Sodium iron phosphate-pyrophosphate positive electrode material, and preparation method therefor and use thereof

Abstract

The present application relates to the technical field of positive electrode materials of sodium-ion batteries, and provides a sodium iron phosphate-pyrophosphate positive electrode material, and a preparation method therefor and the use thereof. The D50 of the sodium iron phosphate-pyrophosphate positive electrode material is 2-10 μm, and the maximum particle size of the sodium iron phosphate-pyrophosphate positive electrode material is less than 25 μm; and the internal porosity of the sodium iron phosphate-pyrophosphate positive electrode material is less than 14%, and the compaction density thereof is greater than 2.17 g / cm3. The present application is conductive to improving the compaction density of the sodium iron phosphate-pyrophosphate positive electrode material.

Description

A sodium iron pyrophosphate cathode material, its preparation method and application

[0001] This application claims priority to Chinese Patent Application No. 202411850524.X, filed on December 13, 2024, entitled "A Sodium Iron Pyrophosphate Cathode Material and Its Preparation Method and Application", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to a sodium iron pyrophosphate cathode material, its preparation method, and its application, belonging to the technical field of sodium-ion battery cathode materials. Background Technology

[0003] Sodium-ion batteries have enormous potential in the energy storage market, and cathode materials (such as sodium iron pyrophosphate cathode material) are one of the important components of sodium-ion batteries. Sodium iron pyrophosphate (Na4Fe3(PO4)2(P2O7), NFPP) has a high specific capacity (129 mAh g / g). -1 With its advantages such as stable structure and cycle life, long lifespan, and low production cost, it is considered the most competitive cathode material in electrochemical energy storage batteries.

[0004] Spray drying combined with carbothermal reduction is a mainstream method for low-cost, large-scale synthesis of NFPP. However, the cathode materials (e.g., NFPP) prepared by this method have large particle sizes and high internal porosity. Large particle size and high internal porosity lead to reduced compaction density of the cathode material, directly affecting battery capacity, internal resistance, and processing difficulty. High internal porosity also results in more intense side reactions, leading to poorer cycle performance; furthermore, sodium deposition may occur under high-rate charging. Summary of the Invention

[0005] This application provides a sodium iron pyrophosphate cathode material, its preparation method, and its application. The sodium iron pyrophosphate cathode material has a small particle size and low internal porosity, which helps to improve the compaction density of the sodium iron pyrophosphate cathode material and improve its capacity (specific capacity, energy density), cycle performance, and rate performance.

[0006] This application provides a sodium iron pyrophosphate cathode material, wherein the sodium iron pyrophosphate cathode material has a D... 50 The diameter is 2-10 μm, and the D of the sodium iron pyrophosphate cathode material is... 100 The porosity of the sodium iron pyrophosphate cathode material is less than 25 μm; the internal porosity is less than 14%, and the compacted density is greater than 2.17 g / cm³. 3 .

[0007] This application provides a sodium iron pyrophosphate cathode material, wherein the sodium iron pyrophosphate cathode material has a D... 50 The porosity of the sodium iron pyrophosphate cathode material is 2-10 μm, the internal porosity is less than 14%, and the compacted density is greater than 2.17 g / cm³. 3 .

[0008] Optionally, the particle size distribution diagram of the sodium iron pyrophosphate cathode material contains m peaks, and the peak intensities of the sodium iron pyrophosphate cathode material corresponding to the m peaks are not equal, where m is an integer greater than or equal to 2.

[0009] Optionally, among the m peaks, at least one peak corresponds to a peak intensity of 1-4 μm for the sodium iron pyrophosphate cathode material, and the mass ratio of the sodium iron pyrophosphate cathode material with a peak intensity of 1-4 μm to the total mass intensity of the sodium iron pyrophosphate cathode material is 10-50%; at least one peak corresponds to a peak intensity of 6-10 μm for the sodium iron pyrophosphate cathode material.

[0010] Optionally, when m is 2, the peak intensity of the sodium iron pyrophosphate cathode material corresponding to one peak is 1-4 μm, and the mass ratio of the sodium iron pyrophosphate cathode material with a peak intensity of 1-4 μm to the total mass of the sodium iron pyrophosphate cathode material is 10%-50%; the peak intensity of the sodium iron pyrophosphate cathode material corresponding to the other peak is 6-10 μm, and the mass ratio of the sodium iron pyrophosphate cathode material with a peak intensity of 6-10 μm to the total mass of the sodium iron pyrophosphate cathode material is 50%-90%.

[0011] Optionally, when m is 3, the peak intensity of the sodium iron pyrophosphate cathode material corresponding to one of the peak values ​​is 1-4 μm, and the mass ratio of the sodium iron pyrophosphate cathode material with a peak intensity of 1-4 μm to the total sodium iron pyrophosphate cathode material is 10%-30%. Alternatively, the peak intensity of the sodium iron pyrophosphate cathode material corresponding to one of the peak values ​​is 4-7 μm, and the mass ratio of the sodium iron pyrophosphate cathode material with a peak intensity of 4-7 μm to the total sodium iron pyrophosphate cathode material is 10%-30%. Furthermore, the peak intensity of the sodium iron pyrophosphate cathode material corresponding to one of the peak values ​​is 7-10 μm, and the mass ratio of the sodium iron pyrophosphate cathode material with a peak intensity of 7-10 μm to the total sodium iron pyrophosphate cathode material is 40-80%.

[0012] Optionally, the sodium iron pyrophosphate cathode material comprises a core and a carbon layer on the surface of the core; the core has the chemical formula Na. x Fe y (PO4) zP2O7, where 3≤x≤5, 2≤y<4, 1≤z≤3.

[0013] This application also provides a method for preparing the sodium iron pyrophosphate cathode material as described above, comprising: subjecting a mixed slurry including a sodium source, an iron source, a phosphorus source, an organic carbon source, and an inorganic carbon to two-fluid spray drying; wherein, during the two-fluid spray drying process, at least m nozzles are used, and by controlling the gas-liquid ratio of the at least m nozzles, the mixed slurry is dried by the two-fluid spray drying to form a mixture comprising m groups of powders, wherein the peak intensities of the m groups of powders are all different; wherein, the D of the mixture comprising the m groups of powders is... 50 The particle size is 2-10 μm, and the mixture comprising m groups of powders has a D... 100 The particle size is less than 25 μm; the mixture comprising m groups of powders is sintered in an inert atmosphere to obtain the sodium iron pyrophosphate cathode material.

[0014] This application also provides a method for preparing the sodium iron pyrophosphate cathode material as described above, comprising: subjecting a mixed slurry including a sodium source, an iron source, a phosphorus source, an organic carbon source, and an inorganic carbon to two-fluid spray drying; wherein, during the two-fluid spray drying process, at least m nozzles are used, and by controlling the gas-liquid ratio of the at least m nozzles, the mixed slurry is dried by the two-fluid spray drying to form a mixture comprising m groups of powders, wherein the peak intensities of the m groups of powders are all different; wherein, the D of the mixture comprising the m groups of powders is... 50 The particle size is 2-10 μm; the mixture comprising m groups of powders is sintered in an inert atmosphere to obtain the sodium iron pyrophosphate cathode material.

[0015] Optionally, the solid content of the mixed slurry is 20% to 40%; and / or, before subjecting the mixed slurry comprising sodium source, iron source, phosphorus source, organic carbon source and inorganic carbon to the two-fluid spray drying, it is first ground to a particle size of 0.2 μm to 0.4 μm, and then the ground mixed slurry is subjected to the two-fluid spray drying; and / or, the sodium source comprises one of sodium dihydrogen phosphate, sodium phosphate, sodium carbonate, sodium oxalate, sodium nitrate, sodium chloride, sodium acetate, sodium sulfate, sodium hydroxide, sodium formate, sodium citrate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate. Or multiple; and / or, the phosphorus source includes one or more of sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, triammonium phosphate, ferric phosphate, ferric pyrophosphate, pyrophosphate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate; and / or, the iron source includes one or more of iron metal powder, ferric citrate, ferrous citrate, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, ferric chloride, ferrous chloride, iron(II,III) oxide, ferric oxide, ferrous oxide, ferric oxalate, ferrous oxalate, ferric acetate, ferric phosphate, ferric pyrophosphate, and ferrous ammonium sulfate; and / or, the organic carbon source includes One or more of the following: oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid, malic acid, oxalic acid, adipic acid, soluble starch, polyethylene glycol, aniline, tannic acid, sucrose, and glucose; and / or, the inorganic carbon includes any one or a combination of at least two of the following: superconducting carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotubes, and graphene; and / or, during the two-fluid spray drying process, the inlet air temperature is 160-240℃, the outlet air temperature is 85-120℃, and the nozzle diameter is 1-mm; and / or, the two-fluid spray... During the fog drying process, the ratio of air consumption to liquid spray volume is 30-100; and / or, the sintering process includes: firstly performing a first gradient sintering at 200-300℃; then performing a second gradient sintering at 500-600℃; the first gradient sintering time is 2-10h; and / or, the second gradient sintering time is 2-10h; and / or, the inert atmosphere includes one or more of argon, nitrogen, and hydrogen, the oxygen content in the inert atmosphere is <50ppm, and the gas flow rate of the inert atmosphere is 2-10L / min.

[0016] This application provides a positive electrode sheet, comprising the sodium iron pyrophosphate positive electrode material as described above or the sodium iron pyrophosphate positive electrode material obtained according to the preparation method described above.

[0017] This application provides a sodium-ion battery, including the positive electrode sheet as described above.

[0018] This application provides a sodium iron pyrophosphate cathode material, its preparation method, and its application. The D... 50With a particle size of 2-10 μm, the internal porosity of this sodium iron pyrophosphate cathode material is less than 10%. Its small particle size and low internal porosity help to improve the compaction density of the sodium iron pyrophosphate cathode material and improve its capacity (specific capacity, energy density), cycle performance and rate performance.

[0019] This application provides a sodium iron pyrophosphate cathode material, its preparation method, and its application. The D... 50 The D of this sodium iron pyrophosphate cathode material is 2-10 μm. 100 The particle size is less than 25 μm; the internal porosity of this sodium iron pyrophosphate cathode material is less than 10%. Its small particle size and small internal porosity help to improve the compaction density of the sodium iron pyrophosphate cathode material and improve its capacity (specific capacity, energy density), cycle performance and rate performance. Attached Figure Description

[0020] Figure 1 is an electron microscope image of the sodium iron pyrophosphate cathode material of Example 1;

[0021] Figure 2 is a cross-sectional electron microscope image of the sodium iron pyrophosphate cathode material of Example 1;

[0022] Figure 3 shows the particle size distribution of the sodium iron pyrophosphate cathode material in Example 1;

[0023] Figure 4 shows the X-ray diffraction (XRD) pattern of the sodium iron pyrophosphate cathode material of Example 1;

[0024] Figure 5 shows the charge-discharge curves of the button cell assembled with sodium iron pyrophosphate cathode material of Example 1 (horizontal axis is Capacity (mAh / g), vertical axis is Voltage (V) vs. Na+ / Na);

[0025] Figure 6 shows the rate performance curves of coin cells assembled with sodium iron pyrophosphate cathode materials of Example 1 and Comparative Example 3.

[0026] Figure 7 shows the electrical performance of coin cells assembled with sodium iron pyrophosphate cathode materials of Example 1 and Comparative Example 3 after 100 cycles at 1C at room temperature.

[0027] Figure 8 shows the particle size distribution of the sodium iron pyrophosphate cathode material in Example 13. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this application clearer, the technical solutions in the embodiments of this application will be clearly and completely described below in conjunction with the embodiments of this application. Obviously, the described embodiments are only some embodiments of this application, not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0029] This application provides a sodium iron pyrophosphate cathode material, the D of which is... 50 With a porosity of 2-10 μm, the internal porosity of this sodium iron pyrophosphate cathode material is less than 14%, and its compacted density is greater than 2.17 g / cm³. 3 .

[0030] The sodium iron pyrophosphate cathode material of this application helps to improve the compaction density of the sodium iron pyrophosphate cathode material, thereby improving its capacity (specific capacity, energy density), cycle performance, and rate performance.

[0031] This application provides a sodium iron pyrophosphate cathode material, the D of which is... 50 The diameter is 2-10 μm, and the D of this sodium iron pyrophosphate cathode material is... 100 The internal porosity of this sodium iron pyrophosphate cathode material is less than 25 μm, and its compacted density is greater than 2.17 g / cm³. 3 .

[0032] According to the inventor's research and analysis, the sodium iron pyrophosphate cathode material of the present application has a small particle size and a small internal porosity, which helps to improve the compaction density of the sodium iron pyrophosphate cathode material and improve its capacity (specific capacity, energy density), cycle performance and rate performance.

[0033] In some embodiments, the particle size distribution diagram of the aforementioned sodium iron pyrophosphate cathode material contains m peaks, each with a different peak intensity, where m is an integer greater than or equal to 2, such as 2, 3, or 4, preferably m is 2. That is, the aforementioned sodium iron pyrophosphate cathode material is formed by blending m groups of secondary particles with different particle size distributions, which can significantly reduce the internal porosity (internal void ratio of secondary particles) between the particles, thereby improving compaction density and enhancing its capacity, cycle performance, and rate performance.

[0034] In this embodiment of the application, the peak intensity refers to the horizontal coordinate corresponding to the peak value in the particle size distribution diagram, that is, the particle size.

[0035] Among the above m peaks, at least one peak corresponds to a peak intensity of sodium iron pyrophosphate cathode material of 1 to 4 μm, for example, 1 μm, 2 μm, 3 μm, 4 μm or any combination thereof, and the mass ratio of sodium iron pyrophosphate cathode material with peak intensity of 1 to 4 μm to sodium iron pyrophosphate cathode material is 10 to 50%, for example, 10%, 20%, 30%, 40%, 50% or any combination thereof; at least one peak corresponds to a peak intensity of sodium iron pyrophosphate cathode material of 6 to 10 μm, for example, 6 μm, 7 μm, 8 μm or any combination thereof. For example, when m is 2, there are two peaks in the particle size distribution diagram of the sodium iron pyrophosphate cathode material. One peak corresponds to a peak intensity of 1–4 μm, and the mass ratio of sodium iron pyrophosphate cathode material with a peak intensity of 1–4 μm to that with the sodium iron pyrophosphate cathode material is 10%–50%. The other peak corresponds to a peak intensity of 6–10 μm, and the mass ratio of sodium iron pyrophosphate cathode material with a peak intensity of 6–10 μm to that with the sodium iron pyrophosphate cathode material is 50%–90%. When m is 3, that is, there are three peaks in the particle size distribution diagram of the sodium iron pyrophosphate cathode material. The peak intensity of the sodium iron pyrophosphate cathode material corresponding to one peak value is 1–4 μm, and the mass ratio of sodium iron pyrophosphate cathode material with a peak intensity of 1–4 μm to sodium iron pyrophosphate cathode material is 10%–30%. Another peak value corresponds to a peak intensity of 4–7 μm, and the mass ratio of sodium iron pyrophosphate cathode material with a peak intensity of 4–7 μm to sodium iron pyrophosphate cathode material is 10%–30%. A third peak value corresponds to a peak intensity of 7–10 μm, and the mass ratio of sodium iron pyrophosphate cathode material with a peak intensity of 7–10 μm to sodium iron pyrophosphate cathode material is 40%–80%. Thorough mixing of secondary particles with different particle size distributions can significantly reduce the internal porosity between the aforementioned sodium iron pyrophosphate cathode material particles, increase the compaction density, and improve the battery's capacity, cycle performance, and rate performance.

[0036] In some embodiments, the sodium iron pyrophosphate cathode material includes a core and a carbon layer on the surface of the core. Under the action of van der Waals forces in the carbon layer, the primary particles are tightly aggregated to form compact secondary particles, which helps to reduce the internal porosity of the sodium iron pyrophosphate cathode material, increase the compaction density, and improve the battery's capacity, cycle performance, and rate performance.

[0037] The chemical formula of the aforementioned kernel can be Na. x Fe y (PO4) z P2O7, where 3≤x≤5, 2≤y<4, 1≤z≤3.

[0038] In some embodiments, the primary particle size of the above-mentioned sodium iron pyrophosphate cathode material is less than 100 nm.

[0039] The particle size of the primary particles in the sodium iron pyrophosphate cathode material can be determined by the following process: after observing 200 primary particles in a 10.0K magnified field of view using a scanning electron microscope, the average particle size is calculated to obtain the particle size of the primary particles.

[0040] In some embodiments, the compaction density of the above-mentioned sodium iron pyrophosphate cathode material is greater than or equal to 2.05 g / cm³. 3 .

[0041] This application embodiment also provides a method for preparing a sodium iron pyrophosphate cathode material, comprising: subjecting a mixed slurry including a sodium source, an iron source, a phosphorus source, an organic carbon source, and an inorganic carbon to two-fluid spray drying; wherein, during the two-fluid spray drying process, at least m nozzles are used, and by controlling the gas-liquid ratio of the at least m nozzles, the mixed slurry is dried to form a mixture comprising m groups of powders, each of the m groups of powders having a different peak intensity; wherein, the D of the mixture comprising the m groups of powders is... 50 The particle size is 2-10 μm; a mixture including m groups of powders is sintered in an inert atmosphere to obtain the above-mentioned sodium iron pyrophosphate cathode material.

[0042] This application embodiment also provides a method for preparing a sodium iron pyrophosphate cathode material, comprising: subjecting a mixed slurry including a sodium source, an iron source, a phosphorus source, an organic carbon source, and an inorganic carbon to two-fluid spray drying; wherein, during the two-fluid spray drying process, at least m nozzles are used, and by controlling the gas-liquid ratio of the at least m nozzles, the mixed slurry is dried to form a mixture comprising m groups of powders, each of the m groups of powders having a different peak intensity; wherein, the D of the mixture comprising the m groups of powders is... 50 The D of the mixture of the above-mentioned powders is 2-10 μm. 100 The particle size is less than 25 μm; the mixture including m groups of powders is sintered in an inert atmosphere to obtain the above-mentioned sodium iron pyrophosphate cathode material.

[0043] According to the inventor's research and analysis: Two-fluid spray drying can simultaneously prepare m groups of powders with different peak intensities. These m groups of powders are then uniformly mixed in a specific ratio to form a mixture. Sintering this mixture yields m groups of secondary particles with different peak intensities. The uniform mixing of these m groups of secondary particles forms a sodium iron pyrophosphate cathode material. The mixing of these m groups of secondary particles with different particle sizes can significantly reduce the porosity between the particles of the sodium iron pyrophosphate cathode material, thereby lowering the internal porosity and increasing the compaction density. Simultaneously, the addition of inorganic carbon can improve the compressibility of the sodium iron pyrophosphate. The conductivity (electrical conductivity) of the cathode material can reduce the amount of organic carbon source used, suppress the formation of pores in the organic carbon source during sintering (high-temperature decomposition), and help reduce the internal porosity of the sodium iron pyrophosphate cathode material. Furthermore, the strong van der Waals forces between inorganic carbon particles (inorganic carbon structure) can be used to tightly aggregate primary particles (polyanionic materials) into compact secondary particles, thereby reducing the internal porosity of the sodium iron pyrophosphate cathode material, increasing compaction density, and improving capacity, cycle performance, and rate performance. In addition, the particle size distribution of the sodium iron pyrophosphate cathode material can be controlled through two-fluid spray granulation, resulting in a more controlled D... 50 It can be freely adjusted within the range of 2 to 10 μm, while ensuring its D 100 (maximum particle size D) max (less than 25μm)

[0044] Therefore, the preparation method provided in this application can prepare sodium iron pyrophosphate cathode material (NFPP material) with stable and controllable particle size, low porosity, high compaction density, high cycle performance and high rate performance. In addition, the above preparation method is simple to operate, green and environmentally friendly, which is conducive to large-scale industrial scale-up and has high industrial applicability.

[0045] The sodium source may include one or more of the following: sodium dihydrogen phosphate, sodium phosphate, sodium carbonate, sodium oxalate, sodium nitrate, sodium chloride, sodium acetate, sodium sulfate, sodium hydroxide, sodium formate, sodium citrate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate. The phosphorus source may include one or more of the following: sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, triammonium phosphate, ferric phosphate, ferric pyrophosphate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate. The iron source may include one or more of the following: iron metal powder, ferric citrate, ferrous citrate, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, ferric chloride, ferrous chloride, iron(II,III) oxide, ferric oxide, ferric oxalate, ferrous oxalate, ferric acetate, ferric phosphate, ferric pyrophosphate, and ferrous ammonium sulfate.

[0046] The carbon and inorganic carbon formed by the sintering of organic carbon sources in an inert atmosphere (oxygen-free high-temperature decomposition) form a carbon layer in the sodium iron pyrophosphate cathode material. This carbon layer exists (covers) the core surface of the sodium iron pyrophosphate cathode material.

[0047] The mass ratio of organic carbon source to sodium iron pyrophosphate cathode material can be 5%-10%, for example, 5%; the mass ratio of inorganic carbon to sodium iron pyrophosphate cathode material can be 0.1%-1%, for example, 0.5%.

[0048] In some embodiments, the organic carbon source includes one or more of the following: oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid, malic acid, oxalic acid, adipic acid, soluble starch, polyethylene glycol, aniline, tannic acid, sucrose, and glucose.

[0049] In some embodiments, the inorganic carbon (inorganic amorphous carbon) includes any one or a combination of at least two of Super-P (SP), acetylene black, Ketjen black, carbon fiber, carbon nanotubes and graphene. The strong van der Waals forces between the structures of inorganic carbon cause the primary particles to aggregate tightly, reducing the internal porosity of the sodium iron pyrophosphate cathode material, increasing its compaction density, and improving its capacity, cycle performance and rate performance.

[0050] The above-mentioned raw materials are all widely distributed, inexpensive and readily available, which helps to reduce production costs.

[0051] In some embodiments, a mixed slurry comprising sodium source, iron source, phosphorus source, organic carbon source and inorganic carbon can be obtained by a process including the following steps: weighing sodium source, iron source, phosphorus source, organic carbon source and inorganic carbon (inorganic carbon source) according to stoichiometric ratio, dispersing them evenly in deionized water or a mixture of anhydrous ethanol and deionized water to obtain a mixed slurry comprising sodium source, iron source, phosphorus source, organic carbon source and inorganic carbon.

[0052] The volume ratio of anhydrous ethanol to deionized water can be (4-6):(94-96), for example, it can be 4:96, 5:95, 6:94 or any combination thereof, preferably 5:95.

[0053] In some embodiments, the solid content of the mixed slurry, including sodium source, iron source, phosphorus source, organic carbon source and inorganic carbon, is 20% to 40%, for example, 20%, 25%, 30%, 35%, 40% or any combination thereof, preferably 25% to 30%. This helps to improve the compaction density of the sodium iron pyrophosphate cathode material. The reason for this is that if the solid content of the mixed slurry is too high, it will reduce the effect of two-fluid spraying, while if the solid content of the mixed slurry is too low, the water content of the mixed slurry will be too high. During the two-fluid spraying process, a large amount of water evaporates, which increases the internal porosity of the sodium iron pyrophosphate cathode material and reduces its compaction density. Therefore, by controlling the solid content of the slurry to be 20% to 40%, it helps to improve the compaction density of the sodium iron pyrophosphate cathode material and improve its capacity, cycle performance and rate performance.

[0054] Before performing two-fluid spray drying on the above-mentioned mixed slurry including sodium, iron, phosphorus, organic carbon, and inorganic carbon sources, it can be ground to a particle size of 0.2 μm to 0.4 μm. Then, the ground mixed slurry is subjected to two-fluid spray drying. This helps to improve the compaction density of the sodium iron pyrophosphate cathode material, thereby improving its capacity, cycle performance, and rate performance. The reason is that if the particle size of the mixed slurry is too small, the viscosity will be too high, reducing the effectiveness of the two-fluid spray. Conversely, if the particle size is too large, the phase formation effect after two-fluid spraying will be poor, both of which are detrimental to improving the compaction density of the sodium iron pyrophosphate cathode material. Therefore, controlling the particle size of the mixed slurry to 0.2 μm to 0.4 μm helps to improve the compaction density of the sodium iron pyrophosphate cathode material, thereby improving its capacity, cycle performance, and rate performance.

[0055] In practice, the above grinding can be carried out in a sand mill. For example, the mixed slurry can be ground in a sand mill by wet sand milling for 2 to 10 hours until the particle size of the mixed slurry is 0.2 μm to 0.4 μm.

[0056] In the two-fluid spray drying process of a mixed slurry including sodium, iron, phosphorus, organic carbon, and inorganic carbon sources, a multi-channel, multi-nozzle device (sprayer) can be used. By adjusting the gas-liquid ratio (the ratio of air consumption to liquid spray volume Qa / Qq) of different nozzles, secondary particles of different sizes can be discharged from different nozzles. The mixing ratio of secondary particles of different sizes can also be controlled by adjusting the feed flow rate of the multi-channel (the feed amount of different nozzles). The two particles of different sizes are uniformly mixed in the sprayer, and after discharge, a mixture of particles of different sizes (the above m groups of powders) can be collected (achieving secondary granulation). That is, through the above two-fluid spray drying, particles with different particle size distributions (the above m groups of powders) can be prepared simultaneously and these particles can be fully mixed.

[0057] In some embodiments, during the two-fluid spray drying process, at least m nozzles are used, and by controlling the gas-liquid ratio (the ratio of air consumption to liquid spray volume Qa / Qq) of the at least m nozzles, the mixed slurry is dried by the two-fluid spray to form a mixture comprising m groups of powders, each group of powders having a different peak intensity; wherein, the D of the mixture comprising the m groups of powders... 50 The particle size is 2-10 μm. During the above two-fluid spray drying process, m groups of powders with different particle size distributions can be prepared simultaneously, and the m groups of powders can be fully mixed to form a mixture including the m groups of powders.

[0058] In some embodiments, during the two-fluid spray drying process, at least m nozzles are used, and by controlling the gas-liquid ratio (the ratio of air consumption to liquid spray volume Qa / Qq) of the at least m nozzles, the mixed slurry is dried by the two-fluid spray to form a mixture comprising m groups of powders, each group of powders having a different peak intensity; wherein, the D of the mixture comprising the m groups of powders... 50 D is a mixture of powders ranging from 2 to 10 μm and including m groups of powders. 100 The maximum particle size (Dmax) is less than 25 μm. During the above two-fluid spray drying process, m groups of powders with different particle size distributions can be prepared simultaneously, and the m groups of powders can be fully mixed to form a mixture including the m groups of powders.

[0059] The particle size (or particle size distribution) of the sprayed particles can be controlled by adjusting the ratio of air consumption to liquid spray volume (the gas-liquid ratio at the nozzle). A higher gas-liquid ratio results in smaller particle size, while a lower ratio results in larger particle size. In some embodiments, during the two-fluid spray drying process described above, the ratio of air consumption to liquid spray volume (the gas-liquid ratio at the nozzle) is 10-200, which facilitates the drying of the sodium iron pyrophosphate cathode material. 50 Controlling the compaction density to 2-10 μm is beneficial for increasing the compaction density of sodium iron pyrophosphate cathode material, thereby improving its capacity, cycle performance, and rate performance.

[0060] In some embodiments, during the two-fluid spray drying process, the inlet air temperature is 160-240°C, for example, 160°C, 180°C, 200°C, 220°C, 240°C or any combination thereof, the outlet air temperature is 85-120°C, for example, 85°C, 95°C, 100°C, 120°C or any combination thereof, and the nozzle diameter is 1-5μm, for example, 1μm, 2μm, 3μm, 4μm, 5μm or any combination thereof.

[0061] Furthermore, in the above-mentioned two-fluid spray drying process, the frequency of the blower can be 35±1Hz, the frequency of the induced draft fan can be 40±1Hz, and the frequency of the atomizer can be 45±1Hz.

[0062] During the above two-fluid spray drying process, the feeding frequency can be adjusted according to the outlet air temperature to achieve the expected mass ratio of powders with different particle size distributions.

[0063] By sintering (calcining) the above mixture including m groups of powders in an inert atmosphere, a sodium iron pyrophosphate cathode material with low internal porosity, high compaction density, high capacity, high cycle performance, and high rate performance can be obtained.

[0064] In some embodiments, the sintering process includes: first performing a first gradient sintering at 200–300°C; and then performing a second gradient sintering at 500–600°C. Exemplarily, the temperature of the first gradient sintering is a range of 200°C, 220°C, 250°C, 280°C, 300°C, or any combination thereof, and the temperature of the second gradient sintering is a range of 500°C, 520°C, 550°C, 580°C, 600°C, or any combination thereof.

[0065] Furthermore, during the first gradient sintering process, the temperature can be increased to the sintering temperature at a rate of 3 to 6 °C / min, for example, 3 °C / min, 5 °C / min, 6 °C / min, or any combination thereof; during the second gradient sintering process, the temperature can be increased to the sintering temperature at a rate of 3 to 6 °C / min, for example, 3 °C / min, 5 °C / min, 6 °C / min, or any combination thereof.

[0066] The sintering time for the first gradient can be 2 to 10 hours, for example, 2 hours, 3 hours, 5 hours, 8 hours, 10 hours or any combination thereof.

[0067] The sintering time for the second gradient can be 2 to 10 hours, for example, 2 hours, 3 hours, 5 hours, 8 hours, 10 hours or any combination thereof.

[0068] The aforementioned inert atmosphere may include one or more of argon, nitrogen, and hydrogen, the oxygen content in the inert atmosphere may be less than 50 ppm, and the gas flow rate of the inert atmosphere may be 2-10 L / min.

[0069] In some embodiments, the inert atmosphere comprises nitrogen and hydrogen, wherein the volume ratio of nitrogen to hydrogen is (94-96):(4-6), for example, 94:6, 95:5, 96:4 or any combination thereof.

[0070] This application also provides a positive electrode sheet, comprising the above-mentioned sodium iron pyrophosphate positive electrode material.

[0071] The positive electrode sheet of this application specifically includes a positive current collector and a positive active layer formed of the above-mentioned sodium iron pyrophosphate positive electrode material disposed on the surface of the positive current collector.

[0072] In the specific preparation of the positive electrode sheet, for example, the sodium iron pyrophosphate positive electrode material of the present application embodiment can be dispersed with a conductive agent and a binder in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and thoroughly stirred to form a uniform positive electrode slurry; the positive electrode slurry is uniformly coated on the positive electrode current collector, and after drying, rolling and slitting, the positive electrode sheet is obtained. In one specific embodiment, the positive electrode active layer comprises, by mass percentage, 70-99 wt% sodium iron pyrophosphate positive electrode material, 0.5-15 wt% conductive agent, and 0.5-15 wt% binder, and further comprises 80-98 wt% sodium iron pyrophosphate positive electrode material, 1-10 wt% conductive agent, and 1-10 wt% binder.

[0073] The positive current collector can be made of at least one of aluminum foil or nickel foil; the conductive agent can be selected from at least one of carbon black, acetylene black, graphene, Ketjen black, and carbon fiber; and the binder can be selected from at least one of polytetrafluoroethylene, polyvinylidene fluoride, polyvinyl fluoride, polyethylene, polypropylene, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, ethylene oxide-containing polymers, polyvinylpyrrolidone, and polyurethane.

[0074] This application also provides a sodium-ion battery, including the above-described positive electrode sheet.

[0075] As can be imagined, the sodium-ion battery of this application embodiment, in addition to the above-mentioned positive electrode, also includes a negative electrode, an electrolyte, and a separator.

[0076] The embodiments of this application are not strictly limited to the negative electrode active material in the negative electrode sheet. It can be the negative electrode active material commonly used in sodium-ion batteries, such as at least one of hard carbon, soft carbon, titanium-based materials, metal oxides and sulfides.

[0077] The embodiments of this application do not strictly limit the selection of electrolyte, and may include one or more of the solvents commonly used in sodium-ion battery electrolytes, as well as the electrolytes commonly used in sodium-ion electrolytes. For example, the solvent may be ethylene carbonate, propylene carbonate, butene carbonate, fluoroethylene carbonate (FEC), dimethyl carbonate (DMC), diethyl carbonate (DEC), difluoroethylene carbonate (DFEC), dipropyl carbonate, methyl ethyl carbonate (EMC), ethyl acetate, ethyl propionate, propyl acetate, propyl propionate, sulfolane, γ-butyrolactone, etc.; the electrolyte may be one or more of sodium hexafluorophosphate, sodium bis(trifluoromethanesulfonyl)imide, sodium bis(fluorosulfonyl)imide, and sodium fluorotrifluoromethanesulfonylimide.

[0078] The embodiments of this application do not strictly limit the choice of membrane material. It can be one of the membrane materials commonly used in sodium-ion batteries, such as polypropylene membrane (PP), polyethylene membrane (PE), polypropylene / polyethylene double-layer composite membrane (PP / PE), polyimide electrospun membrane (PI), polypropylene / polyethylene / polypropylene triple-layer composite membrane (PP / PE / PP), cellulose nonwoven membrane, and membrane with ceramic coating.

[0079] In the preparation of sodium-ion batteries, the positive electrode, separator, and negative electrode are wound or stacked to obtain a bare cell, which is then packaged into a pre-stamped aluminum-plastic film bag. After the packaged battery is dried at 85°C, the electrolyte is injected into the dried battery. The battery undergoes resting, formation, and secondary sealing to complete the preparation of the sodium-ion battery.

[0080] The present application will now be described in more detail through specific embodiments.

[0081] Example 1

[0082] This embodiment provides a method for preparing sodium iron pyrophosphate cathode material, including:

[0083] Iron oxide, iron phosphate, sodium carbonate, glucose, and Ketjen black were weighed and dissolved in deionized water. The molar ratio of iron oxide, iron phosphate, and sodium carbonate was 4:3:4. The mass ratio of glucose to sodium iron phosphate pyrophosphate cathode material was 5%, and the mass ratio of Ketjen black to sodium iron phosphate pyrophosphate cathode material was 0.5%. The mixture was stirred and mixed evenly at room temperature to obtain a mixed slurry containing sodium source, iron source, phosphorus source, organic carbon source, and inorganic carbon, with a solid content controlled at 30%.

[0084] The above-mentioned mixed slurry was then transferred to a sand mill and ground for 5 hours, controlling the slurry particle size to be between 0.2μm and 0.4μm;

[0085] The above-mentioned mixed slurry was subjected to two-fluid spray drying using a dual-channel two-fluid spray dryer. During the two-fluid spray drying process, two nozzles were used, each with a diameter of 5 mm. The gas-liquid ratio Qa / Qq of one nozzle was controlled to be 50, so that the peak intensity of the powder obtained from that nozzle was 8 μm. The gas-liquid ratio Qa / Qq of the other nozzle was controlled to be 100, so that the peak intensity of the powder obtained from that nozzle was 2 μm. The feed rate of the two nozzles was controlled so that the mass ratio of powder with a peak intensity of 8 μm to powder with a peak intensity of 2 μm was 2:1, forming a mixture. The blower frequency was (35±1) Hz, the induced draft fan frequency was (40±1) Hz, the atomizer frequency was (45±1) Hz, the inlet air temperature was controlled to be 190±2℃, and the outlet air temperature was 110±2℃.

[0086] The above mixture was sintered in a nitrogen / hydrogen atmosphere (volume ratio 95:5). First, the temperature was increased to 220℃ at a rate of 5℃ / min for a first gradient sintering of 5h, and then increased to 510℃ at a rate of 5℃ / min for a second gradient sintering of 8h to obtain sodium iron pyrophosphate cathode material (Na4Fe3(PO4)2P2O7).

[0087] Following the procedure in Example 1, sodium iron pyrophosphate cathode materials of Examples 2-16 and Comparative Examples 1-3 were prepared respectively.

[0088] The mass ratios of sodium source, iron source, phosphorus source, organic carbon source, and cathode material in each embodiment and comparative example (mass ratio of organic carbon source to cathode material), the mass ratio of inorganic carbon to cathode material (mass ratio of inorganic carbon to cathode material), the solid content of the mixed slurry, and the particle size of the mixed slurry are summarized in Table 1.

[0089] The inlet air temperature, outlet air temperature, number of nozzles, diameter (nozzle diameter), gas-liquid ratio (Qa / Qq) (gas-liquid ratio of nozzle output), m, particle size distribution peak intensity and mass ratio of m groups of powder in the mixture, temperature and time of first gradient sintering, temperature and time of second gradient sintering, inert atmosphere, etc. of each embodiment and comparative example are summarized in Table 2.

[0090] Table 1

[0091] Table 2

[0092] Experimental Example 1

[0093] 1. The following parameters of the above embodiments and comparative examples were tested:

[0094] 1) D of sodium iron pyrophosphate cathode material 50 and D 100 : Using a laser particle size analyzer, the particle size of the sample particles is analyzed through the spatial distribution of diffraction or scattering;

[0095] 2) Internal porosity of sodium iron pyrophosphate cathode material: Electron micrographs of the cross-section of sodium iron pyrophosphate cathode material were obtained using a profiler and scanning electron microscope, and the porosity of the material was calculated using ImageJ image processing and analysis software;

[0096] 3) Chemical composition of sodium iron pyrophosphate cathode material: The chemical composition of sodium iron pyrophosphate cathode material was determined by inductively coupled plasma atomic emission spectrometry (ICP) and the chemical formula of sodium iron pyrophosphate cathode material is recorded in Table 3.

[0097] 4) Compacted density of sodium iron pyrophosphate cathode material: The compacted density under 3 tons of pressure was tested using a UTM7105 automatic battery powder compaction density tester.

[0098] 5) Scanning electron microscope (SEM) images of sodium iron pyrophosphate cathode material: Figure 1 shows the SEM image of sodium iron pyrophosphate cathode material of Example 1, and Figure 2 shows the cross-sectional SEM image of sodium iron pyrophosphate cathode material of Example 1.

[0099] 6) Particle size distribution diagram of sodium iron pyrophosphate cathode material: The particle size distribution diagram of sodium iron pyrophosphate cathode material in Example 1 is shown in Figure 3, and the particle size distribution diagram of sodium iron pyrophosphate cathode material in Example 13 is shown in Figure 8.

[0100] 7) X-ray diffraction (XRD) pattern of sodium iron pyrophosphate cathode material: The X-ray diffraction (XRD) pattern of sodium iron pyrophosphate cathode material of Example 1 is shown in Figure 4;

[0101] 9) Purity of sodium iron pyrophosphate (NFPP) phase in sodium iron pyrophosphate cathode material: The XRD results were refined using TOPAS.

[0102] 2. Test Results

[0103] Table 3

[0104] Data Analysis:

[0105] The primary particles of the sodium iron pyrophosphate cathode material in Comparative Example 1 have a larger particle size and poorer conductivity.

[0106] Furthermore, Figure 1 is an electron microscope (EM) image of the sodium iron pyrophosphate cathode material of Example 1, showing that the sodium iron pyrophosphate cathode material particles have a near-spherical morphology and controllable particle size; Figure 2 is a cross-sectional EEM image of the sodium iron pyrophosphate cathode material of Example 1, showing that the sodium iron pyrophosphate cathode material has no obvious large pores and a high degree of solidity. The EEM images and cross-sectional EEM images of the sodium iron pyrophosphate cathode materials of other examples are similar to those of Example 1.

[0107] Figure 3 shows the particle size distribution of the sodium iron pyrophosphate cathode material in Example 1. It can be seen that the sodium iron pyrophosphate cathode material particles have two peaks in the particle size distribution diagram, corresponding to D values ​​respectively. 50 The sodium iron pyrophosphate cathode material particles are 3 μm and 8 μm in size. The particle size distribution diagrams of the sodium iron pyrophosphate cathode materials in Examples 2-19 are similar to those in Example 1.

[0108] Figure 4 shows the X-ray diffraction (XRD) pattern of the sodium iron pyrophosphate cathode material of Example 1. It can be calculated that the purity of the NFPP phase of the sodium iron pyrophosphate cathode material is 95.4%, and the NFP phase accounts for 3.1%. The X-ray diffraction (XRD) patterns of the sodium iron pyrophosphate cathode materials of other examples are similar to those of Example 1.

[0109] Figure 5 shows the charge-discharge curves of the coin cell assembled with the sodium iron pyrophosphate cathode material of Example 1 (the horizontal axis represents Capacity (mAh / g), and the vertical axis represents Voltage (V) vs. Na). + / Na (i.e., voltage), Figure 6 shows the rate performance curves of the coin cells assembled with sodium iron pyrophosphate cathode materials of Example 1 and Comparative Example 3, and Figure 7 shows the electrical performance of the coin cells assembled with sodium iron pyrophosphate cathode materials of Example 1 and Comparative Example 3 after 100 cycles at 1C at room temperature; it can be seen that the coin cell assembled with sodium iron pyrophosphate cathode material of Example 1 has high discharge specific capacity, high rate performance, and high cycle performance. The charge-discharge curves of the coin cells assembled with sodium iron pyrophosphate cathode materials of other examples are similar to those of Example 1.

[0110] Experimental Example 2

[0111] After the sodium iron pyrophosphate cathode materials of the examples and comparative examples were respectively fabricated into cathode sheets, they were assembled with a cathode sheet, electrolyte, and separator according to the following method to obtain a coin cell. The method includes:

[0112] Sodium iron pyrophosphate cathode material was mixed with carbon black conductive agent and polyvinylidene fluoride in a mass ratio of 9:0.5:0.5, and then N-methylpyrrolidone solvent was added to prepare a slurry. Subsequently, the slurry was coated onto aluminum foil and rolled to obtain a surface density of 5.5 g / cm³. 3 The positive electrode was obtained by baking it in a vacuum drying oven at 120°C for 12 hours; sodium metal was used as the counter electrode; glass fiber was used as the separator; the electrolyte included EC (ethylene carbonate):DMC (dimethyl carbonate) in a mass ratio of 1:1, and also included NaClO4 and FEC (fluoroethylene carbonate), wherein the concentration of NaClO4 in the electrolyte was 1 mol / L and the mass percentage of FEC was 5%; hard carbon was used as the negative electrode; and a coin cell was assembled in an argon glove box.

[0113] The reversible capacity, capacity retention at 10C, and capacity retention after 100 cycles at 1C of the above-mentioned button cells were tested:

[0114] At 25°C, the capacitor was charged at a constant current rate of 0.2C to 3.75V, then charged at a constant voltage rate of 0.05C to 3.75V, and then allowed to stand for 2 minutes. It was then discharged at a discharge rate of 0.2C to 1.5V, and allowed to stand for another 2 minutes. This charge-discharge cycle was repeated 100 times at 1C. The reversible capacity, discharge capacity Q1 at the first cycle, and discharge capacity Q at the 100th cycle were measured. 100 The capacity retention rate Q after 100 cycles of 1C is calculated using the following formula. 100 / Q1*100%);

[0115] At 25°C, the capacitor was charged at a constant current rate of 0.2C to 3.75V, then charged at a constant voltage rate of 0.05C to 3.75V, and then allowed to stand for 2 minutes. It was then discharged at a discharge rate of 0.2C to 1.5V, and allowed to stand for another 2 minutes. Then, it was charged and discharged at 10C using the same method. The discharge capacity and charge capacity were measured, and the ratio of discharge capacity to charge capacity was calculated to obtain the capacity retention rate at 10C.

[0116] The results are shown in Table 4;

[0117] Table 4

[0118] Data Analysis:

[0119] Compared with Comparative Example 1, the reversible capacity and capacity retention of each embodiment are better, verifying that by adding inorganic carbon, the conductivity of sodium iron pyrophosphate cathode material can be improved, the amount of organic carbon source used can be reduced, the production of pores by organic carbon source under sintering (high temperature decomposition) can be suppressed, which is conducive to reducing the internal porosity of sodium iron pyrophosphate cathode material. Furthermore, the strong van der Waals forces between inorganic carbon (inorganic carbon structure) can be used to make the primary particles (polyanionic materials) tightly aggregate to form compact secondary particles, thereby reducing the internal porosity of sodium iron pyrophosphate cathode material, increasing compaction density, and improving capacity, cycle performance and rate performance.

[0120] Compared with Comparative Examples 2 and 3, the reversible capacity and capacity retention of each embodiment are better, verifying that the above-mentioned sodium iron pyrophosphate cathode material is formed by mixing m groups of secondary particles with different particle size distributions, which can significantly reduce the internal porosity (internal void ratio of secondary particles) between the particles of the above-mentioned sodium iron pyrophosphate cathode material, which is beneficial to improve the compaction density and improve its capacity, cycle performance and rate performance.

[0121] 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 them. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of this application.

Claims

1. A sodium iron pyrophosphate cathode material, characterized in that, The D of the sodium ferric pyrophosphate cathode material 50 The diameter is 2-10 μm, and the D of the sodium iron pyrophosphate cathode material is... 100 Less than 25μm; The internal porosity of the sodium iron pyrophosphate cathode material is less than 14%, and the compacted density is greater than 2.17 g / cm³. 3 .

2. A sodium iron pyrophosphate cathode material, characterized in that, The D of the sodium ferric pyrophosphate cathode material 50 The porosity of the sodium iron pyrophosphate cathode material is 2-10 μm, the internal porosity is less than 14%, and the compacted density is greater than 2.17 g / cm³. 3 .

3. The sodium iron pyrophosphate cathode material according to claim 1 or 2, characterized in that, The particle size distribution diagram of the sodium iron pyrophosphate cathode material contains m peaks, and the peak intensities of the sodium iron pyrophosphate cathode material corresponding to the m peaks are not equal, where m is an integer greater than or equal to 2.

4. The sodium iron pyrophosphate cathode material according to claim 3, characterized in that, Among the m peaks, at least one peak corresponds to a peak intensity of 1-4 μm for the sodium iron pyrophosphate cathode material, and the mass ratio of the sodium iron pyrophosphate cathode material with a peak intensity of 1-4 μm to the total mass intensity of the sodium iron pyrophosphate cathode material is 10-50%; at least one peak corresponds to a peak intensity of 6-10 μm for the sodium iron pyrophosphate cathode material.

5. The sodium iron pyrophosphate cathode material according to claim 4, characterized in that, When m is 2, the peak intensity of the sodium iron pyrophosphate cathode material corresponding to one peak is 1-4 μm, and the mass ratio of the sodium iron pyrophosphate cathode material with a peak intensity of 1-4 μm to the total mass of the sodium iron pyrophosphate cathode material is 10%-50%; the peak intensity of the sodium iron pyrophosphate cathode material corresponding to the other peak is 6-10 μm, and the mass ratio of the sodium iron pyrophosphate cathode material with a peak intensity of 6-10 μm to the total mass of the sodium iron pyrophosphate cathode material is 50%-90%.

6. The sodium iron pyrophosphate cathode material according to claim 4, characterized in that, When m is 3, the peak intensity of the sodium iron pyrophosphate cathode material corresponding to one of the peak values ​​is 1-4 μm, and the mass ratio of the sodium iron pyrophosphate cathode material with a peak intensity of 1-4 μm to the total mass of the sodium iron pyrophosphate cathode material is 10%-30%. The peak intensity of the sodium iron pyrophosphate cathode material corresponding to one of the peak values ​​is 4-7 μm, and the mass ratio of the sodium iron pyrophosphate cathode material with a peak intensity of 4-7 μm to the total mass of the sodium iron pyrophosphate cathode material is 10%-30%. The peak intensity of the sodium iron pyrophosphate cathode material corresponding to one of the peak values ​​is 7-10 μm, and the mass ratio of the sodium iron pyrophosphate cathode material with a peak intensity of 7-10 μm to the total mass of the sodium iron pyrophosphate cathode material is 40-80%.

7. The sodium iron pyrophosphate cathode material according to claim 1 or 2, characterized in that, The sodium iron pyrophosphate cathode material includes a core and a carbon layer present on the surface of the core; The chemical formula of the kernel is Na. x Fe y (PO4) z P2O7, Where 3≤x≤5, 2≤y<4, 1≤z≤3.

8. A method for preparing the sodium iron pyrophosphate cathode material according to any one of claims 1-7, characterized in that, include: A mixed slurry comprising sodium, iron, phosphorus, organic carbon, and inorganic carbon sources is subjected to two-fluid spray drying. During the two-fluid spray drying process, at least m nozzles are used, and the gas-liquid ratio of the at least m nozzles is controlled so that the mixed slurry, after two-fluid spray drying, forms a mixture comprising m groups of powders, each group of powders having a different peak intensity; wherein, the D of the mixture comprising the m groups of powders... 50 The particle size is 2-10 μm, and the mixture comprising m groups of powders has a D... 100 Less than 25μm; The mixture comprising m groups of powders is sintered in an inert atmosphere to obtain the sodium iron pyrophosphate cathode material.

9. A method for preparing the sodium iron pyrophosphate cathode material according to claim 2, characterized in that, include: A mixed slurry comprising sodium, iron, phosphorus, organic carbon, and inorganic carbon sources is subjected to two-fluid spray drying. During the two-fluid spray drying process, at least m nozzles are used, and the gas-liquid ratio of the at least m nozzles is controlled so that the mixed slurry, after two-fluid spray drying, forms a mixture comprising m groups of powders, each group of powders having a different peak intensity; wherein, the D of the mixture comprising the m groups of powders... 50 2-10μm; The mixture comprising m groups of powders is sintered in an inert atmosphere to obtain the sodium iron pyrophosphate cathode material.

10. The preparation method according to claim 8 or 9, characterized in that, The solid content of the mixed slurry is 20% to 40%; And / or, before performing the two-fluid spray drying on the mixed slurry comprising sodium source, iron source, phosphorus source, organic carbon source and inorganic carbon, the slurry is first ground to a particle size of 0.2 μm to 0.4 μm, and then the ground mixed slurry is subjected to the two-fluid spray drying. And / or, the sodium source includes one or more of sodium dihydrogen phosphate, sodium phosphate, sodium carbonate, sodium oxalate, sodium nitrate, sodium chloride, sodium acetate, sodium sulfate, sodium hydroxide, sodium formate, sodium citrate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate; And / or, the phosphorus source includes one or more of sodium dihydrogen phosphate, sodium phosphate, sodium monohydrogen phosphate, phosphoric acid, ammonium dihydrogen phosphate, triammonium phosphate, ferric phosphate, ferric pyrophosphate, pyrophosphate, sodium pyrophosphate, and sodium dihydrogen pyrophosphate. And / or, the iron source includes one or more of the following: iron metal powder, ferric citrate, ferrous citrate, ferric nitrate, ferrous nitrate, ferric sulfate, ferrous sulfate, ferric chloride, ferrous chloride, iron(II) oxide, ferric oxide, ferrous oxide, ferric oxalate, ferrous oxalate, ferric acetate, ferric phosphate, ferric pyrophosphate, and ferrous ammonium sulfate. And / or, the organic carbon source includes one or more of the following: oxalic acid, ascorbic acid, formaldehyde, acetaldehyde, n-butyraldehyde, lactic acid, citric acid, malic acid, oxalic acid, adipic acid, citric acid, soluble starch, polyethylene glycol, aniline, tannic acid, sucrose, and glucose. And / or, the inorganic carbon includes any one or a combination of at least two of the following: superconducting carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotubes, and graphene. And / or, during the two-fluid spray drying process, the inlet air temperature is 160-240℃, the outlet air temperature is 85-120℃, and the diameter of the nozzle is 1-5mm; And / or, during the two-fluid spray drying process, the ratio of air consumption to liquid spray volume is 30-100; And / or, the sintering process includes: first performing a first gradient sintering at 200–300°C; and then performing a second gradient sintering at 500–600°C; The sintering time for the first gradient is 2–10 hours; And / or, the sintering time of the second gradient is 2 to 10 hours; And / or, the inert atmosphere includes one or more of argon, nitrogen, and hydrogen, the oxygen content in the inert atmosphere is <50ppm, and the gas flow rate of the inert atmosphere is 2-10L / min.

11. A positive electrode plate, characterized in that, Includes the sodium iron pyrophosphate cathode material according to any one of claims 1-7 or the sodium iron pyrophosphate cathode material obtained according to the preparation method according to any one of claims 8-10.

12. A sodium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 11.

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