] Further, the average particle size D of the sodium iron phosphate pyrophosphate primary particles ranges from 310 nm to 720 nm; and the D50 of the sodium iron phosphate pyrophosphate secondary particles ranges from 3.4 μm to 8.6 μm, wherein D50 is the particle size value corresponding to 50% of the cumulative volume distribution of the sodium iron phosphate pyrophosphate secondary particles.

[0009] Further, the specific surface area of the sodium iron phosphate pyrophosphate secondary particles ranges from 10 m 2 / g to 18 m 2 / g.

[0010] Further, the sodium iron phosphate pyrophosphate secondary particles are prepared by mixing a sodium source, a phosphorus source, an iron source, a first pore-forming agent and a second pore-forming agent, and then performing first sintering to form pores to obtain an intermediate product; and mixing the intermediate product with a carbon source and then performing second sintering to obtain the sodium iron phosphate pyrophosphate secondary particles; wherein the first pore-forming agent has a linear structure, and the second pore-forming agent has a granular structure.

[0011] In a second aspect, an embodiment of the present application provides a preparation method of sodium iron phosphate pyrophosphate secondary particles, which comprises:

[0012] mixing a sodium source, a phosphorus source, an iron source and a pore-forming agent to obtain a first precursor powder;

[0013] performing first sintering on the precursor powder to form pores to obtain an intermediate product;

[0014] mixing the intermediate product with a carbon source to obtain a second precursor powder; and

[0015] performing second sintering on the second precursor powder to obtain sodium iron phosphate pyrophosphate secondary particles;

[0016] wherein the sodium iron phosphate pyrophosphate secondary particles comprise a plurality of sodium iron phosphate pyrophosphate primary particles, the cross section of the sodium iron phosphate pyrophosphate secondary particles comprises a plurality of pores, the average value of the distance between each pore and the n most adjacent pores is a pore channel parameter I, and the average value of the pore channel parameters I of the plurality of pores ranges from 30 nm to 160 nm, 2≤n≤6, and n is an integer.

[0017] Further, the pore-forming agent comprises a first pore-forming agent and a second pore-forming agent, the first pore-forming agent has a linear structure, and the second pore-forming agent has a granular structure.

[0018] Further, the first pore-forming agent is an inorganic carbon source, the second pore-forming agent is an organic carbon source, and the mass ratio of the first pore-forming agent to the second pore-forming agent ranges from 0.25 to 0.5.

[0019] Furthermore, the first pore-forming agent includes carbon nanotubes; the second pore-forming agent includes at least one of glucose and sucrose; and the carbon source includes at least one of glucose and sucrose.

[0020] Further, the first precursor powder is subjected to a first sintering process to create pores, resulting in an intermediate product, comprising:

[0021] The first precursor powder was placed in an oxygen atmosphere and sintered at a temperature of 400°C to 500°C to obtain an intermediate product.

[0022] Further, the second precursor powder is subjected to a second sintering to obtain secondary particles of sodium iron pyrophosphate, comprising:

[0023] The second precursor powder was placed in an inert atmosphere and sintered at a temperature of 510°C to 620°C to obtain secondary granules of sodium iron pyrophosphate.

[0024] Thirdly, embodiments of this application provide a positive electrode sheet, the positive electrode sheet comprising:

[0025] Positive current collector; and

[0026] The positive electrode active layer includes the sodium iron pyrophosphate secondary particles described in the embodiments of this application.

[0027] Fourthly, this application provides a sodium battery comprising: an electrolyte, a positive electrode, a separator, and a negative electrode as described in this application.

[0028] Fifthly, embodiments of this application provide an energy storage device, which includes:

[0029] include:

[0030] Box; and

[0031] The sodium battery described in this application embodiment is housed within the casing.

[0032] The sodium iron pyrophosphate secondary particles described in this embodiment include multiple sodium iron pyrophosphate primary particles. The cross-section of each secondary particle includes multiple pores. The average distance between each pore and its n nearest neighbors is defined as the pore parameter I. The average value of the pore parameter I ranges from 30 nm to 160 nm, where 2 ≤ n ≤ 6, and n is an integer. By designing the pore parameters of the pores in the cross-section of the sodium iron pyrophosphate secondary particles, this embodiment achieves higher powder compaction density, higher volumetric energy density, and better electrolyte wettability. This facilitates shorter solid-phase transport distances, better capacity utilization of the sodium iron pyrophosphate secondary particles, and improves the rate performance and low-temperature performance of sodium batteries. Attached Figure Description

[0033] 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.

[0034] Figure 1 This is a schematic diagram of the structure of sodium ferric pyrophosphate secondary particles according to an embodiment of this application.

[0035] Figure 2 This is a schematic diagram of the SEM image processing flow when calculating the surface area ratio in this application.

[0036] Figure 3 This is a schematic diagram of the structure of secondary particles of sodium iron pyrophosphate according to another embodiment of this application.

[0037] Figure 4 This is a schematic flowchart of a method for preparing sodium ferric pyrophosphate secondary particles according to an embodiment of this application.

[0038] Figure 5 This is a schematic flowchart of a method for preparing a first precursor powder according to an embodiment of this application.

[0039] Figure 6 This is a cross-sectional structural schematic diagram of the positive electrode sheet according to an embodiment of this application.

[0040] Figure 7 This is a schematic diagram of the structure of a sodium battery according to an embodiment of this application.

[0041] Figure 8 This application describes a sodium battery according to an embodiment of the present application. Figure 7 A schematic diagram of the cross-sectional structure along the AA direction.

[0042] Figure 9This is a cross-sectional structural schematic diagram of the negative electrode sheet according to an embodiment of this application.

[0043] Figure 10 This is a schematic diagram of the structure of an energy storage device according to an embodiment of this application.

[0044] Figure 11 This is a structural block diagram of an energy storage system according to an embodiment of this application.

[0045] Figure 12 This is an application scenario diagram of an energy storage system according to an embodiment of this application.

[0046] Explanation of reference numerals in the attached figures:

[0047] 100-Sodium iron pyrophosphate secondary particles, 10-Sodium iron pyrophosphate primary particles, 11-Sodium iron pyrophosphate core, 12-Carbon layer, 20-Pore, 200-Positive electrode, 210-Positive current collector, 220-Positive active layer, 300-Sodium battery, 320-Separator, 330-Negative electrode, 331-Negative current collector, 332-Negative 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

[0048] 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.

[0049] 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.

[0050] The technical solutions in the embodiments of this application will now be described with reference to the accompanying drawings.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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, in related technologies, it is difficult to achieve both high compaction density and high electrolyte wettability and rate performance with sodium iron pyrophosphate.

[0055] Please see Figure 1 This application provides a secondary sodium ferric pyrophosphate particle 100, which includes a plurality of primary sodium ferric pyrophosphate particles 10. The cross-section of the secondary sodium ferric pyrophosphate particle 100 includes a plurality of pores 20. The average distance between each pore 20 and its n nearest neighbors is a pore parameter I. The average value of the pore parameter I of the plurality of pores 20 ranges from 30 nm to 160 nm, where 2 ≤ n ≤ 6, and n is an integer.

[0056] The sodium iron pyrophosphate secondary particles 100 of this application embodiment can be applied to sodium batteries (such as sodium-ion batteries) as positive active materials of the positive active layer of the positive electrode sheet of sodium batteries.

[0057] The term "secondary particle" refers to a collection of multiple primary particles aggregated through physical or chemical processes, which may contain pores 20 or grain boundaries. The term "primary particle" is the smallest independent crystal unit, typically a single crystal or a dense particle without pores 20.

[0058] It should be noted that the pores 20 of the sodium ferric pyrophosphate secondary particles 100 described in this application are pores 20 left by the accumulation of multiple sodium ferric pyrophosphate primary particles 10. Understandably, the multiple pores 20 are at least partially connected.

[0059] Specifically, n can be 2, 3, 4, 5, or 6. In the calculation of this application embodiment, the calculation is performed using n=4 as an example, which should not be construed as a limitation on the sodium ferric pyrophosphate secondary particles 100 of this application embodiment.

[0060] Specifically, the average value of the pore parameter I of the plurality of pores 20 can be, but is not limited to, 30nm, 35nm, 40nm, 45nm, 50nm, 55nm, 60nm, 65nm, 70nm, 75nm, 80nm, 90nm, 100nm, 110nm, 120nm, 130nm, 140nm, 150nm, 160nm, etc. The pore parameter I of the plurality of pores 20 on the cross-section of the sodium iron pyrophosphate secondary particles 100 can be used to measure the connectivity of the pores 20 and the proportion of the pores 20. When the average value of the pore parameter I of the multiple pores 20 is small, it indicates that the pores 20 inside the sodium iron pyrophosphate secondary particles 100 are highly interconnected and the proportion of pores 20 is large. This allows the electrolyte to better wet the sodium iron pyrophosphate secondary particles 100, greatly shortening the migration path of sodium ions. This is beneficial for the capacity of sodium batteries using sodium iron pyrophosphate secondary particles 100, improving the rate performance and low-temperature performance of sodium batteries. However, if the pore parameter I of the multiple pores 20 is too small, it will reduce the compaction density of the sodium iron pyrophosphate particles, thereby reducing the capacity and volumetric energy density of sodium batteries. When the average value of the pore parameter I of the multiple pores 20 is too large, it indicates that the connectivity of the pores 20 inside the sodium iron pyrophosphate secondary particles 100 is low and the proportion of pores 20 is small. This is not conducive to electrolyte wetting, which is not conducive to the specific capacity of sodium batteries, reducing the rate performance and low-temperature performance of sodium batteries.

[0061] For example, n=4, and the distances between each hole 20 and its four nearest neighbors are Ia, Ib, Ic, and Id, respectively. Then the channel parameter I of the hole 20 is (Ia+Ib+Ic+Id) / 4.

[0062] For example, the cross-section has 100 holes 20, and the channel parameters of these 100 holes 20 are I1, I2, I3, ..., I100, respectively. Then the average value of the channel parameters of the multiple holes 20 is (I1+I2+I3+...+I100) / 100.

[0063] The sodium iron pyrophosphate secondary particles 100 described in this embodiment include a plurality of sodium iron pyrophosphate primary particles 10. The cross-section of the sodium iron pyrophosphate secondary particles 100 includes a plurality of pores 20. The average distance between each pore 20 and its n nearest neighbors is the pore parameter I. The average value of the pore parameter I of the plurality of pores 20 ranges from 30 nm to 160 nm, where 2 ≤ n ≤ 6, and n is an integer. By designing the pore parameters of the pores 20 on the cross-section of the sodium iron pyrophosphate secondary particles 100, this embodiment of the application achieves higher powder compaction density, higher volumetric energy density, and better electrolyte wettability. This is beneficial for shortening the solid-phase transport distance, facilitating the capacity utilization of the sodium iron pyrophosphate secondary particles 100, and improving the rate performance and low-temperature performance of sodium batteries.

[0064] In some embodiments, the pores 20 include a first pore, a second pore, and a third pore. The diameter of the first pore is less than 30 nm, the diameter of the second pore is greater than or equal to 30 nm and less than or equal to 160 nm, and the diameter of the third pore is greater than 160 nm. The proportion of the first pores in the pores 20 of the sodium iron pyrophosphate secondary particles 100 ranges from 25% to 47%, the proportion of the second pores in the pores 20 of the sodium iron pyrophosphate secondary particles 100 ranges from 52% to 74.8%, and the proportion of the third pores in the pores 20 of the sodium iron pyrophosphate secondary particles 100 ranges from 0.2% to 1%.

[0065] In the embodiments of this 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.

[0066] Specifically, the percentage of the first pores in the pores 20 of the sodium iron pyrophosphate secondary particles 100 can be, but is not limited to, 25%, 28%, 30%, 33%, 35%, 38%, 40%, 43%, 45%, 47%, etc.

[0067] Specifically, the percentage of the number of second pores in the pores 20 of the sodium iron pyrophosphate secondary particles 100 can be, but is not limited to, 52%, 55%, 58%, 60%, 63%, 65%, 68%, 70%, 73%, 74.8%, etc.

[0068] Specifically, the percentage of the third pores in the pores 20 of the sodium iron pyrophosphate secondary particles 100 can be, but is not limited to, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, etc.

[0069] In this embodiment, the first pore has a small diameter, making it difficult for the electrolyte to effectively wet the first pore. The first pore occupies the volume of the pore, but it cannot exert the beneficial effect of the pore 20 on the rate performance and specific capacity of the sodium battery. Therefore, if there are too many first pores, it will not have a significant effect on improving the rate performance and specific capacity of the sodium battery, but it will reduce the specific capacity of the sodium iron pyrophosphate secondary particles 100 and reduce the energy density of the sodium battery. The third pore has a large diameter and occupies a large volume. If the number of third pores is too large, it will reduce the compaction density of the sodium iron pyrophosphate secondary particles 100.

[0070] In some embodiments, the total area of ​​the plurality of pores 20 on the cross-section of the sodium iron pyrophosphate secondary particles 100 ranges from 4% to 8%.

[0071] Specifically, the percentage of the total area of ​​the plurality of pores 20 on the cross-section of the sodium iron pyrophosphate secondary particles 100 can be, but is not limited to, 4%, 4.2%, 4.4%, 4.6%, 4.8%, 5%, 5.2%, 5.4%, 5.6%, 5.8%, 6%, 6.2%, 6.4%, 6.6%, 6.8%, 7%, 7.2%, 7.4%, 7.6%, 7.8%, 8%, etc.

[0072] If the total area of ​​the plurality of pores 20 on the cross-section of the sodium iron pyrophosphate secondary particles 100 is too low, the porosity of the sodium iron pyrophosphate secondary particles 100 will be too low. Although this can increase the compaction density of the positive electrode active layer, thereby increasing the overall capacity and volumetric energy density of the sodium battery, it will reduce the wettability of the electrolyte to the positive electrode active layer, thus reducing the specific capacity of the sodium battery and lowering its rate performance and low-temperature performance. Conversely, if the total area of ​​the plurality of pores 20 on the cross-section of the sodium iron pyrophosphate secondary particles 100 is too high, although this is beneficial for electrolyte wetting, shortening the solid-phase transport distance, and improving the capacity of the sodium battery, thus increasing its rate performance and low-temperature performance, the high porosity of the sodium iron pyrophosphate secondary particles 100 will reduce the compaction density of the positive electrode active layer, thereby reducing the overall capacity and volumetric energy density of the sodium battery. When the ratio of the total area of ​​the plurality of pores 20 to the surface area of ​​the sodium iron pyrophosphate secondary particles 100 is in the range of 4% to 8%, the electrolyte can have better wettability to the positive electrode active layer, the sodium battery has higher rate performance and low temperature performance, and the positive electrode sheet can have higher compaction density, the sodium battery has higher capacity and higher volumetric energy density.

[0073] The total area ratio of the plurality of pores 20 on the cross-section of the sodium ferric pyrophosphate secondary particles 100 described in this application can be measured using a scanning electron microscope (SEM). Specifically, an SEM image of the cross-section of the sodium ferric pyrophosphate secondary particles 100 is obtained using a scanning electron microscope. The magnification can be 50k, and five images are taken for each sample. The SEM images are then blacked out, and the contrast is increased to 100% and the brightness to 50% to obtain an image with distinct black and white areas. The percentage of the black area relative to the total image area is calculated by computer to obtain the total area ratio of the plurality of pores 20 on the cross-section of the sodium ferric pyrophosphate secondary particles 100. The area ratio of the plurality of pores 20 for each sample is the average of the five images. The SEM image processing flow is as follows: Figure 2 As shown, the left image has more holes 20, while the right image has fewer holes 20.

[0074] The pore parameter I of hole 20 is measured and calculated based on the average distance between hole 20 (the black dot on the processed SEM image) and the n nearest neighboring holes 20 on the SEM image of the cross-section of sodium iron pyrophosphate secondary particles 100.

[0075] In some embodiments, the compacted density of the sodium ferric pyrophosphate secondary particles 100 ranges from 2.16 g / cm³. 3 Up to 2.38 g / cm 3 .

[0076] Specifically, the compacted density of the sodium iron pyrophosphate secondary particles 100 can be, but is not limited to, 2.16 g / cm³. 3 2.18 g / cm 3 2.20g / cm 3 2.22 g / cm 3 2.24 g / cm 3 2.26 g / cm 3 2.28g / cm 3 2.30g / cm 3 2.32 g / cm 3 2.34 g / cm 3 2.36 g / cm 3 2.38g / cm 3 wait.

[0077] In this embodiment, the powder compaction density of the sodium iron pyrophosphate secondary particles 100 is determined by the porosity and pore parameters of the sodium iron pyrophosphate secondary particles 100. If the powder compaction density of the sodium iron pyrophosphate secondary particles 100 is too low, it indicates that the pore connectivity, pore proportion, and porosity of the sodium iron pyrophosphate secondary particles 100 are high, which will lead to a low compaction density of the positive electrode active layer of the positive electrode sheet and a low energy density of the sodium battery. If the powder compaction density of the sodium iron pyrophosphate secondary particles 100 is too high, it indicates that the pore connectivity, pore proportion, and porosity of the sodium iron pyrophosphate secondary particles 100 are low, which will reduce the wettability of the electrolyte to the positive electrode active layer, hindering the shortening of the solid-phase transport distance, reducing the specific capacity of the sodium battery, and lowering the rate performance and low-temperature performance of the sodium battery.

[0078] The test method for the powder compaction density of sodium iron pyrophosphate secondary particles 100 is as follows: take 2g to 3g of Na4Fe3(PO4)2(P2O7) powder and add it into the mold of a powder resistance instrument with a diameter of 13mm. After applying pressure to 3T, measure its volume and mass, and calculate the powder compaction density of sodium iron pyrophosphate secondary particles 100.

[0079] In some embodiments, the average particle size D of the primary sodium ferric pyrophosphate particles 10 ranges from 310 nm to 720 nm; the D50 of the secondary sodium ferric pyrophosphate particles 100 ranges from 3.4 μm to 8.6 μm, where D50 is the particle size value corresponding to when the cumulative volume distribution of the secondary sodium ferric pyrophosphate particles 100 reaches 50%.

[0080] Specifically, the average particle size D of the sodium iron pyrophosphate primary particles 10 can be, but is not limited to, 310 nm, 330 nm, 350 nm, 380 nm, 400 nm, 430 nm, 450 nm, 480 nm, 500 nm, 530 nm, 550 nm, 580 nm, 600 nm, 630 nm, 650 nm, 680 nm, 700 nm, 720 nm, etc. The particle size (size) of the sodium iron pyrophosphate primary particles 10 is directly related to the length of the sodium ion diffusion path. If the average particle size D of the sodium iron pyrophosphate primary particles 10 is too small, the proportion of surface reaction of the positive electrode active material will increase, leading to an increase in the proportion of side reactions and surface reactions of the positive electrode active material, resulting in a decrease in the initial efficiency of the sodium battery and a deterioration in cycle performance. If the average particle size D of the sodium iron pyrophosphate primary particles 10 is too large, the sodium ion diffusion path process will lead to a deterioration in the rate performance and low-temperature performance of the positive electrode active material.

[0081] Specifically, the D50 of the sodium iron pyrophosphate secondary particles 100 can be, but is not limited to, 3.4 μm, 3.6 μm, 3.8 μm, 4.0 μm, 4.5 μm, 5.0 μm, 5.5 μm, 6.0 μm, 6.5 μm, 7.0 μm, 7.5 μm, 8.0 μm, and 8.6 μm. Without considering the influence of porosity, the sodium iron pyrophosphate secondary particles 100 mainly affect the processing performance of the positive electrode sheet. When the D50 of the sodium iron pyrophosphate secondary particles 100 is too large, the surface of the positive electrode slurry coated with the sodium iron pyrophosphate secondary particles 100 will be uneven and pitted. After rolling, this will damage the foil of the specific positive electrode current collector, resulting in a thinner thickness and a greater risk of damage during winding. When the D50 of the sodium iron pyrophosphate secondary particles 100 is too small, the viscosity of the positive electrode slurry under the same formulation will increase, and the fluidity will decrease, leading to difficulties in coating.

[0082] It should be noted that the average particle size D of the sodium ferric pyrophosphate primary particles 10 is measured in the following way: an X-ray diffraction pattern (XRD pattern) of the sodium ferric pyrophosphate secondary particles 100 is taken, and the XRD pattern is processed and fitted using Jade software to obtain the average particle size (average volume particle size) of the sodium ferric pyrophosphate primary particles 10; the specific operation is as follows: the XRD pattern is subjected to background removal and single smoothing, and then matched with the sodium ferric pyrophosphate (NFPP) standard card for fitting. After fitting, a report on the grain size can be output, thereby obtaining the size value of the sodium ferric pyrophosphate primary particles 10.

[0083] It should be noted that the D50 of the sodium ferric pyrophosphate secondary particles 100 is measured in the following way: 1g of sodium ferric pyrophosphate secondary particles 100 is dispersed in 200ml of water and ultrasonically treated for 5 minutes. Then, the dispersion is added to a Malvern laser particle size analyzer for particle size testing, and a report is output to obtain the D50 value of the sodium ferric pyrophosphate secondary particles 100. The average value of three tests is taken as the D50 range of the material.

[0084] Please see Figure 3 In some embodiments, the primary sodium ferric pyrophosphate particles 10 include a sodium ferric pyrophosphate core 11 and a carbon layer 12, the carbon layer 12 being disposed on the surface of the sodium ferric pyrophosphate core 11, and the mass fraction of the carbon layer 12 in the secondary sodium ferric pyrophosphate particles 100 ranges from 1.4% to 2.2%.

[0085] It should be noted that the sodium ferric pyrophosphate core 11 and carbon layer 12 form a core-shell structure, that is, the sodium ferric pyrophosphate core 11 is the core and the carbon layer 12 is the outer shell. In other words, the primary sodium ferric pyrophosphate particle 10 has a core-shell structure.

[0086] Specifically, the mass fraction of the carbon layer 12 in the sodium iron pyrophosphate secondary particles 100 can be, but is not limited to, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, etc.

[0087] In this embodiment, the mass fraction w of carbon layer 12 in the sodium iron pyrophosphate secondary particles 100 is too low, indicating that the amount of carbon source added during the synthesis of sodium iron pyrophosphate secondary particles 100 is too small. During the formation of carbon layer 12, the carbon source undergoes thermal decomposition, producing too few gaseous products, resulting in too low porosity of sodium iron pyrophosphate secondary particles 100. This reduces the electrolyte wettability and kinetic performance of sodium iron pyrophosphate secondary particles 100, thereby deteriorating the capacity utilization, rate performance, and low-temperature performance of the sodium battery. The high mass fraction w of carbon layer 12 in the sodium iron pyrophosphate secondary particles 100 indicates that too much carbon source was added during the synthesis of the sodium iron pyrophosphate secondary particles 100. During the formation of carbon layer 12, the carbon source undergoes thermal decomposition, producing too many gaseous products, resulting in excessive porosity of the sodium iron pyrophosphate secondary particles 100. This reduces the compaction density of the positive electrode sheet. In addition, the carbon layer 12 itself is inactive, reducing the proportion of active material in the sodium iron pyrophosphate secondary particles 100 and decreasing the capacity and volumetric energy density of the sodium battery.

[0088] Furthermore, the mass fraction of the carbon layer 12 in the sodium iron pyrophosphate secondary particles 100 ranges from 1.6% to 2.0%. This allows the sodium iron pyrophosphate secondary particles 100 to have suitable porosity, which enables the electrolyte to have better wettability to the positive electrode active layer, resulting in higher rate performance and low-temperature performance of the sodium battery. It also allows the positive electrode sheet to have higher compaction density, resulting in higher capacity and higher volumetric energy density of the sodium battery.

[0089] In some embodiments, the specific surface area of ​​the sodium iron pyrophosphate secondary particles 100 ranges from 10 m². 2 / g to 18m 2 / g.

[0090] Specifically, the specific surface area of ​​the sodium iron pyrophosphate secondary particles 100 can be, but is not limited to, 10 m². 2 / g, 10.5m 2 / g、11m 2 / g, 11.5m 2 / g、12m 2 / g, 12.5m 2 / g、13m 2 / g, 13.5m 2 / g、14m2 / g, 14.5m 2 / g, 15m 2 / g, 15.5m 2 / g, 16m 2 / g, 16.5m 2 / g、17m 2 / g, 17.5m 2 / g、18m 2 / g etc.

[0091] In this embodiment, if the specific surface area of ​​the sodium iron pyrophosphate secondary particles 100 is too small, the pores 20 of the sodium iron pyrophosphate secondary particles 100 will be too small, which is not conducive to electrolyte wetting and reduces the rate performance of the sodium battery. If the specific surface area of ​​the sodium iron pyrophosphate secondary particles 100 is too large, the pores 20 of the sodium iron pyrophosphate secondary particles 100 will be too numerous. Under the same formulation conditions, the flowability of the positive electrode slurry will be worse, the processing of the positive electrode sheet will be worse, and it will be easy to cause uneven film surface and material shedding, etc. In addition, the number of side reaction sites increases, which reduces the cycle performance of the sodium battery, but increases the wettability of the electrolyte, thus improving the rate performance of the sodium battery.

[0092] In some embodiments, the sodium iron pyrophosphate secondary particles 100 are prepared by: mixing a sodium source, a phosphorus source, an iron source, a first pore-forming agent, and a second pore-forming agent and then performing a first sintering to form pores, thereby obtaining an intermediate product; and mixing the intermediate product with a carbon source and performing a second sintering to obtain the sodium iron pyrophosphate secondary particles 100; wherein the first pore-forming agent has a linear structure, and the second pore-forming agent is granular.

[0093] In the preparation of the sodium iron pyrophosphate secondary particles 100 described in this embodiment, the sodium source, phosphorus source, iron source, first pore-forming agent, and second pore-forming agent are first mixed and then subjected to a first sintering to create pores, obtaining an intermediate product. The intermediate product is then mixed with a carbon source and subjected to a second sintering to obtain the sodium iron pyrophosphate secondary particles 100. During the first sintering, the second pore-forming agent melts and fully integrates with the first precursor powder, undergoing a reduction reaction and generating a uniform carbon layer 12. Furthermore, the liquid phase state of the second pore-forming agent at high temperatures also facilitates the uniform integration of the first pore-forming agent and the first precursor powder, thus contributing more uniform one-dimensional pores 20. Through the combination of the first and second pore-forming agents, both pore creation and carbon coating can be effectively achieved. Moreover, the first pore-forming agent has a linear structure (i.e., a one-dimensional structure), while the second pore-forming agent has a particulate structure. The granular second pore-forming agent can form spherical or near-spherical pores 20, while the linear first pore-forming agent can connect multiple pores 20 to form channels. This results in high connectivity of the multiple pores 20 in the prepared sodium iron pyrophosphate secondary particles 100, which can significantly shorten the migration path of sodium ions, thus benefiting the capacity utilization of sodium batteries and improving their rate performance and low-temperature performance. Furthermore, by designing the ratio of the first and second pore-forming agents, the sodium iron pyrophosphate secondary particles 100 can have suitable pore parameters, resulting in high compaction density and thus enabling sodium batteries to have high capacity and volumetric energy density.

[0094] The sodium ferric pyrophosphate secondary particles 100 of this application embodiment can be prepared by the methods described in the following embodiments of this application. In addition, they can also be prepared by other methods. The preparation methods of this application embodiment are merely one or more preparation methods of the sodium ferric pyrophosphate secondary particles 100 of this application, and should not be construed as limiting the sodium ferric pyrophosphate secondary particles 100 provided in the embodiments of this application.

[0095] Please see Figure 4 This application also provides a method for preparing sodium ferric pyrophosphate secondary particles 100, which includes:

[0096] S201, a first precursor powder is obtained by mixing a sodium source, a phosphorus source, an iron source and a pore-forming agent;

[0097] S202, the first precursor powder is subjected to a first sintering to create pores, and an intermediate product is obtained;

[0098] S203, mixing the intermediate product with a carbon source to obtain a second precursor powder; and

[0099] S204, the second precursor powder is subjected to a second sintering to obtain sodium iron pyrophosphate secondary particles 100.

[0100] The sodium ferric pyrophosphate secondary particles 100 include a plurality of sodium ferric pyrophosphate primary particles 10. The cross-section of the sodium ferric pyrophosphate secondary particles 100 includes a plurality of pores 20. The average distance between each pore 20 and its n nearest neighbors is a pore parameter I. The average value of the pore parameter I of the plurality of pores 20 ranges from 30 nm to 160 nm, 2 ≤ n ≤ 6, and n is an integer.

[0101] For detailed descriptions of secondary sodium iron pyrophosphate particles 100, primary sodium iron pyrophosphate particles 10, pores 20, pore parameters, and other aspects, please refer to the descriptions in the corresponding sections of the above embodiments, which will not be repeated here.

[0102] The method for preparing sodium iron pyrophosphate secondary particles 100 according to the embodiments of this application involves first sintering a first precursor powder to create pores and obtain an intermediate product, and then mixing the intermediate product with a carbon source and performing a second sintering. By creating pores first and then forming a carbon layer 12, the prepared sodium iron pyrophosphate secondary particles 100 have a large number of interconnected pores 20 or channels with pore parameters between 30 nm and 160 nm. This results in higher powder compaction density, higher volumetric energy density, and better electrolyte wettability, which helps to shorten the solid-phase transport distance, facilitates the capacity utilization of the sodium iron pyrophosphate secondary particles 100, and improves the rate performance and low-temperature performance of sodium batteries.

[0103] 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.

[0104] Optionally, the phosphorus source may include, but is not limited to, at least one of these compounds, such as sodium dihydrogen phosphate, sodium pyrophosphate, ammonium dihydrogen phosphate, and those containing water of crystallization.

[0105] 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. An example of an iron source containing water of crystallization is ferrous oxalate dihydrate.

[0106] In some embodiments, the pore-forming agent includes a first pore-forming agent and a second pore-forming agent, wherein the first pore-forming agent has a linear structure and the second pore-forming agent has a particulate structure.

[0107] Using only the first pore-forming agent for pore formation is insufficient to achieve the desired carbon coating effect on the first precursor powder, and the use of Fe... 3+ When the raw material is reduced, it is difficult to achieve an effective pore-forming effect when only the second pore-forming agent is used for pore-forming.

[0108] In this embodiment, during the first sintering, the second pore-forming agent melts and fully fuses with the first precursor powder to undergo a reduction reaction, generating a uniform carbon layer 12. Furthermore, the liquid state of the second pore-forming agent at high temperature also facilitates the uniform fusion of the first pore-forming agent and the first precursor powder, thus contributing more uniform one-dimensional pores 20. Through the combination of the first and second pore-forming agents, both pore formation and carbon coating can be effectively achieved. Moreover, the first pore-forming agent has a linear structure (i.e., a one-dimensional structure), while the second pore-forming agent has a particulate structure. The granular second pore-forming agent can form spherical or near-spherical pores 20, while the linear first pore-forming agent can connect multiple pores 20 to form channels. This results in high connectivity of the multiple pores 20 in the prepared sodium iron pyrophosphate secondary particles 100, which can significantly shorten the migration path of sodium ions, thus benefiting the capacity utilization of sodium batteries and improving their rate performance and low-temperature performance. Furthermore, by designing the ratio of the first and second pore-forming agents, the sodium iron pyrophosphate secondary particles 100 can have suitable pore parameters, resulting in high compaction density and thus enabling sodium batteries to have high capacity and volumetric energy density.

[0109] In some embodiments, the first pore-forming agent is an inorganic carbon source, the second pore-forming agent is an organic carbon source, and the mass ratio of the first pore-forming agent to the second pore-forming agent ranges from 0.25 to 0.5.

[0110] Specifically, the mass ratio of the first pore-forming agent to the second pore-forming agent can be, but is not limited to, 0.25, 0.26, 0.28, 0.30, 0.32, 0.34, 0.36, 0.38, 0.40, 0.42, 0.44, 0.46, 0.48, 0.5, etc.

[0111] In this embodiment, if the mass ratio of the first pore-forming agent to the second pore-forming agent is too low, the content of the first pore-forming agent with a linear structure will be too small, which will reduce the degree of pore formation and the number of pores 20 in the obtained sodium iron pyrophosphate secondary particles 100. This is not conducive to improving the specific capacity of the sodium iron pyrophosphate secondary particles 100, nor is it conducive to improving the rate performance and low-temperature performance of the battery. If the mass ratio of the first pore-forming agent to the second pore-forming agent is too high, the content of the first pore-forming agent will be too large, resulting in excessive pore formation. This will reduce the compaction density of the obtained sodium iron pyrophosphate secondary particles 100, making the specific surface area of ​​the sodium iron pyrophosphate secondary particles 100 too large, thus reducing the energy density and cycle performance of the battery.

[0112] In some embodiments, the amount of the second pore-forming agent added is 0.12 to 0.16 times the mass of iron (Fe) in the iron source.

[0113] Understandably, the mass ratio of the second pore-forming agent to the iron element in the iron source ranges from 0.12 to 0.16.

[0114] Specifically, the amount of the second pore-forming agent added can be, but is not limited to, 0.12 times, 0.125 times, 0.13 times, 0.135 times, 0.14 times, 0.145 times, 0.15 times, 0.155 times, 0.16 times the mass of iron in the iron source.

[0115] For example, if the iron source is ferrous oxalate dihydrate and the amount of ferrous oxalate dihydrate added is 340g, of which the mass of Fe element is 108g, then the amount of the second pore-forming agent added should be 13.0g to 17.3g.

[0116] In this embodiment, if the amount of the second pore-forming agent added is too small, there will be a lack of a sufficient high-temperature liquid phase environment between the sodium source, iron source, phosphorus source, and the first pore-forming agent during pore formation. This will lead to an increase in impurities and uneven formation of one-dimensional pores 20, reducing the specific weight and compaction density of the obtained sodium iron pyrophosphate secondary particles 100. If the amount of the second pore-forming agent added is too large, the proportion of pores 20 formed by the second pore-forming agent will be too high, resulting in excessively large pores 20 inside the sodium iron pyrophosphate secondary particles 100, thus reducing the compaction density of the sodium iron pyrophosphate secondary particles 100.

[0117] In some embodiments, the first pore-forming agent comprises carbon nanotubes; the second pore-forming agent comprises at least one of glucose and sucrose.

[0118] In this embodiment, by using carbon nanotubes in combination with at least one of glucose and sucrose, during the first sintering process, at least one of glucose and sucrose melts and fully fuses with the first precursor powder, undergoing a reduction reaction and generating a uniform carbon layer 12. Furthermore, the liquid state of at least one of glucose and sucrose at high temperatures also facilitates the uniform fusion of carbon nanotubes and the first precursor powder, thus contributing more uniform one-dimensional pores 20. Through the combination of carbon nanotubes with at least one of glucose and sucrose, both pore formation and carbon coating can be effectively achieved. Moreover, the carbon nanotubes have a linear structure (i.e., a one-dimensional structure), while at least one of glucose and sucrose has a particulate structure. At least one of granular glucose and sucrose can form spherical or near-spherical pores 20. Linear carbon nanotubes can connect multiple pores 20 to form channels, thereby giving the prepared sodium iron pyrophosphate secondary particles 100 a high degree of connectivity among the multiple pores 20. This can greatly shorten the migration path of sodium ions, which is beneficial to the capacity utilization of sodium batteries and improves the rate performance and low-temperature performance of sodium batteries. In addition, by designing the ratio of carbon nanotubes and at least one of glucose and sucrose, the sodium iron pyrophosphate secondary particles 100 can have suitable pore parameters, thereby having a high compaction density, which enables sodium batteries to have high capacity and volumetric energy density.

[0119] Please see Figure 5 In some embodiments, in step S201, a sodium source, a phosphorus source, an iron source, and a pore-forming agent are mixed to obtain a first precursor powder; including:

[0120] S2011, wherein the sodium source, phosphorus source, iron source and pore-forming agent are mixed in a solvent to obtain a slurry; and

[0121] Optionally, the sodium source, phosphorus source, iron source and pore-forming agent are mixed in water and then subjected to sand milling to obtain a slurry. The sand milling time is 0.5 h to 4 h and the sand milling speed is 1000 rpm to 4000 rpm.

[0122] Understandably, the solvent may be, but is not limited to, water.

[0123] In this embodiment, before spray drying, the raw materials (i.e., sodium source, phosphorus source, iron source and pore-forming agent) 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.

[0124] 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 source, phosphorus source, iron source, and pore-forming agent will be too large, resulting in localized growth and crystallization during the subsequent sintering process, thus reducing the sphericity of the obtained sodium ferric pyrophosphate secondary particles 100. If the milling time is too long, it will reduce production efficiency.

[0125] 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 source, phosphorus source, iron source and pore-forming agent will be too large, resulting in localized protruding crystal growth during the subsequent sintering process, which reduces the sphericity of the obtained sodium iron pyrophosphate secondary particles 100. If the milling speed is too high, the slurry is prone to splashing.

[0126] 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 sodium iron pyrophosphate secondary particles 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 first precursor powder, which in turn leads to a reduction in the sphericity of the final sodium iron pyrophosphate secondary particles 100.

[0127] S2012, the slurry is spray-dried to obtain the first precursor powder.

[0128] Optionally, the slurry is spray-dried at a temperature of 102°C to 120°C to obtain a first precursor powder.

[0129] 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.

[0130] Alternatively, spray drying can be carried out in a spray dryer.

[0131] 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 first 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 sodium iron pyrophosphate granules 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 first precursor powder. This reduces the sphericity of the formed first precursor powder.

[0132] In some embodiments, in S202, the first precursor powder is subjected to a first sintering to create pores and obtain an intermediate product. This includes placing the first precursor powder in an oxygen atmosphere and performing a first sintering at a temperature of 400°C to 500°C to obtain the intermediate product. In this embodiment, the first precursor powder is subjected to a first sintering in an oxygen atmosphere. During the first sintering, the first pore-forming agent and the second pore-forming agent decompose, thereby forming pores 20 on the intermediate product. The second pore-forming agent forms larger spherical or near-spherical pores 20, while the first pore-forming agent forms elongated channels or pores 20, connecting adjacent pores 20, thereby improving the connectivity of the pores 20, enhancing the specific capacity of the sodium iron pyrophosphate secondary particles 100, and improving the rate performance and low-temperature performance of the battery.

[0133] It should be noted that an oxygen atmosphere can be a pure oxygen atmosphere, an air atmosphere, or an atmosphere in which oxygen is mixed with other gases (such as inert gases, such as nitrogen).

[0134] Optionally, the first sintering can be carried out in a muffle furnace.

[0135] Specifically, the first sintering temperature can be, but is not limited to, 400℃, 410℃, 420℃, 430℃, 440℃, 450℃, 460℃, 470℃, 480℃, 490℃, 500℃, etc. In this embodiment, if the first sintering temperature is too low, the etching of the first and second pore-forming agents will be incomplete, the formation of pores 20 in the sodium iron pyrophosphate secondary particles 100 will be insufficient, and the residual first and second pore-forming agents will reduce the specific capacity of the sodium iron pyrophosphate secondary particles 100. If the first sintering temperature is too high, the pores 20 formed after etching by the first and second pore-forming agents will reclose under the action of high-temperature sintering, reducing the porosity of the obtained sodium iron pyrophosphate secondary particles 100.

[0136] Optionally, the first sintering time can range from 2 hours to 4 hours. Specifically, the first sintering time can be, but is not limited to, 2 hours, 2.5 hours, 3 hours, 3.5 hours, and 4 hours. If the first sintering time is too short, the etching of the first and second pore-forming agents will be incomplete, resulting in insufficient formation of pores 20 within the sodium iron pyrophosphate secondary particles 100. The residual first and second pore-forming agents will reduce the specific capacity of the sodium iron pyrophosphate secondary particles 100. If the first sintering time is too long, the pores 20 formed after etching by the first and second pore-forming agents will reclose under the action of high-temperature sintering, reducing the porosity of the obtained sodium iron pyrophosphate secondary particles 100.

[0137] Optionally, in S203, mixing the intermediate product with a carbon source to obtain a second precursor powder includes: dispersing the intermediate product and the carbon source in water and spray drying them at a temperature of 102°C to 120°C (e.g., 102°C, 103°C, 104°C, 105°C, 106°C, 107°C, 108°C, 110°C, 112°C, 114°C, 116°C, 118°C, 119°C, 120°C, etc.) to obtain the second precursor powder.

[0138] Alternatively, spray drying can be carried out in a spray dryer.

[0139] Optionally, the carbon source can be an organic carbon source. The carbon source includes at least one of glucose and sucrose.

[0140] Optionally, the mass of the carbon source is 0.04 to 0.08 times the mass of the intermediate product. Specifically, the mass of the carbon source can be, but is not limited to, 0.04, 0.05, 0.06, 0.07, or 0.08 times the mass of the intermediate product. In this embodiment, if the mass of the carbon source is too small, the electronic conductivity of the resulting sodium iron pyrophosphate secondary particles 100 will be poor, affecting the specific capacity of the battery and reducing its rate performance. If the mass of the carbon source is too large, the carbon layer 12 of the sodium iron pyrophosphate secondary particles 100 will be too thick, reducing the specific capacity of the sodium iron pyrophosphate secondary particles 100.

[0141] In some embodiments, in S204, the second precursor powder is subjected to a second sintering to obtain sodium iron pyrophosphate secondary particles 100, comprising:

[0142] The second precursor powder was placed in an inert atmosphere and sintered at a temperature of 510°C to 620°C. After furnace cooling, 100 secondary granules of sodium iron pyrophosphate were obtained.

[0143] Optionally, the second sintering is carried out under the protection of an inert atmosphere. Optionally, the inert atmosphere includes at least one of helium, nitrogen, and argon.

[0144] Alternatively, the second sintering can be carried out in a tube furnace.

[0145] Specifically, the temperature for the second sintering of the second precursor powder can be, but is not limited to, 510℃, 520℃, 530℃, 540℃, 550℃, 560℃, 570℃, 575℃, 580℃, 585℃, 590℃, 595℃, 600℃, 605℃, 610℃, 615℃, 620℃, etc. If the temperature for the second sintering of the second precursor powder is too low, the carbonization degree of the carbon layer 12 in the resulting sodium iron pyrophosphate secondary particles 100 will be too low, and the resistivity of the sodium iron pyrophosphate secondary particles 100 will be too high. When applied to sodium batteries, this will result in excessive internal resistance and reduce the cycle life of the sodium batteries. If the temperature for the second sintering of the second precursor powder is too high, the sodium iron pyrophosphate will easily decompose at high temperatures to generate sodium iron phosphate impurities and sodium iron pyrophosphate impurities, thereby reducing the specific capacity of the sodium iron pyrophosphate secondary particles 100.

[0146] Furthermore, the second sintering temperature of the second precursor powder is in the range of 530°C to 570°C. This allows the carbon layer 12 of the prepared sodium iron pyrophosphate secondary particles 100 to have a higher electron transport rate, and the sodium iron pyrophosphate secondary particles 100 to have a lower powder resistivity. At the same time, it also allows for a lower content of sodium iron phosphate impurities and sodium iron pyrophosphate impurities in the prepared sodium iron pyrophosphate secondary particles 100, thereby achieving a higher specific capacity.

[0147] Optionally, the second sintering time for the second precursor powder can be from 2 hours to 48 hours. Specifically, the second sintering time for the second 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 second sintering time for the second precursor powder is too short, the carbonization degree of the carbon layer 12 of the obtained sodium iron pyrophosphate secondary particles 100 will be too low, increasing the powder resistivity of the sodium iron pyrophosphate secondary particles 100. If the second sintering time for the second 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 sodium iron pyrophosphate secondary particles 100, and reducing the specific capacity of the sodium iron pyrophosphate secondary particles 100.

[0148] Please see Figure 6 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 sodium iron pyrophosphate secondary particles 100 as described in this application embodiment.

[0149] Optionally, the positive current collector 210 can be, but is not limited to, aluminum foil or aluminum sheet.

[0150] Optionally, the positive electrode active layer 220 may also include a positive electrode conductive agent and a positive electrode binder.

[0151] 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.

[0152] 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.

[0153] Please see Figure 7 and Figure 8 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.

[0154] 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.

[0155] Optionally, the sodium battery 300 may be, but is not limited to, at least one of cylindrical, prismatic, and blade 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 secondary sodium iron pyrophosphate particles 100 of the embodiments of this application.

[0156] 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.

[0157] 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.

[0158] Optionally, the electrolyte includes an electrolyte salt, an organic solvent, and a film-forming additive.

[0159] 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.

[0160] 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.

[0161] Optionally, the film-forming additive may include, but is not limited to, at least one of propargylbenzenesulfonic acid, vinylene carbonate (VC), fluoroethylene carbonate (FEC), ethylene sulfate (DTD), methylene disulfonate (MMDS), butyl sulfonate lactone (BS), and 1,3-propenyl sulfonate lactone (PST).

[0162] 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.

[0163] Optionally, the compaction density of the positive electrode active layer 220 (i.e., the compaction density at the electrode layer level) ranges from 2.24 g / cm³. 3 Up to 2.48 g / cm 3 Specifically, it can be, but is not limited to, 2.24 g / cm³. 3 2.25g / cm 3 2.30g / cm 32.35g / cm 3 2.4g / cm 3 2.45g / cm 3 2.48 g / cm 3 wait.

[0164] 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 200 made of sodium iron pyrophosphate secondary particles 100 is measured, and the positive electrode 200 is cut into 12mm round pieces and weighed. The mass and volume of the sodium iron pyrophosphate secondary particles 100 on the positive electrode 200 are calculated, and then the compaction density of the sodium iron pyrophosphate secondary particles 100 on the positive electrode 200 is calculated.

[0165] Please see Figure 9 Optionally, the negative electrode 330 includes a negative current collector 331 and a negative active layer 332.

[0166] Optionally, the negative current collector 331 can be, but is not limited to, at least one of copper foil, copper sheet, aluminum foil, and aluminum sheet.

[0167] Optionally, 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.

[0168] Optionally, the negative electrode active material can be, but is not limited to, hard carbon.

[0169] 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.

[0170] 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.

[0171] Optionally, the negative electrode thickener may be, but is not limited to, at least one of polyacrylamide (PAM) and polymethyl methacrylate (PMA).

[0172] 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 is electrically connected to both the positive electrode 200 and the negative electrode 330, leading them out for electrical connection to external devices or other sodium batteries 300.

[0173] The following specific embodiments further describe the sodium ferric pyrophosphate secondary particles 100 of this application.

[0174] Examples 1 to 10, Comparative Examples 1 to 6

[0175] The sodium ferric pyrophosphate secondary particles 100 of each embodiment and comparative example were prepared by the following steps:

[0176] (1) 178 g of sodium pyrophosphate (sodium source, phosphorus source), 150 g of ammonium dihydrogen phosphate (phosphorus source), 340 g of ferrous oxalate dihydrate (iron source, 1.89 mol), carbon nanotubes (first pore-forming agent), glucose (second pore-forming agent) and 2.5 kg of water (solvent) were stirred evenly and milled at 2000 rpm for 2 hours to obtain a slurry; the amount of first pore-forming agent and second pore-forming agent added in each embodiment and comparative example is shown in Table 1 below.

[0177] (2) The slurry was spray-dried at 105°C to obtain the first precursor powder;

[0178] (3) The first precursor powder was placed in a muffle furnace and sintered at 450°C for 3 hours in an air atmosphere to obtain an intermediate product.

[0179] (4) 200g of intermediate product and 10g of glucose (carbon source) were dispersed in water and spray-dried at 105°C to obtain the second precursor powder. The amount of carbon source added in each example and comparative example is shown in Table 1 below; and

[0180] (5) The second precursor powder was placed in a tube furnace and sintered at 600°C for 12 hours under nitrogen protection. After cooling, 100 secondary granules of sodium iron pyrophosphate were obtained.

[0181] Assembly of sodium batteries 300 in each embodiment and comparative example: (1) Preparation of positive electrode 200: Sodium iron pyrophosphate secondary particles 100 (positive electrode active material), acetylene black (positive electrode conductive agent), and polyvinylidene fluoride (positive electrode binder) are dispersed in N-methylpyrrolidone (NMP) solvent at a mass ratio of 97:1:2 and mixed evenly to obtain positive electrode slurry; the positive electrode slurry is coated on aluminum foil of positive electrode current collector 210, and after drying, cold pressing, slitting, and cutting, (2) Preparation of negative electrode 330: Hard carbon (negative electrode active material), acetylene black (negative electrode conductive agent), styrene-butadiene rubber, and sodium carboxymethyl cellulose are dispersed in deionized water at a mass ratio of 95:2:2:1 and mixed evenly to obtain a negative electrode slurry. The negative electrode slurry is coated on the copper foil of the negative electrode current collector 331. After drying, cold pressing, slitting, and cutting, negative electrode 330 is obtained. (3) Electrolyte preparation process: When the water content is less than or equal to 10 p In an argon atmosphere glove box at pm, sodium hexafluorophosphate was dissolved in an organic solvent, and organic additives fluoroethylene carbonate (FEC) and sodium salt additive sodium difluorosulfonamide (NaFSI) were added to prepare an electrolyte with a sodium hexafluorophosphate mass fraction of 15.5%; wherein, the organic solvent included ethylene carbonate (EC) and diethyl carbonate (DEC) in a mass ratio of 1:1; (4) Preparation of diaphragm 320: a 16μm polyethylene porous film was used as diaphragm 320; (5 Assembly of sodium battery 300: The positive electrode 200, separator 320 and negative electrode 330 are stacked in sequence to form an electrode assembly. After the electrode assembly is wound, a bare cell is obtained. At the end of the bare cell (i.e. the end of the bare cell), the middle area, top area and bottom area of ​​the end area are respectively pasted with termination tape. Then the cell is put into a polypropylene plastic outer packaging and electrolyte is injected. After vacuum sealing, standing, formation and shaping processes, a soft pack sodium battery 300 is obtained.

[0182] The following performance tests were conducted on the sodium iron pyrophosphate secondary particles 100 and sodium battery 300 of each embodiment and comparative example.

[0183] (1) The total area ratio of the multiple pores 20 on the cross-section of the sodium ferric pyrophosphate secondary particles 100, and the number ratio of the first, second, and third pores: Scanning electron microscopy (SEM) images of the cross-section of the sodium ferric pyrophosphate secondary particles 100 were obtained using a scanning electron microscope (SEM). The magnification could be 50k, and five images were taken for each sample. The SEM images were blacked out, and then the contrast was increased to 100% and the brightness was increased to 50% to obtain images with distinct black and white areas. The area ratio of the multiple pores 20 on the cross-section of the sodium ferric pyrophosphate secondary particles 100 was calculated by computer based on the percentage of the black area to the total image area. The area ratio of the multiple pores 20 on the cross-section of each sample was the average of the five images. The number ratio of the first, second, and third pores was calculated based on the size of the black dots on the processed SEM images.

[0184] (2) Average value of pore parameter I: Calculate the average value of the distance between each hole 20 (black dot on the processed SEM image) and the four nearest holes 20 on the SEM image of the cross-section of sodium iron pyrophosphate secondary particles 100 (i.e., pore parameter), and then calculate the average value of pore parameter I of multiple holes 20.

[0185] (3) Powder compaction density: Take 2g to 3g of Na4Fe3(PO4)2(P2O7) powder and add it into the mold of a powder resistance instrument with a diameter of 13mm. After pressing to 3T, measure its volume and mass, and calculate the powder compaction density of sodium iron pyrophosphate secondary particles 100.

[0186] (4) Compacted density of positive electrode active layer 220: The thickness of aluminum foil and the mass of aluminum foil cut into 12mm round pieces are measured in advance. Then, the thickness of positive electrode 200 made of sodium iron pyrophosphate secondary particles 100 is measured, and the positive electrode 200 is cut into 12mm round pieces and weighed. The mass and volume of sodium iron pyrophosphate secondary particles 100 on positive electrode 200 are calculated, and then the compacted density of sodium iron pyrophosphate secondary particles 100 on positive electrode 200 is calculated.

[0187] (5) Specific capacity test of sodium iron pyrophosphate secondary particles 100: Sodium iron pyrophosphate secondary particles 100 were 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 300 coin cell sodium battery was measured using a Blue Electricity Tester. The battery was charged to 3.5V at a rate of 0.1C, and then charged to 1.5V at a rate of 0.1C. Constant current charge-discharge tests were then conducted with a current density of 12mA / g (the actual current value is the current density multiplied by the mass of the 100 sodium iron pyrophosphate secondary particles of the positive electrode 200). The upper and lower limits of the test voltage were 1.5V-3.5V. The measured specific capacity of the discharge battery is the specific capacity of the 100 sodium iron pyrophosphate secondary particles.

[0188] (6) Cycle capacity retention: The sodium battery 300 was subjected to charge-discharge cycle testing on a charge-discharge tester (Nebula Charge-Discharge Test System-BAT-NEEFLCT-05-V010) at a test temperature of 25℃. The sodium battery 300 was charged at a constant power of 1P to the charging cut-off voltage of 3.5V, and the initial charging capacity was recorded. The sodium battery 300 was then left to stand for 10 minutes and discharged at a constant power of 1P to the discharging cut-off voltage of 1.5V, and the discharge capacity was recorded. Here, P refers to the rated charging or discharging power of the sodium battery 300, which is the nominal voltage U of the sodium battery 300 multiplied by the current density of 1C. The nominal voltage of the sodium battery 300 is 2.82V, and 1P refers to 1 times the rated power.

[0189] The formula for calculating the capacity retention rate after the nth cycle is: Capacity retention rate after the nth cycle = (Discharge capacity after the nth cycle / Discharge capacity of the first cycle) × 100%.

[0190] The performance parameters of sodium iron pyrophosphate secondary particles 100 and sodium batteries 300 in each embodiment and comparative example are shown in Tables 1 and 2 below.

[0191] Table 1. Performance parameters of sodium iron pyrophosphate secondary particles 100 and sodium battery 300 in each embodiment and comparative example.

[0192]

[0193]

[0194] Table 2 Performance parameters of sodium iron pyrophosphate secondary particles 100 and sodium battery 300 in each embodiment and comparative example.

[0195]

[0196] The test results from Examples 1 to 5, Comparative Examples 1 and 2 show that as the amount of the second pore-forming agent (glucose) increases, the pore parameters of the prepared sodium ferric pyrophosphate secondary particles 100 gradually decrease, indicating that the spacing between adjacent pores in the sodium ferric pyrophosphate secondary particles 100 gradually decreases. Furthermore, as the amount of the second pore-forming agent (glucose) increases, the area ratio of the pores in the sodium ferric pyrophosphate secondary particles 100 gradually increases, while the proportion of the number of first pores gradually decreases, and the proportions of the number of second and third pores gradually increase. Moreover, as the amount of the second pore-forming agent (glucose) increases, the D50 of the sodium ferric pyrophosphate secondary particles 100 does not change significantly. The average particle size D of the sodium ferric pyrophosphate primary particles 10 generally shows a trend of first decreasing, then increasing, and finally decreasing again. The powder compaction density of the sodium ferric pyrophosphate secondary particles 100 gradually decreases, while the powder specific gravity density does not change significantly. As the amount of the second pore-forming agent (glucose) increases, the compaction density of the positive electrode active layer 220 gradually decreases, the specific capacity of the sodium iron pyrophosphate secondary particles 100 gradually increases, and the cycle capacity retention rate of the sodium battery 300 gradually decreases.

[0197] The test results from Examples 3, 6 to 8, Comparative Examples 3 and 4 show that as the amount of the first pore-forming agent (carbon nanotubes) increases, the pore parameters of the prepared sodium iron pyrophosphate secondary particles 100 gradually decrease, indicating that the spacing between adjacent pores in the sodium iron pyrophosphate secondary particles 100 gradually decreases. Furthermore, as the amount of the first pore-forming agent (carbon nanotubes) increases, the area ratio of the pores in the sodium iron pyrophosphate secondary particles 100 gradually increases, and the proportion of the number of first pores gradually increases, while the proportions of the number of second and third pores gradually decrease. Moreover, as the amount of the first pore-forming agent (carbon nanotubes) increases, the D50 of the sodium iron pyrophosphate secondary particles 100 gradually increases, the average particle size D of the sodium iron pyrophosphate primary particles 10 does not change significantly, and the powder compaction density and powder specific gravity density of the sodium iron pyrophosphate secondary particles 100 gradually decrease. As the amount of the first pore-forming agent (carbon nanotubes) increases, the compaction density of the positive electrode active layer 220 gradually decreases, the specific capacity of the sodium iron pyrophosphate secondary particles 100 gradually increases, and the cycle capacity retention rate of the sodium battery 300 gradually decreases.

[0198] The test results of Examples 7, 9 to 11, Comparative Example 5 and Comparative Example 6 show that in Comparative Example 5, only the first pore-forming agent (carbon nanotubes) was added. The resulting sodium iron pyrophosphate secondary particles 100 had smaller pore parameters, a larger area ratio of pores, a larger proportion of first pores, and the pores were mainly first pores. The number of second and third pores was smaller. The resulting sodium iron pyrophosphate secondary particles 100 and sodium iron pyrophosphate primary particles 10 were both larger. The compaction density and specific gravity density of the sodium iron pyrophosphate secondary particles 100 were both smaller. The compaction density of the positive electrode active layer 220 was lower. The specific capacity of the sodium iron pyrophosphate secondary particles 100 was higher. However, the cycle capacity retention rate of the sodium battery 300 was lower. In Comparative Example 6, only the second pore-forming agent (glucose) was added. The resulting sodium iron pyrophosphate secondary particles 100 had larger pore parameters, a smaller area ratio of pores, a smaller proportion of first pores, and a larger proportion of second and third pores. Both the resulting sodium iron pyrophosphate secondary particles 100 and sodium iron pyrophosphate primary particles 10 were smaller. The compaction density and specific gravity density of the sodium iron pyrophosphate secondary particles 100 were both larger, resulting in a higher compaction density of the positive electrode active layer 220. The specific capacity of the sodium iron pyrophosphate secondary particles 100 was lower, while the cycle capacity retention rate of the sodium battery 300 was higher. Compared to Comparative Examples 5 and 6, Examples 7, 9 to 11 simultaneously added a first pore-forming agent and a second pore-forming agent, resulting in sodium ferric pyrophosphate secondary particles 100 with more moderate pore parameters, moderate pore area ratio, and moderate D50 of sodium ferric pyrophosphate secondary particles 100 and average particle size D of sodium ferric pyrophosphate primary particles 100. The sodium ferric pyrophosphate secondary particles 100 have high powder compaction density and powder specific gravity density, as well as high specific capacity and high cycle capacity retention rate.

[0199] The test results from Examples 7, 9 to 11 show that, with the total amount of the first pore-forming agent and the second pore-forming agent remaining constant, as the mass ratio of the first pore-forming agent to the second pore-forming agent increases,

[0200] The pore parameters of the prepared sodium iron pyrophosphate secondary particles 100 gradually decrease, the area ratio of pores in the sodium iron pyrophosphate secondary particles 100 gradually decreases, the number of first pores shows an increasing trend, the number of second pores shows a decreasing trend, and the number ratio of third pores shows a decreasing trend. Furthermore, with the increase of the mass ratio of the first pore-forming agent to the second pore-forming agent, the D50 of the sodium iron pyrophosphate secondary particles 100 gradually increases, the average particle size D of the sodium iron pyrophosphate primary particles 10 gradually decreases, the powder compaction density of the sodium iron pyrophosphate secondary particles 100 gradually decreases, the powder specific gravity density gradually decreases, the compaction density of the positive electrode active layer 220 gradually decreases, the specific capacity of the sodium iron pyrophosphate secondary particles 100 first increases and then decreases, and the cycle capacity retention rate of the sodium battery 300 gradually decreases.

[0201] Please see Figure 10 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.

[0202] 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.

[0203] Optionally, the energy storage device 400 may include, but is not limited to, battery modules, battery packs, battery systems, energy storage boxes, energy storage cabinets, energy storage containers, etc. 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.

[0204] 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.

[0205] The term "multiple" refers to two or more.

[0206] 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.

[0207] 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.

[0208] Please see Figure 11 and Figure 12 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.

[0209] 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 sodium iron pyrophosphate secondary particles 100, etc., of this application.

[0210] 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.

[0211] 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.

[0212] 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.

[0213] 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.

[0214] 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.

[0215] 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.

[0216] 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 sodium iron phosphate pyrophosphate secondary particle characterized by, The sodium iron phosphate pyrophosphate secondary particles comprise a plurality of sodium iron phosphate pyrophosphate primary particles, a cross section of the sodium iron phosphate pyrophosphate secondary particles comprises a plurality of pores, an average value of a spacing between each of the pores and n most adjacent pores is a pore channel parameter I, an average value of the pore channel parameter I of the plurality of pores ranges from 30 nm to 160 nm, 2≤n≤6, and n is an integer.

2. The secondary particles of sodium iron (III) phosphate pyrophosphate according to claim 1, characterized in that The pores comprise first pores, second pores, and third pores, a pore size of the first pores is less than 30 nm, a pore size of the second pores is greater than or equal to 30 nm and less than or equal to 160 nm, and a pore size of the third pores is greater than 160 nm; a proportion of a number of the first pores in the pores of the sodium iron phosphate pyrophosphate secondary particles ranges from 25% to 47%, a proportion of a number of the second pores in the pores of the sodium iron phosphate pyrophosphate secondary particles ranges from 52% to 74.8%, and a proportion of a number of the third pores in the pores of the sodium iron phosphate pyrophosphate secondary particles ranges from 0.2% to 1%.

3. The secondary particles of sodium iron (III) phosphate pyrophosphate according to claim 1, characterized in that, A total area proportion of the plurality of pores on the cross section of the sodium iron phosphate pyrophosphate secondary particles ranges from 4% to 8%.

4. The secondary particles of sodium iron (III) phosphate pyrophosphate according to claim 1, characterized in that, The powder compaction density of the sodium iron phosphate pyrophosphate secondary particles ranges from 2.16 g / cm 3 to 2.38 g / cm 3 .

5. The secondary particles of sodium iron (III) phosphate pyrophosphate according to claim 1, characterized in that, An average particle size D of the sodium iron phosphate pyrophosphate primary particles ranges from 310 nm≤D≤720 nm; and a D50 of the sodium iron phosphate pyrophosphate secondary particles ranges from 3.4 μm≤D50≤8.6 μm, wherein the D50 is a particle size value corresponding to a cumulative volume distribution of 50% of the sodium iron phosphate pyrophosphate secondary particles.

6. The secondary particles of sodium iron (III) phosphate pyrophosphate according to claim 1, characterized in that, The specific surface area of the sodium iron phosphate pyrophosphate secondary particles ranges from 10 m 2 / g to 18 m 2 / g.

7. The secondary particles of sodium iron (III) phosphate pyrophosphate according to any one of claims 1 to 6, characterized in that The sodium iron phosphate pyrophosphate secondary particles are prepared by mixing a sodium source, a phosphorus source, an iron source, a first pore-forming agent, and a second pore-forming agent, and then performing first sintering to form pores to obtain an intermediate product; and mixing the intermediate product with a carbon source and then performing second sintering to obtain the sodium iron phosphate pyrophosphate secondary particles; wherein the first pore-forming agent is in a linear structure, and the second pore-forming agent is in a granular form.

8. A method for producing secondary particles of sodium iron phosphate pyrophosphate, characterized by, Comprising: mixing a sodium source, a phosphorus source, an iron source, and a pore-forming agent to obtain a first precursor powder; performing first sintering on the precursor powder to form pores to obtain an intermediate product; mixing the intermediate product with a carbon source to obtain a second precursor powder; and performing second sintering on the second precursor powder to obtain sodium iron phosphate pyrophosphate secondary particles; wherein the sodium iron phosphate pyrophosphate secondary particles comprise a plurality of sodium iron phosphate pyrophosphate primary particles, a cross section of the sodium iron phosphate pyrophosphate secondary particles comprises a plurality of pores, an average value of a spacing between each of the pores and n most adjacent pores is a pore channel parameter I, an average value of the pore channel parameter I of the plurality of pores ranges from 30 nm to 160 nm, 2≤n≤6, and n is an integer.

9. The method for producing secondary particles of sodium iron phosphate pyrophosphate according to claim 8, characterized by, performing first sintering on the precursor powder to form pores to obtain an intermediate product, comprising: placing the first precursor powder in an oxygen atmosphere and performing first sintering at a temperature ranging from 400°C to 500°C to obtain the intermediate product.

10. The method for preparing secondary particles of sodium iron phosphate pyrophosphate according to claim 8, characterized by, The pore-forming agent comprises a first pore-forming agent and a second pore-forming agent, the first pore-forming agent is in a linear structure, and the second pore-forming agent is in a granular form.

11. The method for producing secondary particles of sodium iron phosphate pyrophosphate according to claim 10, characterized by, The first pore forming agent is an inorganic carbon source, the second pore forming agent is an organic carbon source, and the mass ratio of the first pore forming agent to the second pore forming agent ranges from 0.25 to 0.

5.

12. The method for preparing secondary particles of sodium iron phosphate pyrophosphate according to claim 10, characterized by, The first pore forming agent comprises carbon nanotubes; the second pore forming agent comprises at least one of glucose and sucrose; and the carbon source comprises at least one of glucose and sucrose.

13. The process for the preparation of secondary particles of sodium iron (III) phosphate pyrophosphate according to any one of claims 8 to 12, characterized in that, The second precursor powder is subjected to second sintering to obtain sodium iron phosphate pyrophosphate secondary particles, comprising: The second precursor powder is subjected to second sintering in an inert atmosphere at a temperature ranging from 510 DEG C to 620 DEG C to obtain sodium iron phosphate pyrophosphate secondary particles.

14. A positive electrode sheet characterized by comprising: The positive electrode tab comprises: A positive electrode current collector; and A positive electrode active layer comprising the sodium iron phosphate pyrophosphate secondary particles of any one of claims 1-7 or the sodium iron phosphate pyrophosphate secondary particles prepared by the method of any one of claims 8-13.

15. A sodium battery, characterized by, Comprise: An electrolyte, the positive electrode tab of claim 14, a separator, and a negative electrode tab.

16. An energy storage device, characterized by Comprise: Comprise: A box; and The sodium battery of claim 15 is accommodated in the box.

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