Coated NFPP positive electrode material and preparation method and application thereof

Abstract

The invention belongs to the technical field of sodium ion batteries, and particularly relates to a coated NFPP positive electrode material as well as a preparation method and application thereof. The preparation method provided by the invention comprises the following steps: mixing an NFPP inner core and a composite sodium supplementing agent, and carrying out heat treatment to obtain a coated NFPP positive electrode material; the composite sodium supplementing agent comprises organic acid sodium salt, a dispersing agent and a conductive carbon material; the percentage dosage of the composite sodium supplementing agent is 3-8wt% based on the NFPP. The stable and conductive interface layer is formed through the specific composite sodium supplementing agent, interface electron conduction and ion transmission are improved, combination is firm, and falling in the electrochemical cycle process is effectively prevented; the two core functions are synergistically realized: 1, irreversible sodium loss caused by factors such as negative electrode SEI formation and the like is efficiently compensated; and 2, the NFPP core is protected, side reaction and structural degradation of the core and electrolyte are inhibited, and the reversible capacity, the first coulombic efficiency and the long cycle stability of the positive electrode material are synergistically improved.

Description

Technical Field

[0001] This invention belongs to the field of sodium-ion battery technology, specifically relating to a coated NFPP cathode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries have shown broad application prospects in the field of large-scale energy storage due to their advantages such as abundant sodium resources, low cost, and high safety.

[0003] Among them, the iron-based polyanionic compound sodium iron pyrophosphate (NFPP) possesses a stable three-dimensional crystal framework structure, with an operating voltage of approximately 3.0V and a theoretical specific capacity of about 129 mAh / g. This material exhibits minimal volume change (<4%) during charge and discharge, and is abundant in iron resources, environmentally friendly, and inexpensive, making it an ideal choice for large-scale energy storage. However, NFPP materials suffer from low initial coulombic efficiency, primarily due to irreversible sodium loss caused by the formation of a solid electrolyte interphase (SEI) film on the negative electrode during the first cycle, which consumes a large amount of sodium ions; and ion transport is hindered by side reactions of residual alkali on the positive electrode surface with the electrolyte and the formation of an interfacial passivation layer.

[0004] Sodium replenishment technology at the cathode is an effective strategy to address the low initial coulombic efficiency. By introducing an additional sodium source, it can offset the sodium ion consumption during the first charging cycle, effectively improving battery capacity, energy density, and cycle performance. Sodium oxalate (Na2C2O4) is considered a promising cathode sodium replenishment agent due to its high sodium replenishment capacity (approximately 400 mAh / g), decomposition products mainly consisting of gas (CO2), environmental friendliness, and low cost. However, current preparation methods typically involve simple mechanical stirring to physically mix sodium oxalate powder with NFPP, resulting in uneven coating and a series of performance defects such as low sodium replenishment efficiency, interfacial instability, and numerous side reactions. Consequently, NFPP cathode materials still suffer from low capacity, poor initial coulombic efficiency, and poor cycle stability. Summary of the Invention

[0005] Therefore, the technical problem to be solved by the present invention is the low initial coulombic efficiency of existing NFPP cathode materials due to irreversible sodium loss, and the decrease in capacity and cycle performance caused by imperfect sodium replenishment technology. Thus, the present invention provides a coated NFPP cathode material, its preparation method and application.

[0006] Therefore, the present invention provides the following technical solution: The first aspect of this invention protects a method for preparing an NFPP-coated cathode material, wherein the preparation method includes the following steps: mixing an NFPP core and a composite sodium supplementer, followed by heat treatment, to obtain an NFPP-coated cathode material; The composite sodium supplement includes sodium salts of organic acids, dispersants, and conductive carbon materials; Based on the NFPP core, the percentage of the compound sodium supplement is 3-8 wt%.

[0007] In one alternative embodiment, the percentage of the compound sodium supplement is 5-7 wt% based on the NFPP core.

[0008] In one optional embodiment, the average particle size of the organic acid sodium salt is in the nanometer range, optionally 20-100 nm.

[0009] In one optional embodiment, the organic acid sodium salt includes at least one of sodium oxalate, sodium citrate, and sodium tartrate, with sodium oxalate being an option.

[0010] In this invention, the organic acid sodium salt can be purchased or prepared. During preparation, an acid-base neutralization reaction is used. Taking sodium oxalate as an example, oxalic acid and sodium hydroxide are mixed and the operating conditions are controlled so that the size of the obtained sodium oxalate meets the requirements.

[0011] In one optional embodiment, the dispersant includes at least one of polyvinylpyrrolidone, polyethylene glycol, and sodium dodecyl sulfate, optionally polyvinylpyrrolidone.

[0012] In one optional embodiment, the conductive carbon material includes at least one of conductive carbon black, carbon nanotubes, graphene, graphitized carbon, carbon fiber, and carbon microspheres, and may be selected as conductive carbon black.

[0013] In one optional embodiment, the mass ratio of the organic acid sodium salt to the dispersant is 60-90:1, optionally 70-85:1.

[0014] In one optional embodiment, the mass ratio of the organic acid sodium salt to the conductive carbon material is 3-10:1, optionally 4-6:1.

[0015] In one optional embodiment, the method for preparing the compound sodium supplement includes the following steps: ① Mix the sodium salt of the organic acid, the dispersant, and the first solvent to obtain a solution; ② After mixing the conductive carbon material and the solution obtained in step ①, let it stand, collect the solid, dry it, and obtain the composite sodium supplement.

[0016] In this invention, the first solvent comprises water and ethanol; the volume ratio of water to ethanol is 1:1-3. The addition of ethanol is used to reduce the solubility of the sodium salt of organic acid, inducing its rapid homogeneous nucleation, thereby forming nanoscale particles.

[0017] In one optional implementation, in step ①, the mixing temperature is 40-60°C, optionally 50-55°C.

[0018] In one alternative embodiment, in step ①, the sodium salt of the organic acid and the first solvent are mixed to obtain solution A, and a dispersant is added to obtain solution B.

[0019] Optionally, in solution A, the concentration of the anion of the organic acid is 0.3-0.8 mol / L, and optionally 0.45-0.5 mol / L.

[0020] In one optional embodiment, in step ②, the settling temperature is -20 to -15°C; the low temperature induces the organic acid sodium salt to fully nucleate and precipitate on the surface and in the pores of the conductive carbon material, thereby forming a tightly bonded composite structure.

[0021] In one optional implementation, in step ②, the drying is freeze-drying.

[0022] In one optional embodiment, the freeze-drying conditions include: first, pre-freezing at -50°C to -45°C for 2-5 hours, then drying at -30°C to -15°C for 18-24 hours, and finally drying at 20-25°C for 4-6 hours. The drying at 20-25°C for 4-6 hours is a desorption drying process to remove water of crystallization.

[0023] In one optional embodiment, the method for preparing the coated NFPP cathode material includes the following steps: S1, mix the NFPP core and the second solvent to obtain a slurry; S2, mix the compound sodium supplement with the third solvent to obtain solution C; S3, mix the slurry and solution C to obtain a mixture, and then perform heat treatment to obtain the coated NFPP cathode material.

[0024] In this invention, in step S1, the NFPP core is vacuum dried before being mixed with the second solvent. Under vacuum conditions, it is dried at 100-120℃ for 8-12 hours. After being mixed with the second solvent, it can also be ultrasonically dispersed to make the dispersion more uniform, so as to obtain a slurry with a concentration of 40-45wt%.

[0025] In this invention, the second solvent is a conventional solvent in the art, typically and non-limitingly including water and / or ethanol, and optionally ethanol.

[0026] In this invention, the solid-liquid ratio of the compound sodium supplement to the third solvent is 1g:(20-50)mL; the third solvent is a conventional solvent in the art, typically and non-limitingly, water and / or ethanol, optionally ethanol, and further optionally, oxalic acid is dissolved in ethanol, and the concentration of oxalic acid in the ethanol-oxalic acid solution is 0.1-0.3mol / L.

[0027] In this invention, the slurry is mixed with solution C. To ensure a more uniform mixture, solution C is typically added dropwise to the slurry at a rate of 3-5 mL / min. The heat treatment is carried out under inert gas or nitrogen protection.

[0028] In this invention, in step S1, the NFPP core is fully dispersed in the second solvent to avoid agglomeration, providing a basis for achieving uniform coating; in step S2, the composite sodium supplement is uniformly dispersed in the third solvent, enabling further precise control of the coating amount; in step S3, the sodium supplement is added dropwise to form a dense and robust coating layer with the NFPP core, reducing interfacial side reactions; combined with subsequent pretreatment and heat treatment, the coating effect is further enhanced, resulting in a cathode material with excellent high capacity, high initial coulombic efficiency, and cycle stability.

[0029] In one optional embodiment, the heat treatment is preceded by separation, washing, and drying steps; all of which are conventional operations in the art. Typically, without limitation, after centrifugation, the solid is collected, washed 3-5 times with anhydrous ethanol, and dried under vacuum at 70-80°C for 8-10 hours. The heat treatment is followed by a sieving step, with the sieve mesh size being 310-330 mesh.

[0030] In one optional embodiment, the heat treatment temperature is 160-280℃, optionally 180-250℃, and the time is 1-3h.

[0031] A second aspect of this invention protects an NFPP-coated cathode material prepared by the aforementioned preparation method.

[0032] A third aspect of this invention protects a secondary battery, wherein the secondary battery comprises the NFPP-coated cathode material prepared by the aforementioned preparation method or the aforementioned NFPP-coated cathode material.

[0033] The technical solution of this invention has the following advantages: 1. This invention provides a method for preparing NFPP coated cathode material, wherein an NFPP core and a composite sodium supplement are mixed and then heat-treated to obtain the NFPP coated cathode material; the composite sodium supplement includes an organic acid sodium salt, a dispersant and a conductive carbon material; the percentage of the composite sodium supplement is 3-8 wt% based on the NFPP core. In this invention, the organic acid sodium salt ensures a tight bond with the NFPP core, significantly improving interfacial electron conduction and reducing its decomposition potential. This allows for the full decomposition and release of sodium ions under the normal charging cutoff voltage of the NFPP, resulting in a significant increase in the utilization rate of the composite sodium supplement. The dispersant inhibits the aggregation of the organic acid sodium salt, ensuring uniform coating, reducing interfacial side reactions, and improving interfacial stability. The conductive carbon material reduces interfacial impedance and improves electron conduction. A specific amount of the composite sodium supplement forms a stable and conductive interfacial layer, which not only significantly improves interfacial electron conduction and ion transport but also provides a strong bond, effectively preventing detachment during electrochemical cycling. This synergistically achieves two core functions: first, it efficiently compensates for irreversible sodium loss caused by factors such as the formation of the negative electrode SEI; second, it protects the NFPP core, inhibiting its side reactions with the electrolyte and structural degradation, thereby synergistically improving the reversible capacity, initial coulombic efficiency, and long-cycle stability of the positive electrode material.

[0034] 2. In this invention, the specific preparation steps of the composite sodium supplement can ensure that a composite sodium supplement with good dispersibility and synergistic composition is obtained, thereby achieving a tight bond between the composite sodium supplement and the NFPP core, making it less prone to detachment, avoiding failure problems caused by poor contact during cycling, and improving the coating effect. Detailed Implementation

[0035] The following embodiments are provided to better understand the present invention and are not limited to the preferred embodiments described. They do not constitute a limitation on the content and scope of protection of the present invention. Any product that is the same as or similar to the present invention, derived by any person under the guidance of the present invention or by combining the features of the present invention with other prior art, falls within the protection scope of the present invention.

[0036] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs; the terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the application; the terms “comprising” and “having” and any variations thereof in the text of this application are intended to cover non-exclusive inclusion.

[0037] In the description of the embodiments of this application, the technical terms "first", "second", etc. are used only to distinguish different objects and should not be construed as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features.

[0038] In this document, the term "embodiment" means that a particular feature, structure, or characteristic described in connection with an embodiment may be included in at least one embodiment of this application. The appearance of this phrase in various places throughout the specification does not necessarily refer to the same embodiment, nor is it a separate or alternative embodiment mutually exclusive with other embodiments. It will be explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0039] The "range" disclosed in this application is defined by a lower limit and an upper limit. A given range is defined by selecting a lower limit and an upper limit, which define the boundaries of the specific range. Ranges defined in this way can include or exclude endpoints and can be arbitrarily combined; that is, any lower limit can be combined with any upper limit to form a range. In this application, unless otherwise stated, the numerical range "ab" represents a shortened representation of any combination of real numbers from a to b, where a and b are real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0-5" have been listed herein, and "0-5" is merely a shortened representation of these numerical combinations. Furthermore, when a parameter is described as an integer ≥ 2, it is equivalent to disclosing that the parameter can be, for example, integers 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0040] In the description of the embodiments of this application, the term "and / or" is merely a description of the relationship between related objects, indicating that there can be three relationships. For example, A and / or B can represent three situations: A exists alone, A and B exist simultaneously, and B exists alone.

[0041] In the description of the embodiments of this application, the term "at least one" refers to one or more (including two).

[0042] For experiments not specifically described in the examples, the procedures or conditions should be followed according to the conventional experimental procedures described in the literature in this field. Reagents or instruments whose manufacturers are not specified are all commercially available conventional reagent products.

[0043] The preparation methods of NFPP used in the examples and comparative examples include the following steps: (1) Add NaH2PO4, FePO4·2H2O, H3PO4 and glucose to deionized water in stoichiometric proportions and stir. Then grind at 1000 rpm for 12 hours to obtain a blended slurry. (2) The mixed slurry was spray-dried at an inlet air temperature of 250°C, an outlet air temperature of 90°C, and a feed rate of 10 mL / min to obtain the precursor. (3) The precursor was heated to 300°C at 5°C / min and held for 2 hours in a nitrogen atmosphere, and then heated to 550°C at 5°C / min and held for 10 hours to obtain the sintered product. (4) After natural cooling, the sample was sieved (400 mesh) to obtain the NFPP core with the chemical formula Na4Fe3(PO4)2P2O7; Conductive carbon black: Cabot's VULCAN® XC72R.

[0044] Example 1 This embodiment provides a coated NFPP cathode material, and the preparation method includes the following steps: Preparation of compound sodium supplement ① At 55℃, oxalic acid and sodium hydroxide are dissolved in a mixed solvent of water and ethanol with a volume ratio of 1:1 in a 1:2 molar ratio to obtain a solution with an oxalate ion concentration of 0.5 mol / L. Polyvinylpyrrolidone (PVP) is added, wherein the mass ratio of nano sodium oxalate to PVP is 80:1. The mixture is magnetically stirred until completely dissolved to obtain a solution. ② Add conductive carbon black to the solution obtained in step ①, wherein the mass ratio of nano sodium oxalate to conductive carbon black is 4:1, ultrasonically disperse for 30 min, and let stand at -20℃ for 12 h; perform freeze-drying treatment, first pre-freezing at -50℃ for 2 h, then drying at -30℃ for 24 h, and finally desorption drying at 25℃ for 4 h to obtain the composite sodium supplement Na2C2O4@C.

[0045] Preparation of NFPP coated cathode material S1, the NFPP core was placed in a vacuum dryer at 120℃ for 12h, mixed with ethanol, and sonicated for 30min to obtain an NFPP-ethanol slurry with an NFPP concentration of 40wt%. S2, based on the NFPP core, the percentage of the compound sodium supplement Na2C2O4@C is 5wt%. The compound sodium supplement Na2C2O4@C and the third solvent are mixed to obtain a solution, wherein the solid-liquid ratio of the compound sodium supplement Na2C2O4@C and the third solvent is 1g:50mL, and the third solvent is an oxalic acid ethanol solution with a concentration of 0.3mol / L. S3. Under magnetic stirring, the solution obtained in step S2 is slowly added dropwise (dropping rate 5 mL / min) to the NFPP-ethanol slurry obtained in S1, and stirred in a water bath at 40℃ for 2 h; centrifuged to separate, the solid is collected, washed 3 times with anhydrous ethanol, dried under vacuum at 80℃ for 10 h, and then heat-treated at 180℃ for 2 h under nitrogen protection. Finally, it is sieved (325 mesh) to obtain the coated NFPP cathode material.

[0046] Example 2 This embodiment provides a coated NFPP cathode material, and the preparation method includes the following steps: Preparation of compound sodium supplement Following the method described in Example 1; Preparation of NFPP coated cathode material S1, according to the method of Example 1; S2, following the method of Example 1, differs in that, based on the NFPP core, the percentage of the composite sodium supplement Na2C2O4@C is 3wt%; S3, following the method in Example 1, the NFPP coated cathode material is obtained.

[0047] Example 3 This embodiment provides a coated NFPP cathode material, and the preparation method includes the following steps: Preparation of compound sodium supplement Following the method described in Example 1; Preparation of NFPP coated cathode material S1, according to the method of Example 1; S2, following the method of Example 1, except that, based on the NFPP core, the percentage of the composite sodium supplement Na2C2O4@C is 8wt%; S3, following the method in Example 1, the NFPP coated cathode material is obtained.

[0048] Comparative Example 1 This comparative example provides a cathode material coated with NFPP, and the preparation method includes the following steps: Preparation of compound sodium supplement Following the method described in Example 1; Preparation of NFPP coated cathode material S1, according to the method of Example 1; S2, following the method of Example 1, differs in that, based on the NFPP core, the percentage of the composite sodium supplement Na2C2O4@C is 1wt%; S3, following the method in Example 1, the NFPP coated cathode material is obtained.

[0049] Comparative Example 2 This comparative example provides a cathode material coated with NFPP, and the preparation method includes the following steps: Preparation of compound sodium supplement Following the method described in Example 1; Preparation of NFPP coated cathode material S1, according to the method of Example 1; S2, following the method of Example 1, except that, based on the NFPP core, the percentage of the composite sodium supplement Na2C2O4@C is 12wt%; S3, under magnetic stirring, the solution obtained in step S2 was slowly added dropwise (5 mL / min) to the NFPP-ethanol slurry obtained in S1, and stirred in a water bath at 40°C for 2 hours. During centrifugation, the material showed severe agglomeration and extremely poor flowability, exhibiting a clear aggregated state, which contrasted sharply with the loose and uniform powder morphology of Example 1. The solid was collected, washed three times with anhydrous ethanol, dried under vacuum at 80°C for 10 hours, and then heat-treated at 180°C for 2 hours under nitrogen protection. In the final sieving (325 mesh) step, a large amount of material could not pass through the sieve and required grinding or forced sieving, indicating that the particles had severely agglomerated.

[0050] Comparative Example 3 This comparative example provides a cathode material coated with NFPP, and the preparation method includes the following steps: The compound sodium supplement was selected from commercially available analytical grade sodium oxalate (Na2C2O4) powder with a D50 of 25μm; The NFPP core was vacuum dried at 120℃ for 12h to remove adsorbed water. The sodium oxalate content was 5wt% based on the NFPP core. The dried NFPP core and sodium oxalate powder were placed in a planetary ball mill and mechanically mixed at 200 rpm for 2h. The resulting mixed powder was heat-treated at 180℃ for 2h under nitrogen protection. Finally, it was sieved (325 mesh) to obtain sodium oxalate-coated NFPP cathode material.

[0051] Comparative Example 4 This comparative example provides an NFPP cathode material, which is directly prepared NFPP without any coating treatment.

[0052] Test case Battery preparation method: The materials obtained in the examples and comparative examples are used as positive electrode active materials. Polyvinylidene fluoride and carbon black are added in a mass ratio of 90:5:5, and N-methylpyrrolidone (NMP) is added to obtain a positive electrode slurry. The solid content of the positive electrode slurry is 48%. The slurry is then homogenized and coated, and the compaction density of the electrode sheet is 2.2 g / cm³. 3 The electrode was fabricated using a sodium metal sheet as the counter electrode, a glass fiber separator as the separator, and a 1 mol / L sodium hexafluorophosphate mixed solution as the electrolyte. The solvent in the mixed solution included ethylene carbonate (EC), dimethyl carbonate (DMC), and fluoroethylene carbonate (FEC) in a volume ratio of 1:1:0.1. The CR2032 coin cell, consisting of a positive electrode, a glass fiber separator, a sodium sheet, a gasket, and a spring, was assembled in an argon-filled glove box and placed in the Blue Electric testing system for electrical performance testing. Test method for charge / discharge capacity: Under 25℃ conditions, charge the battery at a rate of 1C to 4.0V, and obtain the 1C charge specific capacity based on the battery mass; then discharge the battery at a rate of 1C to 2.0V, and obtain the 1C discharge specific capacity based on the battery mass; charge the battery at a rate of 1C to 4.0V, and obtain the 1C charge specific capacity based on the battery mass; then discharge the battery at a rate of 5C to 2.0V, and obtain the 5C discharge specific capacity based on the battery mass. Initial coulombic efficiency: 1C initial discharge capacity / 1C initial charge capacity × 100%; The test method for capacity retention is as follows: At 25°C, the battery is charged at a constant current of 1C to 4.0V, discharged at a constant current of 1C to 2.0V, and cycled for 100 times. The capacity retention rate on the 100th cycle = (1C discharge capacity on the 100th cycle / 1C discharge capacity on the 1st cycle) × 100%. The test results are shown in Table 1; Table 1

[0053] Comparing Examples 1-3 with Comparative Examples 1-2 reveals that the specific amount of composite sodium supplement formed a stable and conductive interfacial layer, significantly improving interfacial electron conduction and ion transport, and ensuring strong adhesion, effectively preventing detachment during electrochemical cycling. In Comparative Example 1, the amount of composite sodium supplement was insufficient, failing to achieve successful coating, resulting in a decrease in electrical performance compared to Example 1. In Comparative Example 2, the excessive amount of composite sodium supplement led to severe agglomeration and uneven coating during subsequent processing. The resulting material particles showed significant adhesion, a decrease in specific surface area, and a decline in electrochemical performance compared to Example 1 due to hindered ion / electron transport and intensified interfacial side reactions.

[0054] A comparison of Examples 1, 3, and 4 reveals a synergistic effect among the components of the composite sodium supplement. The organic acid sodium salt ensures a tight bond with the NFPP core, significantly improves interfacial electron conduction, and significantly reduces its decomposition potential, enabling it to fully decompose and release sodium ions under the normal charging cutoff voltage of the NFPP, thus greatly improving the utilization rate of the composite sodium supplement. The dispersant inhibits the aggregation of the organic acid sodium salt, ensures uniform coating, reduces interfacial side reactions, and improves interfacial stability. The conductive carbon material reduces interfacial impedance and improves electron conduction.

[0055] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of this invention.

Claims

1. A method for preparing a coated NFPP cathode material, characterized in that, The preparation method includes the following steps: mixing the NFPP core and the composite sodium supplementer, and then performing heat treatment to obtain the NFPP coated cathode material; The composite sodium supplement includes sodium salts of organic acids, dispersants, and conductive carbon materials; Based on the NFPP core, the percentage of the compound sodium supplement is 3-8 wt%.

2. The production method according to claim 1, characterized by, Based on the NFPP core, the percentage of the compound sodium supplement is 5-7 wt%.

3. The production method according to claim 1 or 2, characterized by, The average particle size of the organic acid sodium salt is in the nanometer range, and can be selected as 20-100 nm. Optionally, the organic acid sodium salt includes at least one of sodium oxalate, sodium citrate, and sodium tartrate, and sodium oxalate may be selected as the sodium salt. Optionally, the dispersant includes at least one of polyvinylpyrrolidone, polyethylene glycol, and sodium dodecyl sulfate, and may be polyvinylpyrrolidone. Optionally, the conductive carbon material includes at least one of conductive carbon black, carbon nanotubes, graphene, graphitized carbon, carbon fiber, and carbon microspheres, and may be selected as conductive carbon black.

4. The production method according to any one of claims 1 to 3, characterized by, The mass ratio of the organic acid sodium salt to the dispersant is 60-90:1, and can be selected as 70-85:1; Optionally, the mass ratio of the organic acid sodium salt to the conductive carbon material is 3-10:1, and optionally 4-6:

1.

5. The production method according to any one of claims 1 to 4, characterized by, The preparation method of the compound sodium supplement includes the following steps: ① Mix the sodium salt of the organic acid, the dispersant, and the first solvent to obtain a solution; ② After mixing the conductive carbon material and the solution obtained in step ①, let it stand, collect the solid, dry it, and obtain the composite sodium supplement.

6. The preparation method according to claim 5, characterized in that, In step ①, the mixing temperature is 40-60℃, and can be selected as 50-55℃; And / or, in step ①, the sodium salt of the organic acid and the first solvent are mixed to obtain solution A, and a dispersant is added to obtain solution B; And / or, in step ②, the temperature for settling is -20 to -15°C; And / or, in step ②, the drying is freeze-drying; Optionally, the freeze-drying conditions include: first, pre-freezing at -50°C to -45°C for 2-5 hours, then drying at -30°C to -15°C for 18-24 hours, and finally drying at 20-25°C for 4-6 hours.

7. The preparation method according to any one of claims 1-6, characterized in that, The method for preparing the NFPP coated cathode material includes the following steps: S1, mix the NFPP core and the second solvent to obtain a slurry; S2, mix the compound sodium supplement with the third solvent to obtain solution C; S3, mix the slurry and solution C to obtain a mixture, and then perform heat treatment to obtain the coated NFPP cathode material.

8. The preparation method according to claim 7, characterized in that, The heat treatment process also includes separation, washing, and drying steps. And / or, the heat treatment temperature is 160-280℃, optionally 180-250℃, and the time is 1-3h.

9. An NFPP-coated cathode material prepared by the preparation method according to any one of claims 1-8.

10. A secondary battery, characterized in that, The secondary battery includes the NFPP-coated cathode material prepared by the preparation method of any one of claims 1-8 or the NFPP-coated cathode material of claim 9.

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