Positive electrode active material and preparation method therefor, positive electrode sheet, secondary battery and electric device

Abstract

The present application provides a positive electrode active material and a preparation method therefor, a positive electrode sheet, a secondary battery and an electric device. The positive electrode active material has a core-shell structure, comprising a secondary particle core and a second carbon coating layer located on the surface of the secondary particle core, wherein the secondary particle core comprises primary particles and a first carbon coating layer located on the surfaces of the primary particles.

Description

A positive electrode active material and its preparation method, a positive electrode sheet, a secondary battery, and an electrical device thereof.

[0001] This application claims priority to Chinese Patent Application No. 202510466713.5, filed on April 14, 2025, entitled "A positive electrode active material and its preparation method, positive electrode sheet, secondary battery and electrical device", the entire contents of which are incorporated herein by reference. Technical Field

[0002] This application relates to positive electrode active materials, specifically to a positive electrode active material and its preparation method, a positive electrode sheet, a secondary battery, and an electrical device. Background Technology

[0003] In the field of new energy batteries, the performance of cathode active materials plays a crucial role in the overall performance of the battery. In recent years, with the increasing demand for batteries with high energy density and long cycle life, researchers have placed higher requirements on the specific capacity, resistivity, and air stability of cathode active materials.

[0004] Specific capacity is a crucial indicator of a cathode material's ability to store lithium ions. However, traditional cathode materials, such as lithium cobalt oxide (LiCoO2), lithium manganese oxide (LiMn2O4), and lithium iron phosphate (LiFePO4), have relatively low theoretical specific capacities, limiting the battery's energy density. The resistance of the cathode active material directly affects the battery's charge / discharge rate and energy efficiency, and the resistance of existing cathode active materials still needs to be reduced. Air stability refers to a material's ability to maintain its chemical and physical properties when exposed to air, especially in the presence of oxygen and moisture. Improving the air stability of cathode active materials is of great significance for material storage, processing, and the long-term performance of the battery.

[0005] Existing cathode active materials still face some challenges in these aspects. Therefore, a cathode active material that balances high specific capacity and air stability with low resistance is urgently needed in the field. Summary of the Invention

[0006] This invention provides a positive electrode active material and its preparation method, a positive electrode sheet, a secondary battery, and an electrical device. The positive electrode active material balances high specific capacity and air stability with low resistance.

[0007] The present invention provides a positive electrode active material having a core-shell structure, the positive electrode active material comprising a secondary particle core and a second carbon coating layer located on the surface of the secondary particle core, the secondary particle core comprising primary particles and a first carbon coating layer located on the surface of the primary particles.

[0008] Optionally, the positive electrode active material includes sodium iron pyrophosphate.

[0009] Optionally, the positive electrode active material includes doped metal elements.

[0010] Optionally, the chemical formula of the positive electrode active material is Na. x Fe y M z (PO4) w (P2O7) v In this stoichiometry, x is 3.9–4.1, y+z is 2.7–3, w is 1–2, v is 0.8–1, and M includes metallic elements.

[0011] Optionally, M includes M1 and M2, wherein M1 includes one or more of manganese, chromium, vanadium, titanium, cobalt, nickel, copper, zinc, magnesium, and aluminum, and M2 includes one or more of silver, chromium, niobium, and molybdenum, and the doping amount of M1 is higher than that of M2.

[0012] Optionally, the sum of the mass percentages of M1 and M2 in the positive electrode active material is greater than 0% and less than 5%.

[0013] Optionally, the mass percentage of the doped metal element in the positive electrode active material is 2% to 5%.

[0014] Optionally, the sum of the average thicknesses of the first carbon coating layer and the second carbon coating layer is 10–35 nm; and / or, the mass percentage of carbon in the positive electrode active material is 0.59%–2.20%.

[0015] Optionally, the average thickness of the first coating layer is 1 to 10 nm, and the average thickness of the second carbon coating layer is 9 to 25 nm.

[0016] Optionally, the compaction density of the positive electrode active material is 2.0–2.4 g / cm³. 3 ; and / or, the Raman spectrum of the positive electrode active material contains I D with I G The ratio is 1.100 to 1.410.

[0017] The present invention also provides a method for preparing the positive electrode active material as described above, comprising: performing a first sintering on a first raw material system containing a sodium source, an iron source, a phosphorus source, and a first carbon source to obtain a first material; and performing a second sintering on a second raw material system including the first material and a second carbon source to obtain the positive electrode active material.

[0018] Optionally, the first carbon source includes one or more of isopropanolamine, isobutanolamine, and hydroxyethylhydrazine; and / or, the second carbon source includes one or more of conductive polymers, organic covalent polymers-carbon-based polymers, free radical polymers, sulfur-containing polymers, and imino polymers; and / or, the mass percentage of the first carbon source in the first raw material system is 0.05% to 0.85%; and / or, the mass percentage of the second carbon source in the second raw material system is 0.425% to 1.35%; and / or, the first raw material system further includes a metal dopant source.

[0019] Optionally, the first sintering temperature is 200–350°C and the time is 4–5 h; and / or, the second sintering temperature is 500–600°C and the time is 6–12 h.

[0020] The present invention provides a positive electrode sheet, comprising the positive electrode active material as described above or the positive electrode active material prepared by the method described above.

[0021] The present invention provides a secondary battery, comprising the positive electrode sheet as described above.

[0022] The present invention provides an electrical device, including a secondary battery as described above, wherein the secondary battery serves as the power supply for the electrical device.

[0023] This invention provides a positive electrode active material and its preparation method, a positive electrode sheet, a secondary battery, and an electrical device. In the positive electrode active material, a second carbon coating layer coated on the surface of secondary particles and a first carbon coating layer coated on the surface of primary particles form a multi-dimensional network carbon coating layer (carbon network structure). This multi-dimensional network carbon coating layer provides a good electronic conduction path, which helps to improve the electronic conductivity and reduce the resistance of the positive electrode active material. This multi-dimensional network carbon coating layer also protects the positive electrode active material, suppresses interfacial side reactions, and prevents it from getting damp, thereby improving the air stability (environmental stability) of the positive electrode active material. Furthermore, the positive electrode active material with this multi-dimensional network carbon coating layer has less impurities and impurity phases and higher purity, resulting in a higher specific capacity. Therefore, the positive electrode active material of this invention has a high specific capacity, good air stability, low resistance, and excellent rate performance, making it suitable for low-rate energy storage scenarios. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of this application or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0025] Figure 1 shows the scanning electron microscope (SEM) image and elemental distribution map of the positive electrode active material of Comparative Example 1 (Comparative Example 1 shows the SEM image of the positive electrode active material of Comparative Example 1, C is the elemental distribution map of carbon, Na is the elemental distribution map of sodium, and Mn is the elemental distribution map of manganese).

[0026] Figure 2 shows the scanning electron microscope (SEM) image and energy dispersive spectroscopy (EDS) image of the positive electrode active material of Comparative Example 2 (Comparative Example 2 shows the SEM image of the positive electrode active material of Comparative Example 2, where C is the carbon element distribution map, Na is the sodium element distribution map, and Mn is the manganese element distribution map).

[0027] Figure 3 shows the scanning electron microscope (SEM) image and energy dispersive spectroscopy (EDS) image of the positive electrode active material of Example 1 (Example 1 shows the SEM image of the positive electrode active material of Example 1, where C is the carbon element distribution map, Na is the sodium element distribution map, and Mn is the manganese element distribution map).

[0028] Figure 4 shows the scanning electron microscope (SEM) image and energy dispersive spectroscopy (EDS) image of the positive electrode active material of Example 4 (Example 4 shows the SEM image of the positive electrode active material of Example 4, where C is the carbon element distribution map, Na is the sodium element distribution map, Mn is the manganese element distribution map, Ti is the titanium element distribution map, and Ni is the nickel element distribution map).

[0029] Figure 5 is a transmission electron microscope scan of the carbon coating layer of the positive electrode active material in Comparative Example 1, Comparative Example 2, Example 1 and Example 4.

[0030] Figure 6 is a process flow diagram of the preparation process of positive electrode active materials in some embodiments;

[0031] Figure 7 is a schematic diagram of the structure of the positive electrode active material according to an embodiment of the present invention;

[0032] Figure 8 shows electron microscope (EM) images of the first material and the positive electrode active material after argon ion polishing in Example 1 (Example 1: EEM image of the first material after argon ion polishing in Example 1 after one sintering; Example 1: EEM image of the positive electrode active material after argon ion polishing in Example 1 after two sintering).

[0033] Explanation of reference numerals in the attached figures: 001 - primary particles; 002 - first carbon coating layer; 003 - second carbon coating layer; 100 - area exposed on the surface of secondary particles. Detailed Implementation

[0034] To enable those skilled in the art to better understand the present invention, the present invention will be further described in detail below. The specific embodiments listed below are merely descriptions of the principles and features of the present invention, and the examples are only for explaining the present invention and are not intended to limit the scope of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0035] Existing cathode active materials still face some challenges in terms of specific capacity, resistivity, and air stability.

[0036] To overcome the deficiencies in the prior art, this invention provides a positive electrode active material, as shown in FIG7. The positive electrode active material has a core-shell structure, including a secondary particle core and a second carbon coating layer 003 located on the surface of the secondary particle core. The secondary particle core includes a primary particle 001 and a first carbon coating layer 002 located on the surface of the primary particle 001.

[0037] According to research and analysis, in the positive electrode active material, the second carbon coating layer on the surface of the secondary particles and the first carbon coating layer on the surface of the primary particles form a multi-dimensional network carbon coating layer (carbon network structure). This multi-dimensional network carbon coating layer provides a good electronic conduction path, which helps to improve the electronic conductivity and reduce the resistance of the positive electrode active material. This multi-dimensional network carbon coating layer can also protect the positive electrode active material, suppress interfacial side reactions, and prevent it from getting damp, thereby improving the air stability (environmental stability) of the positive electrode active material. Furthermore, the positive electrode active material with this multi-dimensional network carbon coating layer has less impurities and impurity phases and higher purity, resulting in a higher specific capacity. Therefore, the positive electrode active material of this invention has a high specific capacity, good air stability, low resistance, and excellent rate performance, making it suitable for low-rate energy storage scenarios. In addition, the multi-dimensional network carbon coating layer can effectively buffer the volume changes of the positive electrode active material during charge and discharge, which helps to enhance the cycle stability of the positive electrode active material.

[0038] In the positive electrode active material of this invention, the first carbon coating layer substantially covers the entire surface of the primary particles, and the second carbon coating layer substantially covers the entire surface of the secondary particles. In other words, the multi-dimensional network structure carbon coating layer in the positive electrode active material substantially covers the entire surface of the positive electrode active material; that is, the multi-dimensional network structure carbon coating layer in the positive electrode active material provides complete coverage.

[0039] In some embodiments, a first carbon coating layer is uniformly coated (uniformly distributed) on the entire surface of the primary particles, and a second carbon coating layer is uniformly coated (uniformly distributed) on the entire surface of the secondary particles. In the positive electrode active material, a multi-dimensional network structure of carbon coating layer is uniformly coated (uniformly distributed) on the entire surface of the positive electrode active material.

[0040] Furthermore, as shown in FIG7, in region 100 exposed to the surface of secondary particles, at least a portion of the primary particles exposed to the surface of secondary particles have a second coating layer disposed above the first coating layer on their surface.

[0041] Air stability refers to the ability of a material to retain its chemical and physical properties when exposed to air, especially in the presence of oxygen and moisture.

[0042] Specifically, the air stability of the positive electrode active material can be determined by placing it in a dew point environment of -20°C for a certain period of time, testing its specific capacity, observing its capacity retention rate, and judging the specific capacity of the positive electrode active material.

[0043] Sodium iron pyrophosphate (Na4Fe3(PO4)2P2O7) is a novel cathode active material for sodium-ion batteries, which has attracted widespread attention in recent years due to its high theoretical capacity, good thermal stability, and low cost. However, sodium iron pyrophosphate still faces some challenges in practical applications, such as insufficient energy density and cycle stability in sodium-ion batteries. Improving the specific capacity and stability of sodium iron pyrophosphate cathode active material can enhance the energy density and stability of sodium-ion cells, potentially solving these problems. Existing technologies mostly improve the electronic conductivity, reactivity, and potential of sodium iron pyrophosphate by carbon coating, thereby improving its specific capacity and stability. However, these methods suffer from a single carbon coating structure, making it difficult to fully utilize the advantages of carbon material coating and thus failing to effectively improve the specific capacity and air stability of sodium iron pyrophosphate.

[0044] In some embodiments, the positive electrode active material includes sodium iron pyrophosphate.

[0045] That is, the primary particles include sodium iron pyrophosphate, and the secondary particles are formed by the agglomeration of the primary particles.

[0046] As can be seen from the above, this sodium iron pyrophosphate-based positive electrode active material exhibits excellent electronic conductivity, specific capacity, air stability, and low resistance, which helps to improve the battery's specific capacity, rate performance, air stability, and resistance, thus significantly improving the battery's overall performance.

[0047] In some embodiments, the positive electrode active material includes a doped metal element.

[0048] In the positive electrode active material with a carbon coating layer having this multidimensional network structure, the doped metal elements are more easily distributed uniformly. The uniformly distributed doped metal elements can not only improve electronic conductivity, reduce resistance, and improve rate performance, but also help suppress the formation of impurities and impurity phases in the positive electrode active material, improve the purity of the positive electrode active material, and thus help improve the specific capacity of the positive electrode active material and the specific capacity of the battery.

[0049] By synergistically applying doped metal elements and a carbon coating layer (two carbon coating layers) with a multidimensional network structure to the above-mentioned positive electrode active material, the doped metal elements are more easily and uniformly distributed in the positive electrode active material with the above structure, which effectively improves its electronic conductivity, reduces resistance, improves specific capacity, and also helps to improve air stability (environmental stability), thereby helping to improve the battery's specific capacity, rate performance, resistance and air stability, and significantly improving the overall performance of the battery.

[0050] In some embodiments, the chemical formula of the above-mentioned positive electrode active material is Na. x Fe y M z (PO4) w (P2O7) v Where x is 3.9–4.1, y+z is 2.7–3, w is 1–2, v is 0.8–1, and M includes metallic elements. For example, x can be a range of 3.9, 4.0, 4.1, or any two of these. For example, y+z can be a range of 2.7, 2.8, 2.9, 3, or any two of these. Specifically, 2.7 ≤ y+z < 3.

[0051] This positive electrode active material has superior electronic conductivity, specific capacity, air stability, and lower resistance, which helps to improve the battery's specific capacity, rate performance, air stability, and resistance, thus significantly improving the battery's overall performance.

[0052] Specifically, because the above-mentioned positive electrode active material has a multi-dimensional network carbon coating structure, the chemical formula of the positive electrode active material can also be represented as Na. x Fe y M z (PO4) w (P2O7) v @C.

[0053] The aforementioned metallic elements may include one or more of manganese (Mn), chromium (Cr), vanadium (V), titanium (Ti), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), magnesium (Mg), aluminum (Al), silver (Ag), chromium (Cd), niobium (Ni), and molybdenum (Mo), with Ti, Mn, and Ni being preferred. The relatively large size of these metallic elements can increase the cell volume (increase the lattice parameters), which helps to lower the Na+ migration barrier, improve the ionic conductivity of the positive electrode active material, and thus significantly enhance the rate performance. It also helps to further improve the specific capacity, rate performance, resistance, and air stability of the positive electrode active material, thereby further improving the overall performance of the battery.

[0054] In some embodiments, M may include M1 and M2, wherein M1 may include one or more of manganese (Mn), chromium (Cr), vanadium (V), titanium (Ti), cobalt (Co), nickel (Ni), copper (Cu), zinc (Zn), magnesium (Mg), and aluminum (Al); M2 may include one or more of silver (Ag), chromium (Cd), niobium (Ni), and molybdenum (Mo), and the doping amount of M1 is higher than that of M2.

[0055] The doping amount mentioned above refers to the mass percentage of the doped metal element in the positive electrode active material.

[0056] The higher doping concentration of M1 compared to M2 helps to further improve the electronic conductivity, specific capacity, air stability, and resistance of the cathode active material. This is because M1 includes elements with ionic radii close to Fe and possesses multivalent redox potentials, ensuring the integrity of the cathode active material's framework structure. M2 includes multivalent metals with larger ionic radii, increasing redox sites and enhancing the voltage plateau and electrochemical performance of the cathode active material. Controlling the doping concentration of M1 to be higher than that of M2, ensuring a relatively large doping concentration of at least one of M1 elements, helps to further improve the structural stability of the cathode active material. Conversely, ensuring a relatively small doping concentration of at least one of M2 elements significantly improves the doping effect, further enhancing the electronic conductivity and increasing the redox potential of the cathode active material. This, in turn, helps to improve the battery's specific capacity, cycle performance, rate performance, air stability, and resistance, resulting in a significant improvement in the battery's overall performance.

[0057] Understandably, the terms "relatively large" and "relatively small" refer to the results of comparing the doping amounts of M1 and M2.

[0058] In addition, the doping amount of M1 or M2 refers to the mass percentage of M1 or M2 in the positive electrode active material.

[0059] In some embodiments, the sum of the mass percentages of M1 and M2 in the above-mentioned positive electrode active material can be greater than 0% and less than 5%, that is, the sum of the doping amounts of M1 and M2 z satisfies: 0 <z=M1+M2<5%。

[0060] By controlling the sum of the mass percentages of M1 and M2 in the positive electrode active material to meet the above range, it is helpful to improve the cycle stability of the positive electrode active material, improve the stability and integrity of the crystal structure of the positive electrode active material, significantly improve the doping effect, further improve the electronic conductivity of the positive electrode active material, increase the redox potential, thereby helping to improve the specific capacity, cycle performance, rate performance, air stability and resistance of the battery, and significantly improve the overall performance of the battery.

[0061] Furthermore, when the positive electrode active material includes doped metal elements, the mass percentage of the doped metal elements in the positive electrode active material can be 2% to 5%, for example, 2%, 3%, 4%, 5%, or any combination thereof.

[0062] The positive electrode active material with the above doping amount exhibits better electronic conductivity, specific capacity, air stability and lower resistance, which helps to improve the specific capacity, rate performance and resistance of the battery, and significantly improves the overall performance of the battery.

[0063] In the carbon coating layer of the multidimensional network structure, the first carbon coating layer mainly plays the role of improving the electronic conductivity of the positive electrode active material, while the second carbon coating layer can not only improve the electronic conductivity of the positive electrode active material, but also protect the positive electrode active material and improve its air stability.

[0064] In some embodiments, the sum of the average thicknesses of the first carbon coating layer and the second carbon coating layer can be 10–35 nm, for example, a range of 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, or any combination thereof. By controlling the sum of the average thicknesses of the first carbon coating layer and the second carbon coating layer within the above range, the multidimensional network structure of the carbon coating layer can function better, further improving the specific capacity, rate performance, resistance, and air stability of the cathode active material, and further improving the overall performance of the battery.

[0065] In some embodiments, the average thickness of the first carbon coating layer is 1–10 nm, for example, a range of 1 nm, 2 nm, 3 nm, 4 nm, 5 nm, 6 nm, 7 nm, 8 nm, 9 nm, 10 nm, or any combination thereof. The first carbon coating layer with the above-mentioned average thickness can better synergize with the second carbon coating layer, further improving the specific capacity, rate performance, resistance, and air stability of the cathode active material, and further improving the overall performance of the battery. In some embodiments, the average thickness of the second carbon coating layer is 9–25 nm, for example, a range of 9 nm, 10 nm, 15 nm, 20 nm, 25 nm, or any combination thereof. The second carbon coating layer with the above-mentioned average thickness can better synergize with the first carbon coating layer, further improving the specific capacity, rate performance, resistance, and air stability of the cathode active material, and further improving the overall performance of the battery.

[0066] In some embodiments, the mass percentage of carbon (carbon content) in the above-mentioned positive electrode active material is 0.59% to 2.20%, for example, a range of 0.59%, 0.60%, 0.70%, 0.80%, 0.90%, 1.00%, 1.50%, 2.00%, 2.20%, or any combination thereof.

[0067] The aforementioned positive electrode active materials have a low carbon content. This invention, through an innovative carbon coating structure, achieves a networked distribution of the carbon coating layer across multiple dimensions while maintaining a low carbon content. This significantly improves the specific capacity, rate performance, resistivity, and air stability of the positive electrode active material, resulting in a substantial improvement in the overall performance of the battery.

[0068] Furthermore, in the positive electrode active material, the mass percentage of the first carbon coating layer can be 0.05% to 0.85%, for example, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.85%, or any combination thereof, which helps to further improve the specific capacity, rate performance, resistance, and air stability of the positive electrode active material, and further improve the overall performance of the battery.

[0069] In the positive electrode active material, the mass percentage of the second carbon coating layer can be 0.425% to 1.35%, for example, 0.425%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.35%, or any combination thereof. This helps to further improve the specific capacity, rate performance, resistance, and air stability of the positive electrode active material, and further improve the overall performance of the battery.

[0070] The compaction density of the above-mentioned positive electrode active material can be ≥2.0 g / cm³. 3This helps improve the specific capacity of the positive electrode active material, thereby improving the energy density of the battery.

[0071] In some embodiments, the compaction density of the positive electrode active material is 2.0–2.4 g / cm³. 3 For example, 2.0, 2.1, 2.2, 2.3, 2.4 g / cm³ 3 A range of either or both of these can help improve the specific capacity of the positive electrode active material, thereby improving the energy density of the battery.

[0072] In some embodiments, I in the Raman spectrum of the above-mentioned positive electrode active material D with I G The ratio is 1.100 to 1.410. Among them, I D The above-mentioned positive electrode active material is in the range of 1250-1450 cm⁻¹ -1 Peak intensity at I G The above-mentioned positive electrode active material is in the range of 1550-1650 cm⁻¹ -1 Peak intensity at I D with I G The ratio of I to β can characterize the degree of graphitization of the positive electrode active material. The degree of graphitization is an indicator of the graphite content in the positive electrode active material. For carbon materials with a high degree of graphitization, their conductivity and mechanical strength are higher. D with I G The ratio (I) D / I G The smaller the value of I, the higher the degree of graphitization of the positive electrode active material, and the better its conductivity. In the Raman spectrum of the positive electrode active material of this invention embodiment, I... D with I G The ratio of 1.1 to 1.4 indicates that the positive electrode active material of the present invention has a high degree of graphitization and good conductivity.

[0073] In the specific implementation process, the above-mentioned I D The corresponding peak is usually at 1350 cm⁻¹ -1 The above I appears at the location. G The corresponding peak is usually at 1580 cm⁻¹ -1 It appears in the location.

[0074] The discharge specific capacity of the positive electrode active material in this embodiment of the invention can be greater than or equal to 103 mAh / g.

[0075] The above-mentioned discharge specific capacity is detected by a method including at least the following processes:

[0076] 1) Mix the positive electrode active material, acetylene black, and binder (polyvinylidene fluoride) in a mass ratio of 95:3:2 to form a paste. Fix the scraper at 230mm (so that the coating thickness of the dried electrode sheet is 180mm) and coat it evenly on 0.018mm aluminum foil. Then dry it in an oven at 50℃ for 12h to obtain the positive electrode sheet. Then place it in a vacuum drying oven for 10h. Press the dried electrode sheet into a sheet with a pressure of 15MPa to ensure uniform electrode sheet density. After pressing, the electrode sheets are cut into the required size (e.g., 12mm diameter circles) to serve as the positive electrode sheets for subsequent use. Then, the positive electrode sheet (NFPP electrode sheet), negative electrode sheet (sodium metal sheet), glass fiber separator, and electrolyte (1mol / L NaPF, solvents including EC and DEC) are placed in sequence in a glove box (inert atmosphere). Finally, the positive electrode shell, positive electrode sheet, separator, electrolyte, negative electrode sheet, gasket, and spring sheet are placed in sequence. Finally, the 316 steel coin cell shell is placed on top to encapsulate the coin cell.

[0077] 2) After the button cell battery was left to stand at 25°C for 4 hours, the first charge-discharge specific capacity test was performed. The test conditions were: charge to 4V at 0.1C, then charge at constant voltage to 0.025C cutoff, then stand for 3 minutes, and then discharge to 2V at 0.1C. The charge-discharge curves were obtained, and the first discharge specific capacity from 2 to 4V was obtained respectively.

[0078] This invention also provides a method for preparing the above-mentioned positive electrode active material, comprising: performing a first sintering on a first raw material system containing a sodium source, an iron source, a phosphorus source, and a first carbon source to obtain a first material; and performing a second sintering on a second raw material system including the first material and a second carbon source to obtain the positive electrode active material.

[0079] According to research and analysis, the first carbon source helps improve the dispersion effect of the first raw material system, promotes the diffusion of elements such as sodium, iron, and phosphorus, helps suppress the formation of impurities and impurity phases, and forms a first carbon coating layer on the surface of primary particles. The second carbon source can form a uniform and dense second carbon coating layer on the surface of the positive electrode active material. The second carbon coating layer and the first carbon coating layer form a multi-dimensional network structure carbon coating layer (network carbon coating structure) in the positive electrode active material, which is a network structure with high conductivity. Thus, a positive electrode active material with multi-dimensional network structure carbon coating is obtained, which effectively improves the electronic conductivity and specific capacity of the positive electrode active material, improves its air stability, and reduces resistance. This helps to improve the specific capacity, rate performance, air stability, and resistance of the battery, and significantly improves the overall performance of the battery.

[0080] The above preparation method is simple to operate and suitable for industrialization.

[0081] In some embodiments, the first raw material system further includes a metal dopant source.

[0082] The aforementioned metal doping sources provide doping metal elements for the positive electrode active material, specifically including one or more of the following: titanium dioxide, titanium chloride, ilmenite, manganese acetate, manganese carbonate, manganese hydroxide, zinc acetate, zinc carbonate, zinc sulfate, copper nitrate, copper sulfate, copper oxide, nickel nitrate, nickel sulfate, nickel hydroxide, cobalt chloride, cobalt oxalate, cobalt carbonate, chromium oxide, chromium chloride, chromium sulfate, chromium nitrate, chromium sulfide, chromium hydroxide, vanadium pentoxide, vanadium oxide, metavanadate, orthovanadate, magnesium oxide, magnesium hydroxide, magnesium chloride, aluminum chloride, silver oxide, silver nitrate, chromium oxide, chromium chloride, niobium oxide, and molybdenum oxide.

[0083] In some embodiments, the first carbon source may include small-molecule organic matter, and the second carbon source may include polymers. The first carbon source, including small-molecule organic matter, helps improve the dispersion effect of the first raw material system, promoting the uniform distribution of metal elements provided by the metal doping source in the first material (precursor), and subsequently in the positive electrode active material. This improves the electronic conductivity of the positive electrode active material while suppressing the generation of impurities and impurity phases, and forming a first carbon coating layer on the surface of the primary particles. The second carbon source, including polymers, can form a uniform and dense second carbon coating layer on the surface of the positive electrode active material. The second carbon coating layer and the first carbon coating layer form a multi-dimensional network structure carbon coating layer (network carbon coating structure) in the positive electrode active material, a network structure with highly efficient conductivity. This results in a uniformly doped positive electrode active material with a multi-dimensional network carbon coating, effectively improving the electronic conductivity and specific capacity of the positive electrode active material, improving its air stability, and reducing resistance. This helps improve the battery's specific capacity, rate performance, air stability, and resistance, significantly improving the overall performance of the battery.

[0084] The aforementioned first carbon source (first polymer additive) may include one or more of isopropanolamine, isobutanolamine, and hydroxyethylhydrazine, which helps to form a first carbon coating layer on the surface of primary particles, thereby protecting the positive electrode active material. It can also reduce iron ions, ensuring that iron exists in the electrochemically active divalent form. In addition, the aforementioned first carbon source helps to improve the dispersion effect of the raw material system and the uniform distribution effect of the doped metal elements, thereby helping to improve the electronic conductivity and specific capacity of the positive electrode active material, improve its air stability, and reduce resistance. This helps to improve the battery's specific capacity, rate performance, air stability, and resistance, thus significantly improving the overall performance of the battery.

[0085] The aforementioned second carbon source (second polymer additive) may include one or more of conductive polymers (CPs), organic covalent polymers-carbon-based polymers (COPs), free radical polymers (RPs), sulfur-containing polymers (SPs), and imino polymers (IPS), which help improve the effect of carbon coating, enhance the electronic conductivity and specific capacity of the positive electrode active material, improve its air stability, and reduce resistance, thereby helping to improve the battery's specific capacity, rate performance, air stability, and resistance, and significantly improve the overall performance of the battery.

[0086] Specifically, the sodium source may include one or more of sodium acetate, sodium carbonate, sodium phosphate, sodium hydroxide, sodium pyrophosphate, disodium hydrogen phosphate, and sodium dihydrogen phosphate; the iron source may include one or more of iron oxide, ferrous oxalate, ferrous phosphate, ferric phosphate, ferrous sulfate, ferric phosphate, and ferric hydrogen phosphate; and the phosphorus source may include one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, sodium dihydrogen phosphate, sodium phosphate, and sodium pyrophosphate.

[0087] The temperature of the first sintering can be 200 to 350°C, for example, 200, 250, 300, 350°C or any combination thereof, and the time of the first sintering can be 4 to 5 hours, for example, 4, 4.5, 5 hours or any combination thereof, which helps to form a structurally stable positive electrode active material precursor (first material).

[0088] The second sintering temperature can be 500–600℃, for example, 500, 550, 600℃ or any combination thereof, and the second sintering time can be 6–12h, for example, 6, 7, 8, 9, 10h or any combination thereof. This helps to form a multidimensional network carbon coating layer with high conductivity, improves the electronic conductivity and specific capacity of the positive electrode active material, improves its cycle stability and environmental stability, and reduces resistance. This helps to improve the battery's specific capacity, cycle performance, rate performance and resistance, and significantly improves the battery's overall performance.

[0089] The synthesis process of the above preparation method may include steps such as preparation of precursor slurry, preparation of precursor powder, preparation of slurry dispersion and coating, and preparation of finished product, as shown in Figure 6.

[0090] In specific implementation, the first raw material system containing sodium source, iron source, phosphorus source, and first carbon source (polymer additive 1) can be mixed first, then water can be added for dispersion, and then ground (e.g., sand milling) to form a uniform precursor slurry (a slurry with high solid content), thus completing the preparation of the precursor slurry; then the above precursor slurry is dried to remove water (e.g., dried at around 100°C), and then the dried and dehydrated material is ground until the particle size is basically uniform to remove caking and lumps, and then a first sintering (calcination) is carried out in a protective atmosphere to obtain the first material (i.e., a uniformly distributed doped iron-based pyrophosphate material or precursor powder), thus completing the preparation of the precursor powder; then the first material is then added to the mixture. The second raw material system of the second carbon source (polymer additive 2) and a polar solvent (e.g., ethanol) are mixed and prepared into a slurry under water bath heating and stirring conditions of 35-65°C. The stirring speed can be 1000-1400 rpm, for example, 1000, 1100, 1200, 1300, 1400 rpm or any combination thereof, and the stirring time can be 4-8 h, for example, 4, 5, 6, 7, 8 h or any combination thereof, to complete the preparation of the slurry. Then, the slurry is dried (e.g., spray dried) and subjected to a second sintering (high temperature sintering) in a protective atmosphere to obtain the positive electrode active material of the present invention.

[0091] As can be seen from the above, the first raw material system can also include metal doping sources (such as M1 source and M2 source). Correspondingly, the above preparation method can be used to prepare a positive electrode active material doped with metal elements.

[0092] By introducing doped metal elements during the precursor synthesis process, the metal elements and other elements are uniformly distributed in the positive electrode active material, which helps to reduce the formation of impurities and impurity phases. By adding a first carbon source (polymer additive) as a dispersant, carbon source and reducing agent, a core with uniform doped element distribution and a certain carbon network coating is synthesized. By adding a second carbon source to form a second coating layer, a multidimensional network carbon coating structure is formed, which optimizes the carbon coating effect on the surface of the positive electrode active material and realizes the network distribution of the carbon coating layer on a multidimensional scale. A uniformly doped positive electrode active material with a multidimensional network carbon coating structure is obtained. The uniformly distributed doped metal elements and the network carbon coating structure in the positive electrode active material synergistically enhance each other, improve the specific capacity and air stability of the positive electrode active material, and reduce its resistance. The positive electrode sheet prepared by this positive electrode active material has a high specific capacity and compaction density, thereby effectively improving the specific capacity, air stability and resistance of the battery, and also helping to improve the rate performance of the battery.

[0093] In the above preparation method system, the first carbon source (first polymer additive) may include one or more of isopropanolamine, isobutanolamine, and hydroxyethylhydrazine. The first carbon source helps to prepare a precursor slurry with high solids content and uniform dispersion. Compared with conventional precursor sand milling slurry (solids content of 20%–35%) without the first carbon source, the solids content of the precursor slurry with the first carbon source can reach 30%–50%. Even with high solids content, it can still achieve efficient dispersion, ensuring uniform dispersion (distribution) of metal elements in the precursor slurry. Furthermore, the first carbon source (first polymer additive), as a carbon-containing substance, can reduce iron elements, reducing the generation of impurities and other phases. Its addition amount is low and does not hinder particle contact between the main materials (precursor or first material), making the molded material (precursor or first material) more compact. In addition, after the first sintering, it exists as a carbon network structure in the precursor material (first material), achieving a triple effect.

[0094] The precursor material (first material) has a core-shell structure, including a core and a carbon coating layer located on the surface of the core portion.

[0095] Furthermore, in the above preparation method system, the second carbon source (second polymer additive) may include one or more of conductive polymers (CPs) (e.g., polyacetylene), organic covalent polymers-carbon-based polymers (COPs), free radical polymers (RPs), sulfur-containing polymers (SPs), and imino polymers (IPS). A slurry with normal solid content (20%–35%) can be obtained without fiberization. This slurry has a high viscosity, which, on the one hand, utilizes the steric hindrance effect of the polymer molecular chains to achieve dispersion, and on the other hand, it can uniformly coat the material surface to achieve entanglement, which is beneficial to improving the uniformity of coating. It is also safe to produce and has a high degree of sintering and carbonization. The resulting positive electrode active material contains almost no obvious carbon rods. The carbon sources form a graphitized network structure coating the surface of the positive electrode active material, combining with the carbon coating layer formed by the first carbon source to build a three-dimensional multi-dimensional network carbon coating structure (shell) of the positive electrode active material. The positive electrode active material as a whole exhibits more uniform distribution of doped metal elements and better conductivity.

[0096] The positive electrode active material has a two-layer core-shell structure, including a secondary particle core and a second carbon coating layer on the surface of the secondary particle core. The secondary particle core includes primary particles and a first carbon coating layer on the surface of the primary particles.

[0097] The aforementioned protective atmosphere may include at least one of inert gases such as argon and nitrogen.

[0098] Furthermore, the mass percentage of the first carbon source in the first raw material system can be 0.05% to 0.85%, for example, 0.05%, 0.1%, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7%, 0.8%, 0.85%, or any combination thereof, which helps to improve the specific capacity and air stability of the positive electrode active material and reduce its resistance, thereby effectively improving the battery's specific capacity, air stability, rate performance, and resistance.

[0099] Similarly, the mass percentage of the second carbon source in the second raw material system can be 0.425% to 1.35%, for example, 0.425%, 0.5%, 0.6%, 0.7%, 0.8%, 0.9%, 1.0%, 1.1%, 1.2%, 1.3%, 1.35%, or any combination thereof. This helps to improve the specific capacity and air stability of the positive electrode active material and reduce its resistance, thereby effectively improving the battery's specific capacity, air stability, rate performance, and resistance.

[0100] Understandably, the stoichiometric ratios of various raw materials in the first and second raw material systems can be calculated based on the chemical composition of the positive electrode active material, and the amount of each material to be added can be determined according to the stoichiometric ratios.

[0101] This invention also provides a positive electrode sheet, comprising the above-described positive electrode active material or a positive electrode active material prepared according to the above-described method for preparing the positive electrode active material.

[0102] Based on this positive electrode active material, the positive electrode sheet has excellent specific capacity, air stability, rate performance and low resistance, which will not be elaborated here.

[0103] In the specific preparation of the positive electrode sheet, for example, the positive active material of the present invention can be dispersed with a conductive agent and a binder in an appropriate amount of N-methylpyrrolidone (NMP) solvent, and thoroughly stirred and mixed (e.g., using a shear dispersion or degassing stirring device to mix evenly) 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 active layer comprises, by mass percentage, 70-99 wt% of positive active material, 0.5-15 wt% of conductive agent, and 0.5-15 wt% of binder, and more specifically, 80-98 wt% of positive active material, 1-10 wt% of conductive agent, and 1-10 wt% of binder.

[0104] The positive electrode 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, carbon fiber, carbon nanotubes, and conductive graphite; 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.

[0105] This invention also provides a secondary battery, including the above-mentioned positive electrode sheet.

[0106] Based on this positive electrode active material or positive electrode sheet, the secondary battery has excellent specific capacity, air stability, rate performance and low resistance, which will not be elaborated here.

[0107] As can be imagined, the secondary battery of this invention, in addition to the above-mentioned positive electrode, also includes a negative electrode, an electrolyte, and a separator.

[0108] The embodiments of the present invention are not strictly limited to the negative electrode active material in the negative electrode sheet. It can be at least one of the negative electrode active materials commonly used in sodium-ion batteries, such as hard carbon, soft carbon, titanium-based materials, metal oxides and sulfides.

[0109] The embodiments of the present invention 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 electrolyte sodium salts 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.

[0110] The embodiments of the present invention 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.

[0111] In the preparation of secondary batteries (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, the electrolyte is injected into the dried battery. The battery is then left to stand, formed, and resealed to complete the preparation of the secondary battery.

[0112] The present invention also provides an electrical device, including the above-mentioned secondary battery, wherein the secondary battery serves as the power supply for the electrical device.

[0113] The present invention will be further described below through specific embodiments and comparative examples. Unless otherwise specified, the reagents, materials and instruments used below are all conventional reagents, materials and instruments, all of which are commercially available, and the reagents and materials involved can also be synthesized by conventional synthetic methods.

[0114] Example 1

[0115] This embodiment provides a method for preparing a positive electrode active material, including:

[0116] According to Na4Fe 2.94 Mn 0.03 Sodium acetate, ferrous oxalate, ammonium dihydrogen phosphate, and manganese acetate were prepared by stoichiometry of (PO4)2P2O7 / C. Then, a first raw material system containing sodium acetate, ferrous oxalate, ammonium dihydrogen phosphate, manganese acetate, and hydroxyethyl hydrazine was mixed, wherein the mass percentage of hydroxyethyl hydrazine in the first raw material system was 0.35%. Water was then added for dispersion, and the mixture was milled to form a uniform precursor slurry. The precursor slurry was then dried at 100°C. The dried material was then ground until the particle size was basically uniform, and the lumps were removed. The material was then subjected to a first sintering under a nitrogen protective atmosphere at a temperature of 200°C for 4 hours to obtain the first material (a uniformly distributed doped iron-based pyrophosphate material).

[0117] Then, a second raw material system comprising the first material, polyacetylene (conductive polymer), and ethanol is mixed and prepared into a slurry under water bath heating and stirring conditions at 50°C. The stirring speed can be 1200 rpm and the stirring time can be 6 hours. The slurry is then spray-dried and subjected to a second sintering (high-temperature sintering) in a protective atmosphere at a temperature of 500°C for 12 hours to obtain the positive electrode active material of this embodiment. The polyacetylene content in the second raw material system is 0.85% by mass.

[0118] Example 2

[0119] This embodiment is basically the same as Embodiment 1, except that: according to Na4Fe 2.94 Ti 0.03The raw materials were prepared using a stoichiometric ratio of (PO4)2P2O7 / C, with the metal dopant source replaced by titanium dioxide instead of manganese acetate; other conditions remained unchanged.

[0120] Example 3

[0121] This embodiment is basically the same as Embodiment 1, except that: according to Na4Fe 2.94 Ni 0.03 The raw materials were prepared using the stoichiometric ratio of (PO4)2P2O7 / C, with the metal dopant source replaced by nickel hydroxide instead of manganese acetate; other conditions remained unchanged.

[0122] Example 4

[0123] This embodiment is basically the same as Embodiment 1, except that: according to Na4Fe 2.94 Mn 0.01 Ni 0.01 Ti 0.01 The raw materials were prepared using a stoichiometric ratio of (PO4)2P2O7 / C, with metal doping sources including nickel hydroxide, titanium dioxide, and manganese acetate; other conditions remained constant.

[0124] Example 5

[0125] This embodiment is basically the same as Embodiment 1, except that: according to Na4Fe 2.94 Mn 0.01 Ni 0.01 Ti 0.01 The raw materials are prepared by stoichiometry of (PO4)2P2O7 / C, wherein the metal doping sources include nickel hydroxide, titanium dioxide and manganese acetate; the mass percentage of hydroxyethyl hydrazine (HEZ) in the first raw material system is 0.05%, and the mass percentage of polyacetylene in the second raw material system is 0.55%; other conditions remain unchanged.

[0126] Example 6

[0127] This embodiment is basically the same as Embodiment 1, except that: according to Na4Fe 2.94 Mn 0.01 Ni 0.01 Ti 0.01 The raw materials are prepared using the stoichiometric ratio of (PO4)2P2O7 / C, wherein the metal doping sources include nickel hydroxide, titanium dioxide and manganese acetate; the mass percentage of hydroxyethyl hydrazine in the first raw material system is adjusted to 0.85%, and the mass percentage of polyacetylene in the second raw material system is adjusted to 1.35%; other conditions remain unchanged.

[0128] Example 7

[0129] This embodiment is basically the same as Example 1, except that isobutanolamine is used instead of hydroxyethylhydrazine; other conditions remain unchanged.

[0130] Example 8

[0131] This embodiment is basically the same as that of Embodiment 1, except that isobutanolamine and hydroxyethylhydrazine are used instead of hydroxyethylhydrazine. In Embodiment 8, the mass percentage of hydroxyethylhydrazine in the first raw material system is adjusted to 0.175%, and the mass percentage of isobutanolamine in the first raw material system is adjusted to 0.85%; other conditions remain unchanged.

[0132] Example 9

[0133] This embodiment is basically the same as Embodiment 1, except that polyphenylene sulfide (a sulfur-containing polymer) is used instead of polyacetylene (a conductive polymer); other conditions remain unchanged.

[0134] Example 10

[0135] This embodiment is basically the same as Embodiment 1, except that polyphenylene sulfide (sulfur-containing polymer) and polyacetylene (conductive polymer) are used instead of polyacetylene (conductive polymer). In Embodiment 10, the mass percentage of polyacetylene in the second raw material system is adjusted to 0.425%, and the mass percentage of polyphenylene sulfide (sulfur-containing polymer) in the second raw material system is adjusted to 0.425%; other conditions remain unchanged.

[0136] Example 11

[0137] This embodiment is basically the same as Embodiment 1, except that: according to Na4Fe 2.94 Ti 0.02 Cr 0.01 The raw materials are prepared using a stoichiometric ratio of (PO4)2P2O7 / C, with the metal dopant sources including titanium dioxide and chromium chloride; other conditions remain unchanged.

[0138] Example 12

[0139] This embodiment is basically the same as Embodiment 1, except that: according to Na4Fe 2.93 Ti 0.02 Cr 0.02 The raw materials are prepared using a stoichiometric ratio of (PO4)2P2O7 / C, with the metal dopant sources including titanium dioxide and chromium chloride; other conditions remain unchanged.

[0140] Example 13

[0141] This embodiment is basically the same as Embodiment 1, except that:

[0142] According to Na4Fe 2.96 Mn 0.01 The raw materials are prepared using a stoichiometric ratio of (PO4)2P2O7 / C, with manganese acetate serving as the metal dopant source; other conditions remain unchanged.

[0143] Example 14

[0144] This embodiment is basically the same as Embodiment 1, except that:

[0145] According to Na4Fe 2.94 Ti 0.03 Cr 0.02 The raw materials are prepared using a stoichiometric ratio of (PO4)2P2O7 / C, with the metal dopant sources including titanium dioxide and chromium chloride; other conditions remain unchanged.

[0146] Example 15

[0147] This embodiment is basically the same as Embodiment 1, except that the mass percentage of the first carbon source in the first raw material system is 0.03%; other conditions remain unchanged.

[0148] Example 16

[0149] This embodiment is basically the same as Embodiment 1, except that the mass percentage of the first carbon source in the first raw material system is 0.90%; other conditions remain unchanged.

[0150] Example 17

[0151] This embodiment is basically the same as Embodiment 1, except that the mass percentage of the second carbon source in the second raw material system is 0.350%; other conditions remain unchanged.

[0152] Example 18

[0153] This embodiment is basically the same as Embodiment 1, except that the mass percentage of the second carbon source in the second raw material system is 1.45%; other conditions remain unchanged.

[0154] Example 19

[0155] This embodiment is basically the same as Embodiment 1, except that:

[0156] The first carbon source has a mass percentage of 0.03% in the first raw material system, and the second carbon source has a mass percentage of 0.40% in the second raw material system; other conditions remain unchanged.

[0157] Example 20

[0158] This embodiment is basically the same as Embodiment 1, except that:

[0159] The first carbon source has a mass percentage of 0.90% in the first raw material system, and the second carbon source has a mass percentage of 1.40% in the second raw material system; other conditions remain unchanged.

[0160] Example 21

[0161] This embodiment is basically the same as Embodiment 1, except that:

[0162] According to Na4Fe 2.965 Mn 0.005 The raw materials are prepared using a stoichiometric ratio of (PO4)2P2O7 / C, with manganese acetate serving as the metal dopant source; other conditions remain unchanged.

[0163] Example 22

[0164] This embodiment is basically the same as Embodiment 1, except that: according to Na4Fe 2.91 Mn 0.06 The raw materials are prepared using a stoichiometric ratio of (PO4)2P2O7 / C, with manganese acetate serving as the metal dopant source; other conditions remain unchanged.

[0165] Example 23

[0166] This embodiment is basically the same as Embodiment 1, except that:

[0167] According to Na4Fe 2.97 Prepare raw materials using the stoichiometric ratio of (PO4)2P2O7 / C; keep other conditions constant.

[0168] Example 24

[0169] This embodiment is basically the same as Embodiment 1, except that:

[0170] According to Na4Fe 2.96 Mn 0.02 The raw materials were prepared using a stoichiometric ratio of (PO4)2P2O7 / C, with manganese acetate serving as the metal dopant source; other conditions remained constant. Comparative Example 1

[0171] This comparative example provides a method for preparing a positive electrode active material, which is carried out according to the conventional process for synthesizing carbon-doped sodium iron phosphate pyrophosphate (NFPP@C), specifically including:

[0172] Preparation of S1 precursor slurry: Sodium acetate, ferrous oxalate and ammonium dihydrogen phosphate are mixed and then dispersed with water to form a uniform precursor slurry;

[0173] Preparation of S2 precursor powder: The above precursor slurry was dried at 100°C, then ground until the particle size was basically uniform, and the caking lumps were removed. Then it was calcined in a nitrogen protective atmosphere (200°C, holding time 4h) to obtain the first material.

[0174] S3 conventional doping and coating preparation: The first material, manganese acetate, and citric acid are mixed and dispersed in deionized water to prepare a slurry; wherein, the mass ratio of citric acid to the total mass of the first material, manganese acetate, and citric acid is 1.5%;

[0175] S4 Product Preparation: The slurry obtained from S3 is spray-dried and then sintered at high temperature in a nitrogen protective atmosphere at 500℃ for 12 hours to obtain the positive electrode active material.

[0176] Among them, the molecular formula of the positive electrode active material is Na4Fe 2.92 Mn 0.05 The feed was prepared in a stoichiometric ratio of (PO4)2P2O7 / C.

[0177] Comparative Example 2

[0178] This comparative example provides a method for preparing a positive electrode active material, which is carried out according to the conventional process for synthesizing carbon-doped sodium iron phosphate pyrophosphate (NFPP@C), specifically including:

[0179] Preparation of S1 precursor slurry: Sodium acetate, ferrous oxalate, ammonium dihydrogen phosphate and manganese acetate are mixed and then dispersed in water to form a uniform precursor slurry;

[0180] Preparation of S2 precursor powder: The above precursor slurry was dried at 100°C, then ground until the particle size was basically uniform, and the caking lumps were removed. Then it was calcined in a nitrogen protective atmosphere (200°C, holding time 4h) to obtain the first material.

[0181] Preparation of S3 coating slurry: The first material obtained from S2, hydroxyethyl hydrazine (polymer additive 1), conductive polymer-polyacetylene (polymer additive 2), and ethanol are mixed to obtain a second raw material system. The second raw material system is then prepared into a slurry under water bath heating and stirring conditions at 50°C. The stirring speed can be 1200 rpm and the time can be 6 hours. Then, the slurry is spray-dried and subjected to a second sintering (high-temperature sintering) in a protective atmosphere at a temperature of 500°C for 12 hours to obtain the positive electrode active material of this embodiment. The mass ratio of hydroxyethyl hydrazine (HEZ) to the dry matter in the second raw material system can be 0.35%, and the mass ratio of polyacetylene to the dry matter in the second raw material system can be 0.85%.

[0182] Among them, the molecular formula of the positive electrode active material is Na4Fe 2.92 Mn 0.05 The feed was prepared in a stoichiometric ratio of (PO4)2P2O7 / C.

[0183] Experimental Example 1

[0184] The following parameters of the positive electrode active materials of each embodiment and comparative example were tested:

[0185] 1) Uniform elemental distribution: Energy dispersive spectroscopy (EDS) was used for testing and analysis;

[0186] 2) Average thickness of the first and second carbon coating layers: Transmission electron microscopy (TEM) uses an electron beam to bombard the sample and collects the properties of the beam penetrating the sample for imaging. It has a higher resolution, reaching the nanometer level, and can be used to distinguish crystal conditions and carbon layer thickness. In this embodiment of the invention, the JEM-3000 equipment is used to test the average thickness of the first and second coating layers. The specific testing process includes: during the preparation of the positive electrode active material, the average thickness of the carbon layer of the first material (whose average carbon layer thickness is close to or almost the same as the average thickness of the first carbon coating layer in the positive electrode active material) is tested using the above method (which can be approximated as the average thickness of the first carbon coating layer). Then, the average thickness of the carbon coating layer of the positive electrode active material (the sum of the first and second carbon coating layers) is tested using the above method. Then, the average thickness of the carbon coating layer of the positive electrode active material (the sum of the first and second carbon coating layers) is subtracted from the average thickness of the carbon layer of the first material (which can be approximated as the thickness of the first carbon coating layer) to obtain the average thickness of the second coating layer. Specifically, the average thickness can be obtained by measuring the thickness of different regions and then taking the average value.

[0187] 3) Degree of graphitization: This embodiment of the invention utilizes Raman spectroscopy fitting to calculate the degree of graphitization of the material. The specific operation includes uniformly pressing the positive electrode active material powder onto a glass slide, then using a Thermo Scientific DXR2xi high-speed micro-Raman imaging spectrometer to measure the peak intensity (ID) of the D peak and the peak intensity (IG) of the G peak, and comparing them to obtain the ID / IG value. The D and G peaks are both Raman characteristic peaks of C atomic crystals, located at 1250–1450 cm⁻¹, respectively. -1 and 1550~1650cm -1 The D peak represents the defects in the C atom lattice, and the G peak represents the in-plane stretching vibration of the C atom sp2 hybridization. Generally, the smaller the ID / IG ratio, the higher the degree of graphitization of the positive electrode active material and the better its conductivity.

[0188] 4) Compacted density: The compacted density of the positive electrode active material was measured in accordance with GB / T 44330-2024 "Determination of compacted density of lithium-ion battery positive electrode material powder". The pressure used during the compaction test was 3T.

[0189] 5) Resistance: The positive electrode active material, acetylene black, and binder (polyvinylidene fluoride) of each embodiment and comparative example are mixed in a mass ratio of 95:3:2 and made into a paste. The paste is then uniformly coated onto a polyester film (PET film) with a fixed scraper of 230mm (so that the thickness of the electrode coating after drying is 180mm). The resistance (electrode resistivity) is then tested using an FT-331 four-probe sheet resistance resistivity tester.

[0190] 6) Carbon content (mass percentage) in the positive electrode active material: This was determined using an HCS-140 instrument according to YS / T1028.4-2015 "Chemical Analysis Methods for Lithium Iron Phosphate Part 4: Determination of Carbon Content - High-Frequency Combustion Infrared Absorption Method".

[0191] 7) Mass percentage of doped metal elements in the positive electrode active material (doping amount): The content was determined by inductively coupled plasma atomic emission spectrometry (ICPE-9820) according to YS / T1006.2-2014 "Chemical analysis method for lithium nickel cobalt manganese oxide - Part 2: Determination of the content of lithium, nickel, cobalt, manganese, sodium, magnesium, aluminum, potassium, copper, calcium, iron, zinc and silicon".

[0192] Test results

[0193] Table 1. Degree of graphitization (I) D / I G Carbon content, doping amount, and coating thickness

[0194] In the positive electrode active materials of each embodiment, the carbon coating layer is uniformly distributed (coated) on the entire surface of the positive electrode active material, while the carbon coating layer of Comparative Example 1 is distributed in an island-like pattern.

[0195] The overall performance of the positive electrode active material and battery in Comparative Example 2 is superior to that in Comparative Example 1. This is mainly because the carbon coating synthesis process used in Comparative Example 2 is different from that in Comparative Example 1. The carbon coating in Comparative Example 2 is more complete and uniform, which will promote a more complete degree of graphitization, reflected in I. D / I G The lower value is beneficial to improving the overall electronic conductivity of the positive electrode active material, which is reflected in higher rate performance. In addition, the integrity of the coating is also beneficial to improving air stability, reducing the contact between the internal crystal material structure of the coating layer and air and moisture, and reducing the risk of changes in the crystal structure caused by moisture absorption. This is reflected in the improved specific capacity retention rate after 3 days of storage.

[0196] Example 4, I D / I G Example 2 of I D / I G Example 1 of I D / IG Example 3, Part I D / I G <Comparative Example 2 I D / I G This indicates that the positive electrode active material (NFPP@C material) with a specific structure prepared according to the preparation method of the present invention has a better overall carbonization effect and a higher degree of graphitization. The reasons are as follows: on the one hand, it benefits from the improvement of process conditions; on the other hand, the polymer additives help to disperse the raw material system more uniformly and are more conducive to carbonization during high-temperature sintering, and more tightly wrap the main particles, which is more conducive to forming a completely wrapped and highly conductive network, thus improving electronic conductivity. In addition, the metal ion doping in the above-mentioned positive electrode active material structure is more uniform, which also improves the stability of the overall structure and is conducive to improving ionic conductivity.

[0197] Examples 7, 8, 9, and 10, compared to Example 4, use the same process but with different types of carbon source (auxiliary agent). It can be observed that changing the type of auxiliary agent 1 has little impact on carbonization and electrical properties. This is mainly because auxiliary agent 1 functions in three ways: 1) as a reducing agent, 2) as a conductive network for primary interparticle crystal growth, and 3) as a compatibility effect of primary particles. Ensuring the same dosage guarantees sufficient reduction of ferric iron in the structure, and the formation of a conductive network and a similar particle compatibility effect (which can be seen from cross-sectional SEM).

[0198] Examples 11, 12, and 14 are preferred ratios. By doping elements, the electronic conductivity is improved, resulting in lower electrode resistance. The multi-element doping and ratio control also help improve the electrochemical platform and electronic conductivity, as well as the structural stability at different rates. This is reflected in the best specific capacity retention at 2C rate. The carbon coating layer is also more uniform and complete, and its air stability is better, as reflected in the 3-day specific capacity.

[0199] In Examples 1-4 and 13 / 24, the preferred first carbon source and second carbon source (auxiliary agent) are used. Changing the doping element and doping ratio has little effect on the degree of graphitization, mainly at the electrochemical level (such as specific capacity).

[0200] Examples 5 and 6 are small-volume and large-volume carbon-coated samples of preferred first and second carbon sources (auxiliaries), indicating that reducing the carbon content in the preparation method system of the present invention will not affect the degree of carbonization. On the contrary, the degree of carbonization under the preparation method system with high carbon content will be incomplete. This should be mainly caused by the thickening of the carbon layer on the surface of the positive electrode active material, but it still has an improvement effect compared with the comparative example.

[0201] Examples 15-20 involve adjusting the preferred amounts of the first and second carbon sources (additives) within a certain range. Insufficient amounts of the first or second carbon source can slightly degrade the electronic conductivity network and result in insufficient reduction of ferric iron, leading to a slight decrease in the performance of the positive electrode active material and the battery. Excessive amounts of the first or second carbon source will have the opposite effect. While ensuring complete reduction of ferric iron, it will increase the number of carbon dots between primary particles, reducing primary particle compatibility and creating more voids, which is detrimental to increasing ion shuttle distance and thus reducing performance.

[0202] Examples 21 and 22 involve controlling the doping amount at a low or high level. A low doping amount will result in an insignificant doping effect, while an excessive doping amount will easily lead to crystal structure collapse, making the positive electrode active material or battery cycle more unstable, and deteriorating electrochemical performance and air stability.

[0203] The positive electrode active material in Example 23 was not doped with any metal elements.

[0204] Analyzing Figures 1-4, Comparative Example 1 employs a conventional doping and coating process, i.e., first preparing sodium iron pyrophosphate phosphate and then doping and coating with citric acid as a carbon source. Comparative Example 2 involves simultaneous doping during the liquid-phase reaction for preparing sodium iron pyrophosphate phosphate, followed by liquid-phase coating with hydroxyethyl hydrazine and polyacetylene. Figures 1 and 2 show that the carbon coating in Comparative Example 2 is more uniformly distributed on the surface of the positive electrode active material particles compared to the traditional carbon source coating in Comparative Example 1. Examples 1 and 4 both involve doping during the preparation of sodium iron pyrophosphate phosphate (precursor) and coating in liquid phase. In contrast, carbon coating is carried out during the reaction process. As can be seen from Figures 3 and 4, the above process helps to improve the distribution of metal elements, making them uniformly distributed, and also improves the coating effect of the carbon coating layer. In addition, the surface morphology of the positive electrode active material observed by scanning electron microscopy (SEM) in Figures 1-4 shows that the preparation method of the present invention yields a positive electrode active material with higher surface density, which helps to improve its compaction density. The uniform distribution of doped metal elements and the carbon coating with a multi-dimensional network structure in the positive electrode active material help to improve the specific capacity, resistance, rate performance and stability of the battery.

[0205] In each embodiment, a good three-dimensional conductive network structure can be formed on the surface of the positive electrode active material.

[0206] The area circled by the curve in Figure 5 represents the carbon coating layer (first carbon coating layer and second carbon coating layer), and the thickness here is the sum of the thicknesses of the first and second carbon coating layers. Analyzing Figure 5, the second carbon coating layer in Comparative Example 1 shows island-like carbon clusters, while the second carbon coating layer in Comparative Example 2 exhibits overall and relatively uniform carbon coating. It can be seen that the process in Comparative Example 2 is more conducive to the uniform and extensive coating of the carbon source compared to the process in Comparative Example 1, and has a more complete coating effect. Compared with Examples 1 and 4, it can be seen that the process in the embodiments of the present invention has a significant improvement on the carbon coating layer structure, with uniform carbon coating and more graphitized crystal forms, and the overall coating layer thickness is close to 20 nm.

[0207] Figure 8 was analyzed using CP & SEM testing: CP refers to argon ion polishing technology, used to characterize the carbon coating layer of the first material (precursor dried sample) and the finished positive electrode active material in Example 1. Figure 8 shows that after one sintering (first sintering), a filamentous adhesion network exists between the primary particles of the first material, indicating that the hydroxyethyl hydrazine (polymer additive 1) is distributed on the surface and between the primary particles after the first drying and sintering. In the finished positive electrode active material sample after the second sintering (second sintering), the adhesion filamentous network between the primary particles and the coating surface is reduced, but still exists. This is mainly due to complete high-temperature carbonization, increased graphitization of the coating layer, and reduced volume. Furthermore, the primary particles dissolve each other during the second sintering process, leading to a reduction in the thickness of the first coating layer. Figure 8 shows the thickness of the first coating layer. The thickness is generally between 5 and 10 nm. The second coating layer can be compared with the carbon coating layer (first carbon coating layer and second carbon coating layer) in Example 1 in Figure 5. It can be seen that the thickness of the second coating layer is greater than that of the first coating layer. This is mainly due to the different functions and proportions of the first and second coating layers. Specifically, the first coating layer mainly plays the roles of high dispersion, high solid content, forming a three-dimensional conductive network between primary particles (establishing a conductive network inside the overall particles), and acting as a reducing agent. Its usage proportion is significantly lower than that of the second coating layer. The second coating layer has the effect of improving the overall conductivity and improving the material rate and environmental stability.

[0208] Table 2 Compacted Density

[0209] Analyze the compaction density data in Table 2:

[0210] The compaction density of Comparative Example 2 is greater than that of Comparative Example 1, which means that the coating of the first carbon source and the second carbon source is more compact than the traditional carbon source coating, reducing the gaps between carbon coating layers and between the carbon coating layer and the dopant.

[0211] The compaction density of Example 4 > the compaction density of Example 2 > the compaction density of Example 1 > the compaction density of Example 3 > the compaction density of Comparative Example 2, indicating that the NFPP@C material with a specific structure prepared by the preparation method system of the embodiments of the present invention has a denser overall structure and fewer spatial gaps. In addition, it can reduce the total amount of carbon coating without deteriorating or even improving the conductivity. Examples 4, 5, and 6 demonstrate the effect of carbon content control on compaction density. Carbon content affects the contact and fusion between primary particles during secondary sintering. A lower carbon content (such as in Example 5) means fewer carbon barrier points at the primary particle interface, which is conducive to the direct contact and fusion growth of primary particles during sintering, thereby making the internal structure of secondary particles more compact, which is reflected in the increase of material compaction density. Conversely, a higher carbon content (as in Example 6) will form more carbon layer gaps at the primary particle interface, which to some extent hinders the direct fusion of particles and results in insufficient contact between particles. At the same time, since the true density of the carbon layer itself is lower than that of inorganic active materials, its increased proportion will also lead to a decrease in the overall compaction density of the powder. In other words, the reduction in carbon coating will also reduce the physical contact between carbon materials and the main material. During the sintering process, the particles are more likely to cluster and form tightly without obstruction, thereby improving the compactness between the main materials (primary particles) and reflecting the high compaction density of the overall material.

[0212] Examples 4, 5, and 6 demonstrate the effect of carbon content control on powder compaction. In addition to acting as a reducing agent and coating layer, the carbon source also affects the particle contact effect and crystal growth during sintering. Low carbon content (Example 5) affects the contact points (carbon source) for crystal growth, resulting in denser particle growth and increased compaction density. Conversely, high carbon content (Example 6) presents a large number of contact points that hinder crystal growth, leading to looser contact between particles. Furthermore, the carbon layer itself has low density, and the higher its proportion, the lower the overall density of the material.

[0213] Examples 7, 8, 9, and 10, compared to Example 4, use the same process but with different types of carbon sources. The impact on compaction is relatively small, and they are all significantly better than Comparative Example 1 and Comparative Example 2. It can also be considered that changing the type of carbon source can form a better three-dimensional conductive network structure, and also has a positive effect on the crystal growth of NFPP@C.

[0214] Compared with Example 4, Examples 11, 12, 13, and 14 use the same process but change the type and proportion of dopant sources, which has little impact on compaction and is significantly better than Comparative Example 1 and Comparative Example 2.

[0215] The compaction density data for Examples 15 to 22 ranged from 2.17 g / cm³ to 2.23 g / cm³. Overall, the compaction density of these examples was generally higher than that of Comparative Example 1 (2.108 g / cm³). In particular, the compaction density of most examples (e.g., 15-21) (2.19-2.23 g / cm³) was significantly better than that of Comparative Example 2 (2.17 g / cm³), while the compaction density of Example 22 (2.17 g / cm³) was essentially the same as that of Comparative Example 2. This indicates that when the amount of carbon source or doping exceeds the preferred range, its effect on improving compaction density may no longer be significant, or even tend to be consistent. This trend precisely illustrates the necessity of controlling the relevant parameters within the preferred range.

[0216] Data from Examples 1 and 23 show that doping with metal elements helps to further improve compaction density.

[0217] Table 3 Electrode Resistivity

[0218] Analyze the electrode resistivity (resistance) data in Table 3:

[0219] Comparative Examples 1 and 2, using conventional processes, exhibited low graphitization and high electrode resistivity. In contrast, the embodiments of the present invention, by constructing a multidimensional carbon network structure, significantly improved the graphitization of the material and simultaneously reduced the electrode resistivity. This indicates that, after adopting the carbon coating structure of the present invention, the increased graphitization is one of the key factors in reducing resistance. Further observation of Examples 1-4 reveals that while the doping elements in their chemical formulas differ, the electrode resistivity shows a trend correlated with the degree of graphitization. For example, Example 4 has the lowest degree of graphitization and the lowest electrode resistivity, while Example 3 has the highest degree of graphitization and the highest electrode resistivity. This demonstrates that even with variations in doping elements, the degree of graphitization remains a crucial factor affecting the electronic conductivity of the material. The final resistance value exhibited by the material is the result of the synergistic effect of doping modification, carbon network structure, and degree of graphitization.

[0220] The resistance of Example 4 is less than that of Example 5, which is less than that of Example 6, indicating that the resistance of the positive electrode active material is largely correlated with its carbon content.

[0221] As can be seen from the data in Tables 1 and 3, when the doping system remains unchanged (all are Mn-doped), and only the first carbon source (Examples 7 and 8) or the second carbon source (Examples 9 and 10) is changed, the degree of graphitization and the electrode resistivity of the material show a synergistic trend. Example 4 introduces Mn, Ni, and Ti multi-component doping based on Example 1. The degree of graphitization in Example 4 is slightly better than that in Example 1, and the electrode resistivity in Example 4 is also lower than that in Example 1. This indicates that multi-component doping helps to further optimize the electronic conductivity of the material. In summary, the above analysis shows that whether optimizing the carbon source system or adjusting the doping elements, as long as the degree of graphitization can be effectively improved, the electrode resistivity can be reduced accordingly. This fully demonstrates that the high degree of graphitization of the carbon layer is the key intrinsic factor for the excellent electronic conductivity of the positive electrode active material of this invention.

[0222] Compared with Example 4, Examples 11, 12, 13, and 14 use the same process but change the type and proportion of doping sources. The doping of metals will optimize the electronic conductivity of the material to a certain extent, thereby reducing the electrode resistance. All of them are significantly better than Comparative Example 1 and Comparative Example 2.

[0223] Examples 15-22 all have a carbon network structure formed by the first carbon coating layer and the second carbon coating layer, which also promotes the uniform distribution of doped metal elements, optimizes the electronic conductivity of the material, and achieves the effect of reducing the electrode resistance. All of them are significantly better than the effects of Comparative Example 1 and Comparative Example 2.

[0224] Data from Examples 1 and 23 show that the resistance of the positive electrode active material doped with metal elements is further reduced.

[0225] Experimental Example 2

[0226] The positive electrode active material, acetylene black, and binder (polyvinylidene fluoride) of each embodiment and comparative example were mixed evenly in a mass ratio of 95:3:2 to form a paste. A fixed scraper of 230 mm was used to evenly coat the paste onto a 0.018 mm thick aluminum foil. The paste was then dried in a 50°C oven for 12 hours to obtain the positive electrode sheet. This sheet was then placed in a vacuum drying oven and dried for 10 hours. The dried electrode sheet was then pressed, typically at a pressure of 15 MPa, to ensure the electrode sheet density. Uniformly cut the compressed electrode sheets into the required size (e.g., 12mm diameter circles) to serve as the positive electrode sheets for subsequent use. Then, the positive electrode sheet (NFPP electrode), negative electrode sheet (sodium metal sheet), glass fiber separator, and electrolyte (1 mol / L NaPF, solvents including EC and DEC) are placed in sequence in a glove box (inert atmosphere). Finally, a 316 steel coin cell casing is placed on top to encapsulate the coin cell. A battery testing system with a voltage window of 2–4V is used to perform charge-discharge tests at a constant current to evaluate the 0.1C specific capacity and 2C rate performance of the positive electrode active material (NFPP material). The entire process is conducted at a dew point of -40°C.

[0227] Table 4. Charge / discharge specific capacity and initial coulombic efficiency at 0.1C.

[0228] Table 5 Rate performance at 2C

[0229] The charge / discharge specific capacity, first-efficiency performance, and rate performance of each embodiment are superior to those of the comparative example, indicating that the network carbon coating of the present invention and the uniform distribution of doped metal elements in the positive electrode active material of the network carbon coating can suppress the generation of impurities and impurity phases, and enable the large radius and high valence state of metal elements to change the band structure, thus achieving effective doping; it can make the carbon layer coating tighter, the network structure more perfect, and establish a more three-dimensional conductive network, thereby improving the charge / discharge specific capacity, first-efficiency performance, and rate performance of the battery.

[0230] Examples 4, 5, and 6 demonstrate the impact of carbon content regulation on the electronic conductivity of cathode active materials or batteries. Low carbon content inhibits electronic conductivity transport, resulting in a decrease in charge / discharge capacity and rate capability. In addition to its impact on electronic conductivity, the carbon source itself acts as a reducing agent, ensuring the reduction of ferric ions. Insufficient carbon source makes it difficult to ensure the stable existence of ferrous ions, thus reducing the amount of iron that can participate in redox reactions and causing a decrease in specific capacity. While high carbon content improves electronic conductivity and shows better performance in rate capability, materials with high carbon content reduce the proportion of cathode active material (redox raw material) to some extent, which also carries the risk of a decrease in specific capacity.

[0231] Examples 7, 8, 9, and 10, compared to Example 4, use the same process but with different carbon source types. The impact on capacity change is relatively small, and all are significantly better than Comparative Examples 1 and 2. It can also be considered that changing the carbon source type does not affect the formation of a better three-dimensional conductive network structure and has a positive effect on the crystal growth of the positive electrode active material (NFPP@C).

[0232] Compared with Example 4, Examples 11, 12, 13, and 14 use the same process but change the type and proportion of dopant sources. In addition to optimizing the electronic conductivity of the material, the doping of metals also results in a high redox potential, which improves the specific capacity and the 2C rate capacity retention rate. The electrochemical performance is significantly better than that of Comparative Example 1 and Comparative Example 2.

[0233] Examples 15-22 all have a carbon network structure formed by the first carbon coating layer and the second carbon coating layer, which also promotes the uniform distribution of doped metal elements and optimizes the charge-discharge specific capacity, first efficiency and rate performance of the material, which are significantly better than those of Comparative Example 1 and Comparative Example 2.

[0234] Although the charge / discharge specific capacity, first-efficiency and rate performance of Example 23 (without metal doping) were slightly worse than those of Example 1 (positive electrode active material with metal doping), they were still significantly better than those of Comparative Example 1 and Comparative Example 2.

[0235] Experimental Example 3

[0236] Air stability testing

[0237] After the positive electrode active material was placed in a dew point environment of -20℃ and left to stand for 3 days, its electrochemical performance was tested according to the method of Experiment Example 2. The 0.1C constant current specific capacity was tested, and the specific capacity after standing for 3 days was compared with the specific capacity before standing for 3 days (initial charging specific capacity or initial discharging specific capacity). The capacity retention rate was calculated to determine the stability of the positive electrode active material.

[0238] Table 6. Results of Air Stability Test

[0239] The air stability of each embodiment is better than that of the comparative example, indicating that the network carbon coating of the present invention and the uniform distribution of doped metal elements in the positive electrode active material of the network carbon coating can suppress the generation of impurities and impurity phases, so that the effect of the large radius and high valence state of metal elements changing the band structure can be realized, achieving effective doping, and thus synergistically improving the air stability of the positive electrode active material or battery.

[0240] The capacity loss (capacity loss = 100% - capacity retention) satisfies the following order: Example 4 < Example 2 < Example 1 < Example 3 < Comparative Example 2 < Comparative Example 1, which is consistent with the carbon coating. The positive electrode active material (NFPP material) itself is prone to absorbing moisture, resulting in an irreversible phase transition and reducing specific capacity. A well-developed carbon coating layer tightly encapsulates the main material, reducing the risk of moisture absorption and ensuring the overall stability of the material.

[0241] Examples 4, 5, and 6 demonstrate the effects of carbon content control, or more specifically, the degree and thickness of carbon coating, on stability. A lower carbon source proportion reduces the overall carbon coating effect, causing the hygroscopic NFPP crystal phase to remain on the periphery of the carbon coating, thus reducing stability. Conversely, a higher carbon content results in more complete particle coating and an increased carbon layer thickness, reducing the contact between the NFPP main phase and the external environment, thereby improving the overall material stability.

[0242] Examples 7, 8, 9, and 10, compared to Example 4, use the same process but with different carbon source types, and all show significantly better results than Comparative Examples 1 and 2. It can also be considered that changing the carbon source type does not affect the formation of a good three-dimensional conductive network structure, and a complete coating effect is achieved, positively improving the stability of the material.

[0243] Compared with Example 4, Examples 11, 12, 13, and 14 use the same process but change the type and proportion of dopant sources, which does not affect the coating effect. Therefore, they have little impact on environmental stability and their overall performance is significantly better than Comparative Example 1 and Comparative Example 2.

[0244] Examples 15-22 all have a carbon network structure formed by the first carbon coating layer and the second carbon coating layer, which also promotes the uniform distribution of doped metal elements and optimizes the air stability of the material. The effects are all better than the comparative examples.

[0245] Although the air stability of Example 23 (without metal doping) was slightly worse than that of Example 1 (with metal doping positive electrode active material), it was still significantly better than that of Comparative Example 1 and Comparative Example 2. Based on the above test results, compared to Comparative Example 1 and Comparative Example 2, the preparation method of the present invention can achieve the effects of increasing powder compaction density, improving graphitization degree (reducing electrode resistance), increasing specific capacity, and improving stability. This also proves that electrodes or batteries made with the positive electrode active material of the present invention have higher energy density, stability, and safety.

[0246] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention 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; and these 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 the present invention.

Claims

1. A positive electrode active material, characterized in that, The positive electrode active material has a core-shell structure, and the positive electrode active material includes a secondary particle core and a second carbon coating layer located on the surface of the secondary particle core. The secondary particle core includes primary particles and a first carbon coating layer located on the surface of the primary particles.

2. The positive electrode active material according to claim 1, characterized in that, The positive electrode active material includes sodium iron pyrophosphate.

3. The positive electrode active material according to claim 1 or 2, characterized in that, The positive electrode active material includes doped metal elements.

4. The positive electrode active material according to claim 3, characterized in that, The chemical formula of the positive electrode active material is Na. x Fe y M z (PO4) w (P2O7) v In this stoichiometry, x is 3.9–4.1, y+z is 2.7–3, w is 1–2, v is 0.8–1, and M includes metallic elements.

5. The positive electrode active material according to claim 4, characterized in that, M includes M1 and M2, where M1 includes one or more of manganese, chromium, vanadium, titanium, cobalt, nickel, copper, zinc, magnesium, and aluminum, and M2 includes one or more of silver, chromium, niobium, and molybdenum, and the doping amount of M1 is higher than that of M2.

6. The positive electrode active material according to claim 5, characterized in that, The sum of the mass percentages of M1 and M2 in the positive electrode active material is greater than 0% and less than 5%.

7. The positive electrode active material according to any one of claims 3-6, characterized in that, The mass percentage of the doped metal element in the positive electrode active material is 2% to 5%.

8. The positive electrode active material according to any one of claims 1-7, characterized in that, The sum of the average thicknesses of the first carbon coating layer and the second carbon coating layer is 10–35 nm; And / or, the carbon content in the positive electrode active material is 0.59% to 2.20% by mass.

9. The positive electrode active material according to any one of claims 1-8, characterized in that, The average thickness of the first carbon coating layer is 1–10 nm, and the average thickness of the second carbon coating layer is 9–25 nm.

10. The positive electrode active material according to any one of claims 1-9, characterized in that, The compaction density of the positive electrode active material is 2.0–2.4 g / cm³. 3 ; And / or, the Raman spectrum of the positive electrode active material contains I D with I G The ratio is 1.100 to 1.

410.

11. A method for preparing the positive electrode active material according to any one of claims 1-10, characterized in that, include: A first raw material system containing sodium source, iron source, phosphorus source and a first carbon source is subjected to a first sintering to obtain a first material; a second raw material system including the first material and a second carbon source is subjected to a second sintering to obtain the positive electrode active material.

12. The method for preparing the positive electrode active material according to claim 11, characterized in that, The first carbon source includes one or more of isopropanolamine, isobutanolamine, and hydroxyethylhydrazine; And / or, the second carbon source includes one or more of the following: conductive polymers, organic covalent polymers-carbon-based polymers, free radical polymers, sulfur-containing polymers, and imino polymers; And / or, the mass percentage of the first carbon source in the first raw material system is 0.05% to 0.85%; And / or, the mass percentage of the second carbon source in the second raw material system is 0.425% to 1.35%; And / or, the first raw material system further includes a metal doping source.

13. The method for preparing the positive electrode active material according to claim 11 or 12, characterized in that, The first sintering temperature is 200–350℃ and the time is 4–5 hours; And / or, the second sintering temperature is 500-600℃ and the time is 6-12h.

14. A positive electrode plate, characterized in that, The positive electrode active material includes the positive electrode active material according to any one of claims 1-10 or the positive electrode active material prepared by any one of claims 11-13.

15. A secondary battery, characterized in that, Includes the positive electrode sheet as described in claim 14.

16. An electrical appliance, characterized in that, The device includes the secondary battery as described in claim 15, wherein the secondary battery serves as the power supply for the electrical equipment.

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