A sodium ferric pyrophosphate phosphate positive electrode material, a preparation method and application thereof

Abstract

This invention provides a sodium iron pyrophosphate cathode material, its preparation method, and its applications. The cathode material comprises a main phase of sodium iron pyrophosphate and a heterogeneous phase of NaFePO4, with a molar ratio of sodium iron pyrophosphate to NaFePO4 of 1:(0.3-0.9). The sodium iron pyrophosphate cathode material provided by this invention achieves in-situ activation of the amorphous inert NaFePO4 heterogeneous phase by controlling the molar ratio of the main phase of sodium iron pyrophosphate to the heterogeneous phase of NaFePO4. This avoids the problem of uncontrollable heterogeneous phase content in traditional methods and effectively improves the electrochemical performance of the final synthesized material, mainly reflected in the synergistic optimization of "high capacity, high rate capability, and long cycle life".

Description

Technical Field

[0001] This invention relates to the field of sodium-ion batteries, specifically to a sodium iron pyrophosphate cathode material, its preparation method, and its application. Background Technology

[0002] Sodium-ion batteries have become a highly attractive candidate for large-scale energy storage due to the abundance and low cost of sodium resources. The cathode material is crucial in determining the performance of sodium-ion batteries, and sodium iron pyrophosphate (NFPP), as a core representative of polyanionic systems, has become a strong candidate for iron-based polyanionic cathode materials for sodium-ion batteries due to its high theoretical specific capacity, suitable operating potential, low cost, and "zero strain" characteristic with a volume change rate of only 2%-4% during charge and discharge. However, the low electronic conductivity and slow ion transport capability of NFPP cathode materials limit their practical application.

[0003] Currently, the preparation methods of NFPP cathode materials (such as solid-state reaction, sol-gel, spray drying, and combustion synthesis) generally suffer from problems such as high energy consumption, complex processes, uneven raw material mixing, high residual carbon content, low compaction density, and difficulty in scale-up. Furthermore, they easily introduce inert NaFePO4 impurities and lack effective elemental doping synergistic mechanisms (such as Mn stabilizing the framework and F stabilizing the structure) and quantitative detection and stepwise adjustment methods for the impurity content in the precursor. This makes it impossible to precisely control the ratio of the main phase to the impurities during synthesis, resulting in the inert impurities never being activated in situ. While current mainstream strategies focus on elemental doping (Ta... 5+ 、Nb、V、K + Three types of methods have been used to reduce the impurity phase content to below 1%: precursor and process optimization (controlling the Na / Fe ratio, selecting citric acid or organophosphonic acid, two-step heat treatment, dual iron source synergy) and surface engineering (Al2O3 or composite carbon source coating). However, these "passive defense" measures have only improved electronic conductivity and ion transport capability to a certain extent, but have never been able to achieve the functionalization of the impurity phase. In the end, the material still has to make a trade-off between cycle stability and discharge specific capacity. Summary of the Invention

[0004] This invention provides a sodium iron pyrophosphate cathode material, its preparation method, and its application to solve the above-mentioned problems.

[0005] In a first aspect, the present invention provides a sodium iron pyrophosphate cathode material, the cathode material comprising a main phase sodium iron pyrophosphate and a heterogeneous phase NaFePO4, wherein the molar ratio of the main phase sodium iron pyrophosphate to the heterogeneous phase NaFePO4 is 1:(0.3-0.9).

[0006] In some optional embodiments, the chemical formula of the sodium iron pyrophosphate cathode material is Na₄Fe₂O₃.3-x Mn x (PO4) 2-y P2O7F y Where x = 0.2-0.4, y = 0.1-0.2; optionally, x = 0.3, y = 0.15.

[0007] In some optional embodiments, the molar ratio of the main phase sodium iron pyrophosphate to the impurity phase NaFePO4 is 1:0.5.

[0008] Secondly, the present invention also provides a method for preparing a sodium iron pyrophosphate cathode material, comprising the following steps: S1, a sodium source, an iron source, a manganese source, a phosphorus source, and a fluorine source are mixed and heat-treated to obtain a precursor A solution; S2, mix sodium source, iron source and phosphorus source, heat treat to obtain precursor B solution; S3. Mix the precursor B solution and the precursor A solution, heat treat to form a mixture, dry, sinter in two stages, and cool to obtain sodium iron pyrophosphate cathode material, wherein the molar ratio of precursor A to precursor B is 1:0.3-0.9.

[0009] In some optional implementations, in step S1, the molar ratio of Na, Fe, Mn, P, and F in the sodium source, iron source, manganese source, phosphorus source, and fluorine source is (3.95-4.15):(2.6-2.8):(0.2-0.4):4:(0.1-0.2).

[0010] In some optional implementations, in step S2, the molar ratio of Na, Fe, and P in the sodium source, iron source, and phosphorus source is (0.9-1.1):(0.9-1.1):(0.9-1.1).

[0011] In some alternative implementations, step S1 further includes a chelating agent in the precursor A solution.

[0012] Optionally, the mass ratio of the chelating agent to the total amount of metal ions in the sodium source, iron source, manganese source, and phosphorus source is (0.005-0.015):1.

[0013] In some optional implementations, in step S3, the precursor B solution is added dropwise to the precursor A solution at a rate of 5-10 mL / min.

[0014] In some optional implementations, step S3 specifically includes raising the temperature to 350-420℃ at a heating rate of 2-5℃ / min and holding for 2-4 hours under an inert atmosphere, and raising the temperature to 600-650℃ at a heating rate of 2-5℃ / min and holding for 6-10 hours.

[0015] In some optional implementations, in step S3, the drying includes spray drying; optionally, the inlet temperature of the spray dryer is 180-220°C; optionally, the outlet temperature of the spray dryer is 90-110°C; optionally, the feed rate of the spray dryer is 5-15 mL / min. In some optional implementations, the solid content of the mixture before drying is 10-30 wt%.

[0016] In some optional implementations, in step S1, when forming the precursor A solution, the heat treatment temperature is 60-80℃ and the heat treatment time is 1.5-2.5h; In some optional implementations, in step S2, when forming the precursor B solution, the heat treatment temperature is 45-55°C and the heat treatment time is 25-30 min. In some optional implementations, in step S3, when forming the mixture, the temperature of the precursor B solution and the precursor A solution for heat treatment is 45-55°C, and the heat treatment time is 0.8-1.2h.

[0017] In some alternative embodiments, the sodium source forming the precursor A solution and / or precursor B solution includes a sodium salt; Optionally, the soluble salt of sodium includes at least one of sodium carbonate, sodium hydroxide, sodium bicarbonate, sodium acetate, sodium oxalate, and sodium dihydrogen phosphate. In some alternative implementations, the iron source forming the precursor A solution and / or precursor B solution includes at least one of ferrous salts and ferric salts; Optionally, the ferrous salt includes at least one of ferrous sulfate, ferrous oxalate, and ferrous acetate; Optionally, the trivalent iron salt includes ferric nitrate; In some alternative implementations, the manganese source forming the precursor A solution and / or precursor B solution includes manganese salts; Optionally, the manganese salt includes at least one of manganese acetate, manganese carbonate, and manganese nitrate; In some alternative embodiments, the phosphorus source forming the precursor A solution and / or precursor B solution includes at least one of phosphoric acid and phosphate. Optionally, the phosphate includes at least one of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and sodium phosphate; In some alternative implementations, the fluorine source forming the precursor A solution includes fluoride salts; Optionally, the fluoride salt includes at least one of sodium fluoride, ammonium fluoride, and lithium fluoride; In some optional implementations, the chelating agent forming the precursor A solution includes organic acid chelating agents; Optionally, the organic acid chelating agent includes at least one of citric acid, ethylenediaminetetraacetic acid tartaric acid, and gluconic acid.

[0018] Thirdly, the present invention also provides a positive electrode sheet comprising the above-mentioned sodium iron pyrophosphate positive electrode material or the sodium iron pyrophosphate positive electrode material prepared by the above-mentioned preparation method.

[0019] Fourthly, the present invention also provides a sodium-ion battery, including the above-mentioned positive electrode sheet.

[0020] Those skilled in the art will understand that the sodium-ion battery provided by this invention, in addition to including a positive electrode sheet containing the aforementioned sodium iron pyrophosphate positive electrode material, also includes structural components such as a negative electrode sheet, electrolyte, separator, and casing. During the battery charging and discharging process, sodium ions (Na... + The electrolyte moves back and forth between the positive and negative electrodes, inserting and removing itself. The electrolyte acts as a conductor for sodium ions between the positive and negative electrodes. The diaphragm is placed between the positive and negative electrodes, mainly to prevent short circuits between the positive and negative electrodes, while allowing sodium ions to pass through.

[0021] As an example, the positive electrode sheet includes a positive current collector (such as aluminum foil) and a positive active material layer. The positive current collector has two opposing surfaces in its own thickness direction, and the positive active material layer is disposed on either or both of the opposing surfaces of the positive current collector. Other materials, compositions, and manufacturing methods of the positive electrode sheet used in the sodium-ion battery of the present invention may include any techniques disclosed in the prior art suitable for sodium-ion batteries. For example, the positive active material may also include, but is not limited to, layered transition metal oxides, polyanionic compounds, Prussian blue analogues, etc.

[0022] As an example, the negative electrode sheet includes a negative electrode current collector (such as copper foil or aluminum foil) and a negative electrode active material layer. The negative electrode current collector has two opposing surfaces in its own thickness direction, and the negative electrode active material layer is disposed on either or both of the opposing surfaces of the negative electrode current collector. The materials, composition, and manufacturing methods of the negative electrode sheet used in the sodium-ion battery of the present invention may include any techniques disclosed in the prior art suitable for sodium-ion batteries (e.g., the negative electrode active material may include, but is not limited to, hard carbon, soft carbon, alloy-based materials, titanium-based materials, etc.).

[0023] The materials (such as polyolefins) and shapes (such as porous films) of the separator used in the sodium-ion battery of the present invention are not particularly limited, and may include any techniques disclosed in the prior art, provided that they can effectively isolate the positive and negative electrodes and allow sodium ion conduction.

[0024] The electrolyte used in the sodium-ion battery of the present invention is not particularly limited and may include any technology disclosed in the prior art, provided that it can effectively conduct sodium ions between the positive and negative electrodes (e.g., sodium salts such as NaPF6 and NaClO4 dissolved in organic solvents such as carbonate solvents, and may contain additives).

[0025] This invention does not specifically limit the preparation method of sodium-ion batteries. Conventional methods applicable to sodium-ion batteries in the art can be used to prepare sodium-ion batteries. For example, a positive electrode, a separator, and a negative electrode are sequentially stacked, with the separator positioned between the positive and negative electrodes. A battery cell is obtained through stacking or winding processes. Then, the sodium-ion battery of this invention is obtained through baking (to remove moisture), injection of sodium-ion battery electrolyte, formation (to form a stable solid electrolyte interphase (SEI) film), and encapsulation.

[0026] Fifthly, the present invention also provides an electrical device including the aforementioned sodium-ion battery.

[0027] It is understood that in the electrical equipment provided by this invention, the sodium-ion battery can be used as a power source for the electrical equipment, or as an energy storage unit for the electrical equipment. The electrical equipment may be, but is not limited to, stationary energy storage systems (including grid peak shaving, renewable energy support, and industrial and commercial energy storage), light electric vehicles (including electric bicycles, electric scooters, and site work vehicles), short-to-medium range electric vehicles (pure electric vehicles / plug-in hybrid vehicles), backup power supply devices (including communication base station UPS and emergency power supplies), and energy density-insensitive equipment (stationary power tools, outdoor power supplies), etc.

[0028] Compared with the prior art, the present invention has the following beneficial effects: 1. This invention provides a sodium iron pyrophosphate cathode material, comprising a main phase sodium iron pyrophosphate and a heterogeneous phase NaFePO4. By controlling the molar ratio of the main phase sodium iron pyrophosphate to the heterogeneous phase NaFePO4 to be 1:(0.3-0.9), in-situ activation of the amorphous inert NaFePO4 heterogeneous phase is achieved, avoiding the problem of uncontrollable heterogeneous phase content in traditional methods, and effectively improving the electrochemical performance of the final synthesized material, mainly reflected in the synergistic optimization of "high capacity, high rate capability, and long cycle life".

[0029] 2. The present invention provides a method for preparing sodium iron pyrophosphate cathode material, comprising the following steps: S1, mixing sodium source, iron source, manganese source, phosphorus source and fluorine source, and heat-treating to obtain precursor A solution; S2, mixing sodium source, iron source and phosphorus source, and heat-treating to obtain precursor B solution; S3, mixing precursor B solution and precursor A solution, heat-treating to form a mixture, drying, two-stage sintering and cooling to obtain sodium iron pyrophosphate cathode material, wherein the molar ratio of precursor A to precursor B is 1:0.3-0.9. This invention constructs a "two-way regulation" mechanism through Mn / F dual doping: Mn preferentially stabilizes the structural framework, effectively suppressing the uncontrollable formation of the NaFePO4 impurity phase during synthesis, while F stabilizes the crystal structure and enhances cycle stability; at the same time, based on the actual content of the impurity phase in the precursor A solution, it is introduced into the precursor B solution in step S3 to achieve precise control of the ratio of the two phases, so that the originally inert impurity phase is activated in situ and participates in the electrochemical reaction, thereby significantly improving the material capacity while ensuring stability.

[0030] 3. The present invention provides a method for preparing sodium iron pyrophosphate cathode material. In step S1, the molar ratio of Na, Fe, Mn, P, and F in the sodium source, iron source, manganese source, phosphorus source, and fluorine source is 4:(2.6-2.8):(0.2-0.4):4:(0.1-0.2), wherein the molar ratio of Mn and F is (0.2-0.4):(0.1-0.2). Mn occupies lattice sites, thereby inhibiting the nucleation of impurity phases at the source; while F stabilizes the crystal structure, preventing the transformation to impurity phases during growth. The two work together to control the ratio within a specific range, thereby maximizing the yield of high-purity sodium iron pyrophosphate cathode material.

[0031] 4. The present invention provides a method for preparing sodium iron pyrophosphate cathode material, wherein the dropping rate is 5-10 mL / min, which can achieve uniform mixing of two precursor solutions, control the dropping rate as the core, prevent excessively high local reactant concentrations, and thus avoid uneven distribution of transition metal elements such as Fe and Mn in the generated precipitate particles. Detailed Implementation

[0032] The following embodiments are provided to better understand the present invention, but the following embodiments 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 scope of protection of the present invention.

[0033] 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.

[0034] 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. 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.

[0035] 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.

[0036] In the description of the embodiments of this application, the term "at least one" refers to one or more (including two).

[0037] Unless otherwise specified, all steps in this application may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), indicating that the method may include steps (a) and (b) performed sequentially, or it may include steps (b) and (a) performed sequentially. For example, the mention that the method may also include step (c) indicates that step (c) may be added to the method in any order. For example, the method may include steps (a), (b), and (c), or it may include steps (a), (c), and (b), or it may include steps (c), (a), and (b), etc.

[0038] Unless otherwise specified, all experimental steps or conditions in the examples were performed according to conventional experimental procedures and conditions in the art. Reagents or instruments whose manufacturers are not specified are all commercially available products.

[0039] Example 1 This embodiment provides a sodium iron pyrophosphate cathode material, and the specific steps and operating parameters are as follows: S1, dissolve sodium source (sodium carbonate), iron source (ferrous oxalate), manganese source (manganese acetate), phosphorus source (ammonium dihydrogen phosphate), fluorine source (sodium fluoride), and chelating agent (citric acid monohydrate) in deionized water, and stir at 70°C for 2 hours to perform the first mixing, to obtain precursor A solution; the molar ratio of Na, Fe, Mn, P, and F in the sodium, iron, manganese, phosphorus, and fluorine sources is 4.15:2.7:0.3:4:0.15; the mass ratio of the chelating agent to the total amount of metal ions in the sodium, iron, manganese, and phosphorus sources is 0.005:1; S2, dissolve sodium dihydrogen phosphate and ferrous oxalate in deionized water, stir at 50°C for 30 min for a second mixing to obtain precursor B solution; the molar ratio of Na, Fe and P in the sodium source, iron source and phosphorus source is 1:1:1; S3, the precursor B solution is added dropwise to the precursor A solution at a rate of 8 mL / min, and the mixture is stirred at 50 °C for 1 h to obtain a third mixture. The molar ratio of precursor A to precursor B is 1:0.5. The mixture is spray-dried at an inlet temperature of 200 °C, an outlet temperature of 100 °C, and a feed rate of 10 mL / min. Then, under an Ar atmosphere, it is first sintered at 350 °C at a rate of 5 °C / min for 3 h, and then sintered at 620 °C at a rate of 5 °C / min for 8 h. Finally, it is cooled in the furnace at a rate of 2 °C / min, pulverized, and sieved to obtain the sodium iron pyrophosphate cathode material, wherein the molar ratio of the main phase sodium iron pyrophosphate to the impurity phase NaFePO4 is 1:0.5.

[0040] Example 2 This embodiment provides a sodium iron pyrophosphate cathode material, which differs from Embodiment 1 only in that, in step S3, the molar ratio of precursor A to precursor B is 1:0.9, and the molar ratio of the main phase sodium iron pyrophosphate and the impurity phase NaFePO4 in the obtained sodium iron pyrophosphate cathode material is 1:0.9.

[0041] Example 3 This embodiment provides a sodium iron pyrophosphate cathode material, which differs from Embodiment 1 only in that, in step S3, the molar ratio of precursor A to precursor B is 1:0.3, and the molar ratio of the main phase sodium iron pyrophosphate and the impurity phase NaFePO4 in the obtained sodium iron pyrophosphate cathode material is 1:0.3.

[0042] Example 4 This embodiment provides a sodium iron pyrophosphate cathode material, which differs from Embodiment 1 only in that, in step S3, the molar ratio of precursor A to precursor B is 1:0.6, and the molar ratio of the main phase sodium iron pyrophosphate and the impurity phase NaFePO4 in the obtained sodium iron pyrophosphate cathode material is 1:0.6.

[0043] Example 5 This embodiment provides a sodium iron pyrophosphate cathode material, and the specific steps and operating parameters are as follows: S1, dissolve sodium source (sodium hydroxide), iron source (ferric nitrate), manganese source (manganese carbonate), phosphorus source (diammonium hydrogen phosphate), fluorine source (lithium fluoride), and chelating agent (citric acid) in deionized water, and stir at 60°C for 2.5 h to perform the first mixing, to obtain precursor A solution; the molar ratio of Na, Fe, Mn, P, and F in the sodium, iron, manganese, phosphorus, and fluorine sources is 3.95:2.8:0.2:4:0.2; the mass ratio of the chelating agent to the total amount of metal ions in the sodium, iron, manganese, and phosphorus sources is 0.005:1; S2, dissolve sodium source (sodium bicarbonate), iron source (ferrous acetate), and phosphorus source (sodium phosphate) in deionized water, stir at 55°C for 25 min for a second mixing to obtain precursor B solution; the molar ratio of Na, Fe, and P in the sodium source, iron source, and phosphorus source is 1.1:0.9:1.1; In step S3, precursor B solution is added dropwise to precursor A solution at a rate of 5 mL / min, and the mixture is stirred at 55°C for 1.5 h to obtain a third mixing material. In step S3, the molar ratio of precursor A to precursor B is 1:0.9. The mixture is spray-dried at an inlet temperature of 220°C, an outlet temperature of 90°C, and a feed rate of 15 mL / min. Then, under a N2 atmosphere, it is first sintered at 420°C at a rate of 2°C / min for 2 h, and then sintered at 600°C at a rate of 5°C / min for 10 h. Finally, it is cooled in the furnace at a rate of 2°C / min, pulverized, and sieved to obtain the sodium iron pyrophosphate cathode material with a molar ratio of sodium iron pyrophosphate as the main phase and NaFePO4 as the impurity phase, which is 1:0.9.

[0044] Example 6 This embodiment provides a sodium iron pyrophosphate cathode material, and the specific steps and operating parameters are as follows: S1, dissolve sodium source (sodium oxalate), iron source (ferrous sulfate), manganese source (manganese nitrate), phosphorus source (sodium phosphate), fluorine source (ammonium fluoride), and chelating agent (ethylenediaminetetraacetic acid) in deionized water, and stir at 80°C for 1.5 h to obtain precursor A solution; the molar ratio of Na, Fe, Mn, P, and F in the sodium, iron, manganese, phosphorus, and fluorine sources is 4.15:2.6:0.4:4:0.1; the mass ratio of the chelating agent to the total amount of metal ions in the sodium, iron, manganese, and phosphorus sources is 0.015:1; S2, dissolve sodium source (sodium oxalate), iron source (ferrous oxalate), and phosphorus source (diammonium hydrogen phosphate) in deionized water, stir at 45°C for 30 min for a second mixing to obtain precursor B solution; the molar ratio of Na, Fe, and P in the sodium source, iron source, and phosphorus source is 0.9:1.1:0.9; In step S3, precursor B solution is added dropwise to precursor A solution at a rate of 10 mL / min, and the mixture is stirred at 45°C for 2.5 h to obtain a third mixing material. In step S3, the molar ratio of precursor A to precursor B is 1:0.3. The mixture is spray-dried at an inlet temperature of 180°C, an outlet temperature of 110°C, and a feed rate of 5 mL / min. Then, under a N2 atmosphere, it is first sintered at 350°C at a rate of 5°C / min for 4 h, and then sintered at 650°C at a rate of 2°C / min for 6 h. Finally, it is cooled in the furnace at a rate of 5°C / min, pulverized, and sieved to obtain the sodium iron pyrophosphate phosphate cathode material with a molar ratio of sodium iron pyrophosphate phosphate as the main phase and NaFePO4 as the impurity phase, which is 1:0.3.

[0045] Example 7 This embodiment provides a sodium iron pyrophosphate cathode material, which differs from Example 5 only in that the precursor B solution and the precursor A solution are directly mixed instead of being added dropwise. The specific operation is as follows: S3, the precursor B solution and the precursor A solution are stirred at 45°C for 2.5 h to obtain a third mixture. In step S3, the molar ratio of precursor A to precursor B is 1:0.9. After spray drying, the mixture is sintered in a first sintering and a second sintering under a N2 atmosphere, cooled, crushed, and sieved to obtain the sodium iron pyrophosphate cathode material.

[0046] Example 8 This embodiment provides a sodium iron pyrophosphate cathode material, which differs from Embodiment 5 only in that S3 is a one-step sintering process. The specific operation is as follows: In step S3, the precursor B solution is added dropwise to the precursor A solution for a third mixing to obtain a mixture. In step S3, the molar ratio of precursor A to precursor B is 1:0.9. After spray drying, the mixture is sintered at 650°C for 10 hours under N2 atmosphere at a temperature of 5°C / min. After cooling, it is pulverized and sieved to obtain the sodium iron pyrophosphate cathode material.

[0047] Comparative Example 1 This comparative example provides a sodium iron pyrophosphate cathode material, which differs from Example 1 only in that it is free of manganese and fluorine source doping and the addition of precursor B solution. The specific operation is as follows: S1, dissolve sodium source (sodium carbonate), iron source (ferrous oxalate), phosphorus source (ammonium dihydrogen phosphate), and chelating agent (citric acid monohydrate) in deionized water, and stir at 70°C for 2 hours to perform the first mixing, to obtain precursor A solution; the molar ratio of Na, Fe, and P in the sodium, iron, and phosphorus sources is 4:3:4; the mass ratio of the chelating agent to the total amount of metal ions in the sodium, iron, and phosphorus sources is 0.005:1; S2, after spray drying the precursor A solution, perform a first sintering and a second sintering, cool, crush, and sieve to obtain the sodium iron pyrophosphate cathode material.

[0048] Comparative Example 2 This comparative example provides a sodium iron pyrophosphate cathode material, which differs from Example 1 only in that the precursor B solution is not added. The specific operation is as follows: No step S2; S3, after spray drying the precursor A solution, perform a first sintering and a second sintering, cool, pulverize, and sieve to obtain the sodium iron pyrophosphate cathode material.

[0049] Comparative Example 3 This embodiment provides a sodium iron pyrophosphate cathode material, which differs from Embodiment 1 only in that, in step S3, the molar ratio of precursor A to precursor B is 1:1, resulting in a molar ratio of sodium iron pyrophosphate as the main phase and NaFePO4 as the impurity phase in the sodium iron pyrophosphate cathode material being 1:1.

[0050] Comparative Example 4 This embodiment provides a sodium iron pyrophosphate cathode material, which differs from Embodiment 1 only in that, in step S3, the molar ratio of precursor A to precursor B is 1:0.1, and the molar ratio of the main phase sodium iron pyrophosphate and the impurity phase NaFePO4 in the precursor B solution and precursor A solution obtained from the sodium iron pyrophosphate cathode material is 1:0.1.

[0051] Experimental Example 1 The sodium iron pyrophosphate cathode materials provided in the various embodiments and comparative examples were applied to sodium-ion batteries, and then their electrical performance was tested.

[0052] The method for preparing the sodium-ion battery includes the following steps: A positive electrode sheet was prepared by uniformly coating the positive electrode material, PVDF binder, and SP conductive carbon black onto aluminum foil at a mass ratio of 95:2.5:2.5, with an active material loading of 2 mg / cm³. 2 Using a sodium metal sheet as the counter electrode, glass fiber as the separator, and a 1 mol / L NaPF6 EC / DMC (volume ratio 1:1) solution as the electrolyte, CR2032 coin cells were assembled in an argon glove box. After assembly, the cells were allowed to stand for 10 hours to allow the electrolyte to fully impregnate them. Finally, the cells were placed in the Blue Electric testing system for electrical performance testing.

[0053] The specific method for the electrical performance test is as follows: The test temperature was 25±2℃, the voltage range was 2.0-4.0 V, and a constant current charge-discharge mode was used. After each cycle, the device was left to stand for 10 minutes. The rate performance test was conducted in the order of 0.1C→0.2C→0.5C→1C→2C→5C→0.1C, with each rate cycled 3 times. The 1C current was calibrated based on the actual discharge capacity of the 3rd cycle of 0.1C. Long-cycle performance testing involved 100 charge-discharge cycles at a 1C rate, recording the discharge specific capacity and coulombic efficiency for each cycle.

[0054] (1) The calculation method for the first-cycle coulomb efficiency is as follows: ; Where: ICE is the initial coulomb efficiency; D1 is the initial discharge capacity (mAh) at a specified rate (e.g., 0.1C). C1 represents the initial charge capacity (mAh) at the same charging rate.

[0055] (2) The calculation method for 0.1C discharge specific capacity (mAh / g) is as follows: ; Where m is the mass (g) of the active material (i.e., sodium iron pyrophosphate cathode material) in the cathode sheet.

[0056] (3) The calculation method for cycle capacity retention (%) is as follows: .

[0057] The specific test results are shown in the table below: Table 1 Electrical performance test data

[0058] As shown in Table 1, the sodium iron pyrophosphate cathode material provided in Example 1 of this application exhibits relatively superior overall performance, with high capacity, cycle retention, and initial efficiency. This may be related to the favorable ratio of the main phase to the impurity phase NaFePO4. Furthermore, the dropwise mixing and two-step sintering processes may have positively impacted the crystal structure and interfacial stability. The sodium iron pyrophosphate cathode materials provided in Examples 3 and 4 also showed good performance. The performance of the three samples from Examples 1, 3, and 4 was relatively similar, indicating that fluctuations in the impurity ratio within a certain range may not have a single linear relationship with the electrochemical behavior of the material. The performance of the sodium iron pyrophosphate cathode material provided in Example 6 was slightly lower than the aforementioned three groups. Although its impurity ratio also fell within a similar range, the different types of raw materials such as sodium and iron sources may have affected the reaction pathway of the precursor or the distribution morphology of the impurity phase, thus affecting the final performance. The sodium iron pyrophosphate cathode materials provided in Examples 5 and 2 had a higher impurity ratio, resulting in a significant performance decline. It is speculated that a higher impurity content may have diluted the active component or introduced additional interfacial side reactions to some extent. The difference between Example 7 and Example 5 is that the precursor B solution was directly mixed instead of added dropwise. Under the same impurity phase, its performance was further weakened, possibly reflecting the influence of the mixing method on the uniformity of the impurity phase and the particle morphology. Example 8 used one-step sintering instead of two-step sintering. Its cycle retention rate and first-time efficiency were relatively low compared to all examples. The lack of a low-temperature pre-sintering stage may have resulted in insufficient precursor decomposition and crystal growth, thus affecting structural integrity and interface properties.

[0059] Compared to Example 1, Comparative Example 1, which neither added manganese / fluorine doping nor introduced precursor B solution for impurity phase control, had the lowest capacity and cycling performance among all samples, indicating that the basic formulation itself may not achieve ideal electrochemical performance. Comparative Example 2, although introducing manganese / fluorine doping, still did not add precursor B solution, resulting in an uncontrolled impurity phase ratio. Its performance was improved compared to Comparative Example 1 but was still significantly lower than all examples, indicating that while doping can bring some improvement, without proper impurity phase control, it is difficult to achieve optimal performance. Comparative Examples 3 and 4, on the other hand, adjusted the impurity ratio to be too high and too low, respectively. Both resulted in performance inferior to the examples. The former may have hindered ion transport due to excessive impurity phase becoming an inactive component, while the latter may have resulted in significant structural degradation during cycling due to the lack of stabilizing or modifying effect of the impurity phase on the main phase structure. These four comparative examples, from different perspectives, verify the necessity of the process of this invention: the lack of impurity phase control and deviation from the ratio window both prevent the achievement of ideal electrochemical performance.

[0060] 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 sodium iron pyrophosphate cathode material, characterized in that, The cathode material comprises a main phase of sodium iron pyrophosphate and a heterogeneous phase of NaFePO4, wherein the molar ratio of the main phase of sodium iron pyrophosphate to the heterogeneous phase of NaFePO4 is 1:(0.3-0.9).

2. The sodium iron pyrophosphate cathode material according to claim 1, characterized in that, The chemical formula of the sodium iron pyrophosphate cathode material is Na₄Fe₂O₃. 3-x Mn x (PO4) 2-y P2O7F y Where x = 0.2 - 0.4, y = 0.1 - 0.2; And / or, the molar ratio of the main phase sodium iron pyrophosphate and the impurity phase NaFePO4 is 1:0.

5.

3. The sodium iron pyrophosphate cathode material according to claim 2, characterized in that, x=0.3, y=0.

15.

4. A method for preparing the sodium iron pyrophosphate cathode material according to any one of claims 1-3, characterized in that, Includes the following steps: S1, a sodium source, an iron source, a manganese source, a phosphorus source, and a fluorine source are mixed and heat-treated to obtain a precursor A solution; S2, mix sodium source, iron source and phosphorus source, heat treat to obtain precursor B solution; S3. Mix the precursor B solution and the precursor A solution, heat treat to form a mixture, dry, sinter in two stages, and cool to obtain sodium iron pyrophosphate cathode material, wherein the molar ratio of precursor A to precursor B is 1:0.3-0.

9.

5. The preparation method according to claim 4, characterized in that, In step S1, the molar ratio of Na, Fe, Mn, P, and F in the sodium source, iron source, manganese source, phosphorus source, and fluorine source is (3.95-4.15):(2.6-2.8):(0.2-0.4):4:(0.1-0.2). And / or, in step S2, the molar ratio of Na, Fe, and P in the sodium source, iron source, and phosphorus source is (0.9-1.1):(0.9-1.1):(0.9-1.1). And / or, in step S1, the precursor A solution further includes a chelating agent; Optionally, the mass ratio of the chelating agent to the total amount of metal ions in the sodium source, iron source, manganese source, and phosphorus source is (0.005-0.015):

1.

6. The preparation method according to claim 4 or 5, characterized in that, In step S3, the precursor B solution is added dropwise to the precursor A solution at a rate of 5-10 mL / min. And / or, in step S3, the specific steps of the two-stage sintering include, under an inert atmosphere, first raising the temperature to 350-420℃ at a heating rate of 2-5℃ / min and holding for 2-4h, then raising the temperature to 600-650℃ at a heating rate of 2-5℃ / min and holding for 6-10h. And / or, in step S3, the drying includes spray drying; Optionally, the inlet temperature of the spray dryer is 180-220°C; Optionally, the outlet temperature of the spray dryer is 90-110°C; Optionally, the feed rate for the spray drying is 5-15 mL / min; And / or, the solid content of the mixture before drying is 10-30 wt%.

7. The preparation method according to any one of claims 4-6, characterized in that, In step S1, when forming the precursor A solution, the heat treatment temperature is 60-80℃ and the heat treatment time is 1.5-2.5h. And / or, in step S2, when forming the precursor B solution, the heat treatment temperature is 45-55℃ and the heat treatment time is 25-30min; And / or, in step S3, when forming the mixture, the temperature of the heat treatment of the precursor B solution and the precursor A solution is 45-55℃, and the heat treatment time is 0.8-1.2h.

8. The preparation method according to any one of claims 4-7, characterized in that, The sodium source forming precursor A solution and / or precursor B solution includes sodium salt; Optionally, the soluble salt of sodium includes at least one of sodium carbonate, sodium hydroxide, sodium bicarbonate, sodium acetate, sodium oxalate, and sodium dihydrogen phosphate. And / or, the iron source forming the precursor A solution and / or precursor B solution includes at least one of ferrous salts and ferric salts; Optionally, the ferrous salt includes at least one of ferrous sulfate, ferrous oxalate, and ferrous acetate; Optionally, the trivalent iron salt includes ferric nitrate; And / or, the manganese source forming the precursor A solution and / or precursor B solution includes manganese salts; Optionally, the manganese salt includes at least one of manganese acetate, manganese carbonate, and manganese nitrate; And / or, the phosphorus source forming the precursor A solution and / or precursor B solution includes at least one of phosphoric acid and phosphate; Optionally, the phosphate includes at least one of sodium dihydrogen phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, and sodium phosphate; And / or, the fluorine source forming the precursor A solution includes a fluoride salt; Optionally, the fluoride salt includes at least one of sodium fluoride, ammonium fluoride, and lithium fluoride; And / or, the chelating agents forming the precursor A solution include organic acid chelating agents; Optionally, the organic acid chelating agent includes at least one of citric acid, ethylenediaminetetraacetic acid tartaric acid, and gluconic acid.

9. A positive electrode sheet, characterized in that, The material comprises sodium iron pyrophosphate cathode material according to any one of claims 1-3 or sodium iron pyrophosphate cathode material prepared by any one of claims 4-8.

10. A sodium-ion battery, characterized in that, Includes the positive electrode sheet as described in claim 9.

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