Sodium ferric phosphate positive electrode material of sodium-ion battery and preparation method of sodium ferric phosphate positive electrode material

Abstract

The invention relates to a sodium iron phosphate positive electrode material of a sodium ion battery and a preparation method thereof, the positive electrode material sequentially comprises a Na4Fe7 (PO4) 6 core, a Mn doping layer, a Mg doping layer and a conductive carbon coating layer from inside to outside, and the structural general formula of the positive electrode material can be abbreviated as Na4Fe7 (PO4) 6 (at) Mn (at) Mg (at) C; wherein the chemical general formula of the Mn doped layer is Na < 4 > Fe < 7-x > Mn < x > (PO < 4 >) < 6 >; the general chemical formula of the Mg-doped layer is Na4Fe7-yMgy (PO4) 6, x and y are both greater than 0, x does not exceed 2.1, and y does not exceed 1.75. According to the positive electrode material disclosed by the invention, the rate capability and the capacity are improved through the Mn doping layer, and the interface side reaction is inhibited through the Mg doping layer, so that the technical problems that the Na4Fe7 (PO4) 6 positive electrode material with a thermodynamic stable phase in the prior art is relatively short in cycle life and too low in capacity and charge-discharge rate are solved.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of sodium ion batteries, in particular to a sodium ion battery sodium iron phosphate positive electrode material and a preparation method thereof. BACKGROUND

[0002] Under the impetus of the rapid development of new energy storage and electric vehicles, sodium ion batteries have become an important supplement or alternative to lithium ion batteries due to their abundant sodium resource reserves, low cost, excellent safety and other advantages, and have been widely studied and concerned. As a core component of sodium ion batteries, the performance of the positive electrode material directly determines the energy density, cycle life and rate characteristics of the battery.

[0003] Among the many candidate materials, Na4Fe7(PO4)6, as a thermodynamically stable iron phosphate polyanion compound, has attracted attention due to its unique structural characteristics. This material is a thermally stable phase that can be directly synthesized by conventional high-temperature solid-phase method, thereby avoiding the complex ion exchange process required for metastable phase materials such as olivine-type NaFePO4, and has a significant advantage in industrialized preparation. At the same time, as an iron-based phosphate material, it has a wide range of raw material sources, low cost, and is environmentally friendly, further enhancing its application potential.

[0004] However, this thermodynamic stability brings corresponding constraints on the crystal structure and electrochemical performance, resulting in many key performance shortfalls of Na4Fe7(PO4)6 material in practical application, which seriously restricts its industrial application process. First, the stable polyanion framework of Na4Fe7(PO4)6 can obtain high structural durability, but at the same time, it also limits the electron escape ability of the lattice, resulting in extremely low intrinsic electronic conductivity. The stable crystal structure also makes the sodium ion migration channel relatively narrow and has a high diffusion energy barrier, making the sodium ion migration slow. The dual disadvantages of electronic and ionic conduction together result in poor rate performance of the material. And compared with other phosphate positive electrode materials (such as NaFePO4), the theoretical specific capacity of Na4Fe7(PO4)6 (about 110 mAh·g -1 ) is at a relatively low level, and combined with its high ion transport and rate limitation, the already low capacity is more difficult to fully exert, resulting in low energy density. Studies have shown that its discharge capacity at 10C high rate is usually less than 70 mAh·g -1This severely limits the application of this material in practical devices, especially in scenarios requiring rapid energy storage / release, such as large-scale energy storage and grid frequency regulation. Furthermore, during charge-discharge cycles, particularly at high rates, the abundant iron ions in the electrodes readily undergo interfacial side reactions with the electrolyte, leading to problems such as the dissolution of transition metal ions and localized collapse of the electrode structure. This significantly shortens the battery's cycle life, preventing it from reaching a high level.

[0005] While existing technologies have attempted to improve the electrochemical performance of Na4Fe7(PO4)6 using physical or chemical modification strategies, these approaches often only address a limited extent of the aforementioned defects while preserving its thermal stability, making comprehensive breakthroughs difficult. For example, simple carbon coating can construct an external conductive network and improve electronic conduction, but its effect on enhancing bulk ionic conductivity and suppressing interfacial side reactions is limited; while bulk doping with metal ions (such as Mo)... 6+ K + Co 2+ Even with these methods, the cycling stability or initial capacity can only be slightly improved. They cannot simultaneously solve the problems of poor electronic conductivity and slow ion diffusion, nor can they prevent the rapid capacity decay at high rates.

[0006] Therefore, there is an urgent need for a sodium iron phosphate cathode material that, while possessing good thermodynamic stability, also has relatively high capacity, discharge rate, and cycle life. Summary of the Invention

[0007] (a) Technical problems to be solved In view of the above-mentioned shortcomings and deficiencies of the prior art, the present invention provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method, which solves the technical problems of short cycle life, low capacity and charge / discharge rate of Na4Fe7(PO4)6 cathode material with thermodynamically stable phase in the prior art.

[0008] (II) Technical Solution To achieve the above objectives, the main technical solutions adopted by the present invention include: In a first aspect, the present invention provides a sodium iron phosphate cathode material for sodium-ion batteries, characterized in that it comprises, from the inside out, a Na4Fe7(PO4)6 core, a Mn doped layer, a Mg doped layer, and a conductive carbon coating layer, the general formula of which can be simplified as Na4Fe7(PO4)6@Mn@Mg@C; wherein, the chemical formula of the Mn doped layer is Na4Fe 7-x Mn x (PO4)6; the general chemical formula of the Mg-doped layer is Na4Fe. 7-y Mg y(PO4)6; x and y are both stoichiometric coefficients that satisfy charge balance, both x and y are greater than 0 and x does not exceed 2.1 and y does not exceed 1.75.

[0009] According to a preferred embodiment of the present invention, the molar ratio of Fe to Mn in the Mn-doped layer is Fe:Mn=80-90:10-20, calculated by stoichiometry; and the molar ratio of Fe to Mg in the Mg-doped layer is Fe:Mg=85-95:5-15.

[0010] According to a preferred embodiment of the present invention, the particle size of the sodium iron phosphate cathode material is 1-12 micrometers; the mass ratio of the Na4Fe7(PO4)6 core, Mn doped layer, Mg doped layer and conductive carbon coating layer is 55-70:13-25:10-20:2-10.

[0011] In a second aspect, the present invention also provides a method for preparing sodium iron phosphate cathode material for sodium-ion batteries as described in any one of the first aspects, comprising the following steps: S1: Provide core precursor materials; weigh sodium source, iron source, manganese source and phosphorus source, mix and spray granulate to obtain Mn-doped precursor materials; weigh sodium source, iron source, magnesium source and phosphorus source, mix and spray granulate to obtain Mg-doped precursor materials; S2: Mix the core precursor material with the Mn-doped precursor material and granulate to obtain primary particles; mix the primary particles with the Mg-doped precursor material and granulate to obtain secondary particles; mix the secondary particles with a carbon source and granulate to obtain a gradient composite precursor material. S3: Sinter the gradient composite precursor material to crystallize the core precursor material, Mn-doped precursor material and Mg-doped layer precursor material, and carbonize the carbon source to form a conductive carbon coating layer, thus obtaining sodium iron phosphate cathode material.

[0012] According to a preferred embodiment of the present invention, in S1, the particle size of the core precursor material is 1-13 micrometers; the particle size of the Mn-doped precursor material is 0.1-4 micrometers; and the particle size of the Mg-doped precursor material is 0.01-3 micrometers.

[0013] According to a preferred embodiment of the present invention, in S1, the sodium source includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, sodium carbonate, sodium nitrate, sodium oxalate, sodium sulfate, sodium sulfite, sodium acetate, sodium citrate, and sodium hydroxide. The iron source is at least one of ferrous sulfate, ferric sulfate, ferric phosphate, ferrous phosphate, ferrous oxalate, ferrous acetate, ferrous oxide, ferric oxide, iron tetroxide, and ferrous carbonate. The phosphorus source is at least one of the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, phosphoric acid, ferric phosphate, triammonium phosphate, pyrophosphate, or sodium pyrophosphate. The carbon source is at least one of sucrose, glucose, polyacrylic acid, malic acid, oxalic acid, citric acid, and ascorbic acid. The manganese source includes at least one of manganese sulfate, manganese acetate, manganese carbonate, and manganese dioxide; The magnesium source includes at least one of magnesium sulfate, magnesium acetate, magnesium oxide, and basic magnesium carbonate. The adhesive is at least one of polymethyl methacrylate, polyvinylpyrrolidone, polyethylene glycol, polyacrylonitrile, polyacrylic acid, polystyrene, hydroxypropyl methylcellulose, methylcellulose, asphalt, dextrin, and sucrose.

[0014] According to a preferred embodiment of the present invention, in S2, after mixing the core precursor material and the Mn-doped precursor material, a binder of 1%-2% of the total weight of the two is added, followed by high-speed shearing granulation and drying to obtain the primary particles; The primary particles are mixed with the Mg-doped precursor material, and then 1%-2% of a binder is added according to their total weight. After high-speed shearing and granulation, the mixture is dried to obtain the secondary particles. The secondary particles are mixed with a solution containing a carbon source and then spray-granulated to obtain a gradient composite precursor material.

[0015] According to a preferred embodiment of the present invention, in S2, the ratio of the core precursor material: Mn-doped precursor material: Mg-doped precursor material by mass is 60-70:15-25:10-20.

[0016] According to a preferred embodiment of the present invention, in S2, the weight of the carbon source is 5%-20% of the total weight of the core precursor material, the Mn-doped precursor material, and the Mg-doped layer precursor material; the weight of the binder is 1%-6% of the total weight of the core precursor material, the Mn-doped precursor material, and the Mg-doped layer precursor material.

[0017] According to a preferred embodiment of the present invention, in step S3, sintering includes the following steps: S31: Under an inert atmosphere, the temperature of the environment in which the gradient composite precursor material is located is raised to 250-350℃ at a heating rate not exceeding 3℃ / min for the first stage of heat preservation. S32: Raise the temperature of the environment where the gradient composite precursor material is located to 500-550℃ at a heating rate of 3-6℃ / min, and carry out the second stage of heat preservation. S33: The temperature of the environment in which the gradient composite precursor material is located is increased to 600-700℃ at a heating rate not exceeding 3℃ / min, and the third stage of heat preservation is carried out to obtain sodium iron phosphate cathode material.

[0018] (III) Beneficial Effects The beneficial effects of this invention are as follows: This invention provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method. The general formula of the sodium iron phosphate cathode material is Na4Fe7(PO4)6@Mn@Mg@C. Compared with the prior art, because this invention uses the thermodynamically stable Na4Fe7(PO4)6 as the core and doped base material, it can fundamentally avoid the synthesis problems of unstable phases such as olivine-type NaFePO4 structure sodium iron phosphate. This allows the material to be synthesized by conventional solid-state methods, which is simple and easy to scale up. Furthermore, Na4Fe7(PO4)6 also possesses "zero strain" characteristics, which gives the cathode material of this invention a longer cycle life.

[0019] Furthermore, since the present invention independently constructs an intermediate functional layer doped with Mn outside the Na4Fe7(PO4)6 core, by introducing a redox couple of manganese ions with a higher potential, the average operating voltage of the cathode material of the present invention is significantly improved, and the capacity utilization rate of the core material is fully improved, thereby improving the energy density and rate performance of the cathode material of the present invention.

[0020] Furthermore, this invention constructs a Mg-doped interfacial functional layer on the outer layer of the cathode material, utilizing its electrochemical inertness to form a stable solid solution on the surface of the cathode material. This effectively reduces the direct contact between the active material in the cathode material and the electrolyte, reduces the problems of severe interfacial side reactions and transition metal dissolution during long-term use of the cathode material, slows down the rate of interfacial degradation, and further improves the long-term cycle capacity retention rate of the cathode material of this invention.

[0021] Furthermore, this invention also promotes the directional diffusion of elements (Mn, Mg) by first preparing and granulating multiple precursors, and then co-sintering them once, forming a concentration gradient transition region with a non-sharp interface. This buffers interlayer stress, enhances the structural integrity and inter-interface bonding strength of the cathode material of this invention, provides a smooth channel for sodium ion migration across layers, reduces interfacial impedance, and steadily improves the rate performance of this invention.

[0022] The multi-layered structure of the cathode material of this invention enables it to be prepared by high-temperature curing while possessing relatively high energy density, good rate performance, and ultra-long cycle life. Even under frequent, high-rate charge and discharge conditions, it can maintain a long service life, meeting the needs of large-scale energy storage, backup power, and microgrids. Attached Figure Description

[0023] Figure 1 This is a microscopic electron microscope image of the sodium iron phosphate cathode material obtained in Example 1 of the present invention. Detailed Implementation

[0024] To better explain and facilitate understanding of the present invention, the present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0025] This invention also provides a method for preparing sodium iron phosphate cathode material for sodium-ion batteries, comprising the following steps: S1: Three functional precursors were prepared separately. By independently synthesizing three precursor materials with controllable chemical components and physical morphology, the stability of the hierarchical structure and the effective formation of the gradient structure during subsequent granulation and compounding were ensured, laying the foundation for the effective combination of different hierarchical functions.

[0026] Specifically, this includes S11: Preparation of core precursors: Sodium source, iron source, and phosphorus source are weighed according to the metering, and then these raw materials are dissolved in water or organic solvent, mixed evenly, and then sprayed into granules to obtain core precursor materials.

[0027] S12: Preparation of Mn-doped precursor for improving voltage: Sodium source, iron source, manganese source and phosphorus source are weighed according to the metric (i.e., the manganese source is used to replace part of the iron source in S11 according to the proportion), these raw materials are dissolved in water or organic solvent, mixed evenly and then sprayed into granules to obtain Mn-doped precursor material.

[0028] S13: Preparation of Mg-doped precursor for interface stabilization: Sodium source, iron source, magnesium source and phosphorus source are weighed according to the stoichiometric ratio (i.e., magnesium source replaces part of the iron source in S11 according to the ratio). These raw materials are dissolved in water or organic solvent, mixed evenly and then sprayed into granules to obtain Mg-doped precursor material.

[0029] S2: Mixed Granulation: The core precursor material is mixed and granulated with the Mn-doped precursor material to obtain primary particles. The primary particles are then mixed and granulated with the Mg-doped precursor material to obtain secondary particles. The three-layer composite particles obtained by mixed granulation are mixed with a carbon source solution and then subjected to a second granulation by spray drying, so that the carbon source is uniformly attached to the surface and interstices of the composite particles to obtain a gradient composite precursor material.

[0030] S3. Co-sintering: Under an inert atmosphere, or protective atmosphere, the gradient composite precursor material is sintered (co-sintered) to crystallize the core precursor material, Mn-doped precursor material, and Mg-doped layer precursor material, forming a Na4Fe7(PO4)6 core, Mn-doped layer, and Mg-doped layer. The carbon source is also carbonized to form a conductive carbon coating layer, ultimately yielding the sodium iron phosphate cathode material. During the crystallization process of the three precursors, the doped Mn and Mg will further diffuse into adjacent doped layers and / or the core through solid-phase diffusion, forming a concentration gradient transition region without sharp interfaces, i.e., a gradient doped structure. Therefore, the sodium iron phosphate cathode material of this invention can also be called a gradient-doped sodium iron phosphate cathode material.

[0031] Specifically, the sodium iron phosphate cathode material finally prepared according to the method of this invention comprises, from the inside out, a Na4Fe7(PO4)6 core (referred to as the core layer), a Mn doped layer, a Mg doped layer, and a conductive carbon coating layer, the general formula of which can be simplified as Na4Fe7(PO4)6@Mn@Mg@C. The Na4Fe7(PO4)6 core is obtained by crystallizing a core precursor. The Mn doped layer is obtained by crystallizing a Mn doped precursor material, the general formula of which is Na4Fe7(PO4)6@Mn@Mg@C. 7-x Mn x (PO4)6. The Mg-doped layer is obtained by crystallizing the Mg-doped layer precursor material, and its general structural formula is Na4Fe. 7-y Mg y (PO4)6. Where x and y are stoichiometric coefficients that satisfy charge balance, both x and y are greater than 0 and x does not exceed 2.1 and y does not exceed 1.75.

[0032] In this invention, the thermodynamically stable phase Na4Fe7(PO4)6 serves as the structural core and doping basis. Because its crystal structure exhibits minimal changes in the framework structure (FeO6 octahedrons and PO4 tetrahedrons) and an overall cell volume change rate typically below 0.3% during repeated sodium ion insertion / extraction processes, it demonstrates zero-strain characteristics. This effectively ensures that the sodium iron phosphate cathode material of this invention (hereinafter referred to as the cathode material of this invention) will not experience structural fatigue or collapse due to accumulated mechanical stress during long-term cycling, nor will it undergo irreversible phase transitions due to repeated sodium ion insertion / extraction. It can maintain the unobstructed flow of sodium ion channels for a long period, providing stable reversible capacity and providing a structural basis for the long cycle life of the cathode material of this invention. Furthermore, its thermodynamic stability allows it to be directly formed during high-temperature sintering, without the need for unconventional pathways such as ion exchange, making the preparation method relatively simple.

[0033] The core function of Mn-doped layers is to increase the operating voltage and optimize reaction kinetics. Among these, manganese ions (Mn)... 2+ / Mn 3+ / Mn 4+ The redox couple, especially Mn 3+ / Mn 4+ The redox couple has a higher charge-to-iron ratio than the Fe ion in the phosphate framework. 2+ / Fe 3+ The redox potential of Mn is increased. Introducing Mn directly increases the average operating voltage of the material, thereby improving the energy density of the cathode material of this invention. More importantly, Mn doping is not limited to providing a high voltage platform. During charge and discharge processes, Mn in a high valence state, such as Mn... 4+ Manganese ions possess a strong electron-withdrawing ability and can exert an electron-induced effect on neighboring Fe ions through pathways such as Fe-O-Mn in the crystal structure. This effect can locally reduce the Fe concentration. 2+ / Fe 3+ The energy barrier required for oxidation allows some Fe ions to be oxidized more deeply and rapidly within the high-voltage reaction range of Mn. This enables additional activation and extraction of more sodium ions from the Na4Fe7(PO4)6 lattice and deep within the core layer, thereby increasing the response speed. Consequently, while increasing the voltage, this further enhances the reversible capacity and rate performance of the cathode material of this invention.

[0034] The core function of the Mg-doped layer is to construct a stable electrode / electrolyte interface, where the standard electrode potential of Mg ions is much lower than that of Fe. 2+ / Fe 3+ It can maintain a certain electrochemical inertness within the working voltage window of the electrode material of the present invention. By setting the Mg doping layer on the outermost layer of the positive electrode material particles of the present invention, excluding the carbon layer, a more stable passivation layer can be constructed on the basis of Na4Fe7(PO4)6, reducing the direct contact area between the highly active transition metals (Fe, Mn) and the electrolyte. In principle, this inhibits the catalytic decomposition of the electrolyte under high voltage, the dissolution and loss of transition metal ions, and the irreversible side reactions such as the continuous thickening of the solid electrolyte interphase (SEI) film caused by this.

[0035] Furthermore, since the ionic radii of Mn and Mg are similar to those of Fe, and each tetrahedral cell of Mn and Mg has seven Fe sites, they can provide a large number of equivalent substitutable sites for Mn and Mg. In a Fe-dominated crystal structure, Mn and Mg can substitute Fe sites in solid solution form without significantly affecting the stability of the crystal structure. Therefore, the Mn-doped and Mg-doped layers of this invention, after doping, still possess a crystal structure similar to the Na4Fe7(PO4)6 core, maintaining a high degree of consistency with the Na4Fe7(PO4)6 core in terms of thermodynamic stability, zero strain, and sodium ion insertion / extraction stability, thus minimizing the likelihood of stress-related conflicts. Under these conditions, the different layered structures of this invention can crystallize simultaneously during a single sintering process, and through solid-phase diffusion at high temperatures, naturally form a gradient transition region with continuously changing composition, avoiding the high impedance and stress concentration problems caused by sharp interfaces.

[0036] The multilayer structure of this invention further confines the Mn element, which may cause problems such as lattice distortion, in the middle, so that its negative impact is constrained and buffered by the stable inner core and the inert outer Mg shell. At the same time, the capacity-sacrificing Mg element is strictly limited to the outermost layer, achieving the maximum interface protection effect with the minimum capacity cost. This allows the gradient-doped sodium iron phosphate material of this invention to maintain a small overall volume change throughout the entire charge and discharge process, and to maintain the stability of the sodium ion channel for a long time, thus having a long cycle life.

[0037] Preferably, in S1, based on stoichiometric ratios, the ratio of Fe to Mn in the Mn-doped layer is Fe:Mn=70-90:10-30, more preferably Fe:Mn=80-90:10-20; the ratio of Fe to Mg in the Mg-doped layer is Fe:Mg=75-95:5-25, more preferably Fe:Mg=85-95:5-15; or, in the final gradient-doped sodium iron phosphate cathode material, x ranges from 0.7 to 2.1, more preferably 0.7 to 1.4, and y ranges from 0.35 to 1.75, more preferably 0.35 to 1.05.

[0038] Specifically, the Mn doping level needs to reach a certain threshold (>10%) to produce a significant voltage boost effect. Excess Mn (>20%) may lead to voltage boosting due to differences in ionic radii (the radii of manganese and iron ions are similar but not identical; the general radius of manganese ions is larger than that of iron ions) and Mn... 3+Excessive doping can exacerbate lattice distortion, potentially leading to problems such as blocked sodium ion migration channels, decreased cycle stability, and even the formation of impurity phases. Similarly, sufficient Mg doping (>5%) is required to form a continuous surface cover and ensure effective interface protection. However, since the radius of Mg ions is smaller than that of Fe, excessive use (>15%) may cause excessive lattice shrinkage in the non-carbon layer of the cathode material of this invention, resulting in increased stress between it and the intermediate Mn-doped layer, which may lead to problems such as cracks. At the same time, excessive occupation of surface active sites may also lead to a decrease in the reversible capacity of the cathode material of this invention.

[0039] Preferably, in S1, the sodium source includes at least one selected from sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, sodium carbonate, sodium nitrate, sodium oxalate, sodium sulfate, sodium sulfite, sodium acetate, sodium citrate, and sodium hydroxide. The iron source is at least one selected from ferrous sulfate, ferric sulfate, ferric phosphate, ferrous phosphate, ferrous oxalate, ferrous acetate, ferrous oxide, ferric oxide, ferric oxide, ferric oxide, and ferrous carbonate. The phosphorus source is at least one selected from ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, phosphoric acid, ferric phosphate, triammonium phosphate, pyrophosphate, or sodium pyrophosphate. The manganese source is at least one selected from manganese sulfate, manganese acetate, manganese carbonate, and manganese dioxide. The magnesium source is at least one selected from magnesium sulfate, magnesium acetate, magnesium oxide, and basic magnesium carbonate.

[0040] This invention does not have any particular requirements for the specific selection of sodium, iron, phosphorus, manganese and magnesium sources. Those skilled in the art can arbitrarily select and combine from the conventional materials listed above according to cost, process compatibility and the purity requirements of the final product, as long as Na4Fe7(PO4)6 material can be obtained by sintering and corresponding doping can be performed without introducing harmful impurities that are difficult to remove.

[0041] Preferably, in S1, water is used as a solvent to dissolve other materials, thus avoiding organic solvent residue.

[0042] More preferably, in S1, the Mg-doped precursor material is prepared by ultrasonic spray drying granulation. The high-frequency vibration of the ultrasonic spray generates micron to submicron-sized droplets, resulting in a small and narrowly distributed particle size of the precursor material, or precursor powder, after drying. This provides finer particles that are easier to disperse evenly in subsequent granulation, ensuring a dense and continuous outermost coating and preventing the overall particle size of the cathode material of this invention from becoming excessively large.

[0043] Preferably, in S1, the particle size of the core precursor material is 1-13 micrometers, the particle size of the Mn-doped precursor material is 0.1-4 micrometers, and the particle size of the Mg-doped precursor material is 0.01-3 micrometers. Furthermore, the average particle size of the core precursor material is greater than that of the Mn-doped precursor material, which in turn is greater than that of the Mg-doped precursor material. This ensures that during subsequent high-speed shear granulation, smaller particles can effectively fill and adhere to the surface of larger particles, thereby achieving layer-by-layer coating.

[0044] More preferably, in S1, the core precursor material has a particle size of 2-9 micrometers to ensure that the final cathode material has a good tap density and a short ion diffusion distance, avoiding affecting the electrode processing performance or rate performance of the cathode material. The Mn-doped precursor material has a particle size of 1-3.5 micrometers to ensure the effectiveness of filling and coating. The Mg-doped precursor material has a particle size of 0.5-2 micrometers to ensure uniform and effective coverage. If its particle size is similar to that of the Mn-doped precursor material, it may be difficult to form an effective surface coating, and may lead to a significant decrease in the capacity of the cathode material.

[0045] It should be noted that, in practice, due to factors such as raw material batches, machine errors, and fluctuations in granulation effect, the particle size of the precursor material obtained by granulation may not entirely fall within the aforementioned particle size range. In actual operation, as long as the D90 and D10 values ​​of a certain precursor material fall within the aforementioned preferred range, and the D50 value is stable within the aforementioned more preferred range, the basic coating requirements can be met. Furthermore, in actual operation, regardless of the type of precursor material being prepared, after granulation, conventional methods such as cryogenic grinding and fine ball milling can be used for grinding to further control the particle size of the precursor material. However, it is necessary to avoid introducing impurities such as dispersants, which will not be elaborated upon in this invention. Preferably, in S2, the mass ratio of the core precursor material, the Mn-doped precursor material, and the Mg-doped precursor material is: core precursor material: Mn-doped precursor material: Mg-doped precursor material = 60-70:15-25:10-20.

[0046] Specifically, it is necessary to ensure a high weight ratio of the core precursor material. This provides space for other doped materials while ensuring that the core layer of the final cathode material particle dominates, guaranteeing the most basic stable capacity of the cathode material. By controlling the amount of Mn-doped precursor material used and the thickness of the formed Mn-doped layer, the stability and continuity of the subsequently formed Mn-doped layer are ensured, while avoiding problems such as interface instability and distortion caused by excessive Mn-doped layer thickness. This ensures that the Mn-doped layer can stably increase the voltage without affecting the lifespan of the cathode material. By controlling the amount of Mg-doped precursor material used, the stability and continuity of the Mg-doped layer are ensured, guaranteeing stable physical isolation and interface stability. This avoids the formation of an excessively thick electrochemical inert layer, ensuring the ion conduction effect of the Mg-doped layer and preventing dilution of the cathode material's capacity.

[0047] More preferably, the mass of the Mn-doped precursor material used is greater than the mass of the Mg-doped precursor material to ensure the effectiveness of the Mn-doped layer.

[0048] Preferably, in S3, the particle size of the final sodium iron phosphate cathode material is 1-20 micrometers, more preferably 2-8 micrometers, which ensures the tap density and avoids affecting the processing performance of the cathode material of the present invention, while avoiding an excessively long diffusion path of sodium ions inside the cathode material.

[0049] Furthermore, it should be noted that during actual operation, the particle size of the final cathode material may not fall within the above-mentioned preferred range or may fluctuate due to factors such as raw material batch, equipment status, granulation effect, particle size distribution during granulation, potential breakage during granulation, impregnation effect, bonding effect, and sintering effect. In actual operation, the preparation method of this invention can obtain batch-stable materials, with the final cathode material particles having a stable D50 value within the range of 2-8 micrometers, and D90 and D10 values ​​falling within the overall range of 1-20 micrometers, thus meeting the performance requirements of this invention. Similarly, in a specific cathode material particle, the specific thickness, composition, and other parameters of each doped layer may fluctuate depending on the actual coating effect and particle size. This invention does not impose further specific limitations. In actual operation, by feeding the core precursor material, Mn-doped precursor material, and Mg-doped precursor material according to the above-mentioned mass ratio, the parameters of most particles can be controlled within the above-mentioned preferred range.

[0050] Preferably, in S2, the carbon source is at least one of sucrose, glucose, polyacrylic acid, malic acid, oxalic acid, citric acid, and ascorbic acid. There are no special requirements for the specific material selection of the carbon source in this invention. Conventional materials that can form a conductive carbon layer on the surface of the Mg doped layer by sintering can be used.

[0051] Preferably, in the final sodium iron phosphate cathode material, the thickness of the conductive carbon coating layer (hereinafter referred to as the carbon layer) does not exceed 0.3 micrometers, more preferably 10-100 nanometers, and even more preferably 20-50 nanometers. While ensuring the formation of a stable and continuous carbon layer, it is also necessary to avoid an excessively thick carbon layer that hinders the transfer of sodium ions.

[0052] Preferably, in S2, the weight of the carbon source is 5%-20% of the total weight of the core precursor material, the Mn-doped precursor material, and the Mg-doped layer precursor material. The specific content needs to be determined based on the specific type of carbon source selected and the desired thickness of the carbon layer, as long as a carbon layer of appropriate thickness can be formed.

[0053] Preferably, in S2, the binder is at least one selected from polymethyl methacrylate, polyvinylpyrrolidone, polyethylene glycol, polyacrylonitrile, polyacrylic acid, polystyrene, hydroxypropyl methylcellulose, methylcellulose, asphalt, dextrin, and sucrose. More preferably, the binder is one selected from polymethyl methacrylate, polyvinylpyrrolidone, polyethylene glycol, polyacrylonitrile, polyacrylic acid, polystyrene, and hydroxypropyl methylcellulose. It is necessary to ensure that the selected binder has basic bonding effect, can bond the precursor material to form a preliminary coating structure, has good affinity with the surface of the inorganic precursor, does not easily generate impurity phases during sintering, and can be completely or mostly thermally decomposed in subsequent sintering into gases, small molecule hydrocarbons, and a small amount of dispersed carbon, avoiding the generation of harmful residues.

[0054] Preferably, the residual carbon content after thermal decomposition of the adhesive is less than 5 wt%, ensuring that no residues that significantly affect electrical performance are introduced.

[0055] Preferably, in step S2, the binder weight is 1%-6% of the total weight of the core precursor material, Mn-doped precursor material, and Mg-doped layer precursor material. More preferably, in step S2, after mixing the core precursor material and the Mn-doped precursor material, 1%-2% of the total weight of the binder is added, or a binder solution containing 1%-2% of the total weight of the binder is added. High-speed shear granulation is then performed. Utilizing the strong mechanical and shear forces generated by a high-speed rotating stirring paddle, the fine Mn-doped precursor material and the larger core precursor material undergo intense collision, compression, and kneading under the action of the binder. This allows the smaller Mn-doped precursor material to be firmly embedded and coated onto the surface of the larger particles. The mixture is then dried to obtain primary particles. Similarly, after mixing the primary particles with the Mg-doped precursor material, a binder solution containing 1%-2% of the total weight of the binder is added, and high-speed shear granulation is performed again to coat the surface of the primary particles with the Mg-doped layer precursor material. The mixture is then dried to obtain secondary particles.

[0056] By controlling the amount of binder, it is ensured that granulation can form stable composite particles, avoiding problems such as pulverization or stratification of primary or secondary particles during the drying process, and avoiding problems such as agglomeration of primary or secondary particles due to excessive use of binder, excessive binder residue, excessive pores, and particle cracking after sintering of cathode materials.

[0057] Preferably, in step S2, after obtaining primary or secondary particles, they are dried at 70-120°C for 1-12 hours to ensure the adhesive cures and that the moisture content of the primary and secondary particles is less than 2%, preferably less than 1%. During high-speed shear granulation, the stirring speed is controlled at 200-500 rpm, the cutting speed is controlled at 1000-3000 rpm, and the total granulation time is 10-30 minutes, ensuring that the primary and secondary particles are stably formed.

[0058] Furthermore, it should be understood that in S2, during drying, fluidized bed dryers, hollow paddle dryers, and other drying equipment with anti-adhesion properties suitable for granular materials can be selected for drying to maintain the particle morphology of primary and secondary particles and prevent the primary or secondary particles from clumping. After drying, while ensuring the integrity of the particle structure, appropriate grinding can also be performed to separate the adhered particles. Although this invention provides parameters such as the range of drying temperature and drying time, these are only preferred ranges. In actual practice, specific details such as the stirring speed during high-speed shear granulation, the amount of water or other solvent used to dissolve the binder to form a binder solution, and the selection of equipment for drying after granulation, as well as the specific drying temperature and drying time, can be further confirmed by selecting the type and performance parameters of the binder and high-speed granulator, combined with preliminary small-scale experiments, to ensure the formation of primary and secondary particles and prevent the resulting gradient composite precursor material from pulverizing or cracking before sintering. This invention will not elaborate on these details. Preferably, in S2, the secondary particles are mixed with a solution containing a carbon source and then spray-granulated to obtain the gradient composite precursor material.

[0059] Specifically, the carbon source is first dissolved in an organic solvent such as water or ethanol, preferably in water, to form a solution containing the carbon source. Then, the secondary particles are dispersed in the carbon source solution to form a suspension. The suspension is then spray-dried to deposit and uniformly coat the surface of the secondary particles and their pores. This facilitates the subsequent formation of a continuous conductive carbon layer that partially enters the pores of the doped layer and interlocks with the doped layer, ensuring the formation of a uniform, effective, and robust conductive network.

[0060] Preferably, in S2, during spray drying, the inlet air temperature is 180-250℃, the outlet air temperature is 100-140℃, and the atomization pressure is 0.2-0.4MPa.

[0061] Of course, it should be noted that in actual operation, the specific inlet air temperature, atomization pressure, outlet air temperature, etc. during spray drying also need to be confirmed based on parameters such as the particle size distribution of the specific gradient composite precursor material and related preliminary experiments. The above range is only a preferred range.

[0062] Preferably, in S3, the sintering process specifically includes the following steps: S31: Under an inert atmosphere, the temperature of the environment in which the gradient composite precursor material is located is raised to 200-400℃, preferably 250-350℃, at a heating rate not exceeding 3℃ / min. Then, a first-stage heat preservation is carried out to decompose the adhesive and carbon source. The heat preservation time is preferably 30-120min.

[0063] By using a relatively low heating rate, the volatile components within the gradient composite precursor material undergo a gradual and orderly thermal decomposition and / or release during the heating process, avoiding problems such as violent decomposition of organic matter and preventing issues like expansion, cracking, or even pulverization of the gradient composite precursor material. Subsequent heat preservation ensures that the organic matter within the gradient composite precursor material also decomposes sufficiently.

[0064] S32: Under an inert atmosphere, the temperature of the environment containing the gradient composite precursor material is increased to a suitable temperature range for element diffusion at a heating rate of 3-6℃ / min, preferably 500-600℃, more preferably 500-550℃. Then, a second stage of heat preservation is performed, preferably for 30-120 min, to allow for the diffusion and pre-crystallization of the doping elements. In S32, Mn and Mg ions can obtain sufficient energy to spontaneously diffuse into adjacent layers (Mn to the core layer and Mg doped layer, Mg to Mn doped layer), forming a concentration gradient transition region at the interface rather than a sharp interface. This achieves interlayer fusion and reduces potential stress conflicts between different layers, resulting in a more robust interface transition and further ensuring the long-term performance and service life of the cathode material of this invention.

[0065] At the same time, it is necessary to control the temperature and holding time during sintering in S32. While ensuring active solid-phase diffusion, it is also necessary to avoid excessive crystallization of the gradient composite precursor material due to excessively high temperature or excessively long holding time, which could lead to microstructure freezing, insufficient diffusion, or incomplete crystallization. Furthermore, it is necessary to avoid excessive diffusion of Mn and Mg ions due to excessively long holding time, which could result in the loss of gradient structure and indistinct multilayer structure, making it impossible to effectively increase voltage and maintain the long-term stability of the structure through multilayer structure.

[0066] Furthermore, since the organic matter has been basically decomposed and its structure has been basically stable in S31, the temperature of the environment where the gradient composite precursor material is located can be quickly raised to a temperature range suitable for element diffusion through a relatively fast heating rate in S32. This avoids problems such as the decrease in sintering activity of the gradient composite precursor material caused by staying in a low temperature environment below 300℃ for too long.

[0067] S33: Under an inert atmosphere, the temperature of the environment containing the gradient composite precursor material is raised to 600-700℃ at a heating rate not exceeding 3℃ / min for a third-stage heat preservation period, preferably 3-20h, more preferably 8-18h. This allows the gradient composite precursor material to undergo complete crystallization, and the amorphous carbon layer generated after the decomposition of the carbon source is graphitized and densified to form a conductive carbon coating layer, thus obtaining the sodium iron phosphate cathode material. The relatively low heating rate (not exceeding 3℃ / min) ensures that during the transition from pre-crystallization in S32 to complete crystallization, the grains grow uniformly and the internal stress is fully released, preventing an increase in lattice defects or the formation of microcracks due to sudden temperature changes.

[0068] Preferably, in S3, the inert atmosphere is a nitrogen or argon atmosphere, or an atmosphere.

[0069] Preferably, in step S3, after obtaining the sodium iron phosphate cathode material of the present invention, it can be further processed by grinding, sieving, etc., to further ensure that the particle size of the obtained sodium iron phosphate cathode material meets the requirements and to ensure its performance.

[0070] This invention also provides a sodium iron phosphate cathode material for sodium-ion batteries, comprising, from the inside out, a Na4Fe7(PO4)6 core, a Mn doped layer, a Mg doped layer, and a conductive carbon coating layer, the general formula of which can be simplified as Na4Fe7(PO4)6@Mn@Mg@C. The Mn doped layer is Na4Fe... 7-x Mn x (PO4)6, Mg doping layer is Na4Fe 7-y Mg y (PO4)6. Where x and y are stoichiometric coefficients that satisfy charge balance, both x and y are greater than 0 and x does not exceed 2.1 and y does not exceed 1.75.

[0071] The Mn-doped layer is a solid solution structure formed by partially replacing the Fe sites in Na4Fe7(PO4)6 with Mn, and its function is to improve the working voltage and effective capacity of the gradient-doped sodium iron phosphate cathode material of the present invention. The Mg-doped layer is a solid solution formed by partially replacing the Fe sites in Na4Fe7(PO4)6 with Mg, and its function is to suppress interfacial side reactions between the material and the electrolyte and improve interfacial stability. The conductive carbon layer is a carbon structure layer used to improve the electronic conductivity of the cathode material of the present invention.

[0072] Preferably, the mass ratio of the Na4Fe7(PO4)6 core, Mn doped layer, Mg doped layer, and conductive carbon coating layer is 55-70:13-25:10-20:2-10, to avoid any one layer having an excessively high weight ratio or excessive thickness, which would affect the performance balance of the cathode material of this invention. To better understand the above technical solution, exemplary embodiments of the present invention will be described in more detail below with reference to the accompanying drawings. Although exemplary embodiments of the present invention are shown in the drawings, it should be understood that the present invention can be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to enable a clearer and more thorough understanding of the present invention and to fully convey the scope of the invention to those skilled in the art.

[0073] Example 1 This embodiment provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method. The preparation method includes the following steps: S1: Preparation of three functional precursor materials: Preparation of core precursor material: Sodium carbonate, ferric nitrate nonahydrate, and ammonium dihydrogen phosphate were weighed according to the stoichiometric ratio of Na4Fe7(PO4)6, added to deionized water, and stirred until completely dissolved to form a homogeneous solution with a solid content of 20%. The solution was spray-dried at an inlet air temperature of approximately 220°C and an outlet air temperature of approximately 110°C to obtain the core precursor material with a D50 of 4.0 micrometers.

[0074] Preparation of Mn-doped precursor material: Sodium carbonate, ferric nitrate, manganese acetate, and ammonium dihydrogen phosphate were weighed according to a Fe to Mn molar ratio of 85:15 and dissolved in deionized water to form a homogeneous solution with a solid content of approximately 20 wt%. The solution was spray-dried at an inlet air temperature of 220℃ and an outlet air temperature of 110℃ to obtain the Mn-doped precursor material with a D50 of 1.5 micrometers.

[0075] Preparation of Mg-doped precursor material: Sodium carbonate, ferric nitrate, magnesium acetate, and ammonium dihydrogen phosphate were weighed according to a Fe:Mg molar ratio of 92:8 and dissolved in deionized water to form a homogeneous solution (solid content of approximately 15 wt%). The solution was treated by ultrasonic spray drying at an inlet air temperature of 200℃ and an outlet air temperature of 100℃ to obtain the Mg-doped precursor material with a D50 of 0.9 micrometers.

[0076] S2: Mixing and Granulation with Carbon Source Introduction; Three precursors were prepared according to a mass ratio of core precursor material: Mn-doped precursor material: Mg-doped precursor material = 64:19:17. Then, primary particles were prepared: The core precursor material and Mn-doped precursor material were mixed in a high-speed shear granulator, and then a binder (polyvinylpyrrolidone, prepared as a 5wt% aqueous solution) equivalent to 1.5% of the total mass of the two was added. Granulation was carried out for 15 minutes at a stirring speed of 300 rpm and a cutting speed of 1500 rpm. The resulting wet particles were then dried at 100℃ for 10 hours to obtain primary particles. Secondary particles were then prepared: The primary particles and Mg-doped precursor material were placed in a high-speed shear granulator. A binder (polyvinylpyrrolidone, prepared as a 10wt% aqueous solution) equivalent to 1.5% of the total mass of the two precursors was added, and granulation was carried out for 15 minutes at a stirring speed of 300 rpm and a cutting speed of 1500 rpm. The resulting wet granules were then dried at 100°C for 10 hours to obtain secondary granules. Then, 15% of the total mass of the three precursors (sucrose) was dissolved in deionized water to prepare a carbon source solution with a solid content of 15wt%. The secondary granules were dispersed in the carbon source solution and stirred evenly, followed by spray drying at an inlet air temperature of 220°C, an outlet air temperature of 120°C, and an atomization pressure of 0.3 MPa to obtain a gradient composite precursor material.

[0077] S3: The gradient composite precursor material was placed in a tube furnace and heated to 300℃ at a rate of 2℃ / min under nitrogen protection, and held for 60 minutes. Then, the temperature was increased to 550℃ at a rate of 5℃ / min and held for 60 minutes. Finally, the temperature was increased to 650℃ at a rate of 2℃ / min and held for 10 hours. After sintering, the material was cooled in the furnace to obtain the final product as shown below. Figure 1 The final product shown can be represented by the chemical formula of the resulting cathode material as: Na4Fe7(PO4)6@Na4Fe 5.95 Mn 1.50 (PO4)6@Na4Fe 6.44 Mg 0.56 (PO4)6@C, whose general structural formula can be simplified to Na4Fe7(PO4)6@Mn@Mg@C.

[0078] Example 2 This embodiment provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method. The difference from Embodiment 1 is that in S3, under a nitrogen atmosphere, the temperature is increased to 300°C at 2°C / min and held for 60 minutes, then increased to 520°C at 5°C / min and held for 90 minutes, and finally increased to 600°C at 2°C / min and held for 15 hours to obtain the sodium iron phosphate cathode material.

[0079] Example 3 This embodiment provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method. The difference from Embodiment 1 is that in S1, the molar ratio of Fe:Mn in the Mn-doped precursor material is 75:25. The chemical formula of the sodium iron phosphate cathode material obtained in this embodiment can be represented as Na4Fe7(PO4)6@Na4Fe 5.25 Mn 1.75 (PO4)6@Na4Fe 6.44 Mg 0.56 (PO4)6@C.

[0080] Example 4 This embodiment provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method. The difference from Embodiment 1 is that in S1, the molar ratio of Fe:Mg is 85:15. The chemical formula of the sodium iron phosphate cathode material obtained in this embodiment can be represented as Na4Fe7(PO4)6@Na4Fe 5.95 Mn 1.05 (PO4)6@Na4Fe 5.95 Mg 1.05 (PO4)6@C.

[0081] Comparative Example 1 This embodiment provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method. The difference from Example 1 is that in S1, the molar ratio of Fe:Mn is 50:50. The chemical formula of the sodium iron phosphate cathode material obtained in this comparative example can be represented as Na4Fe7(PO4)6@Na4Fe 3.5 Mn 3.5 (PO4)6@Na4Fe 6.44 Mg 0.56 (PO4)6@C.

[0082] Comparative Example 2 This embodiment provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method. The difference from Embodiment 1 is that in S2, under a nitrogen atmosphere, the temperature is increased to 300°C at 2°C / min and held for 60 minutes, then increased to 600°C at 2°C / min and held for 15 hours to obtain the sodium iron phosphate cathode material.

[0083] Comparative Example 3 This comparative example provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method. The difference from Example 1 is that in S1, the raw material of the core precursor material is also doped with Mn, with a doping ratio of Fe:Mn molar ratio of 75:25. In other words, the raw material of the core precursor material is the same as that of the Mn-doped precursor material. The sodium iron phosphate cathode material obtained in this comparative example is actually a bulk-doped three-layer structure, and its chemical formula can be represented as Na₄Fe₂O₃. 5.25 Mn 1.75 (PO4)6@Na4Fe 6.44 Mg 0.56 (PO4)6@C.

[0084] Comparative Example 4 This embodiment provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method. The difference from Example 1 is that, in S1, no Mg-doped precursor material is prepared. The resulting cathode material does not include a Mg-doped layer. The chemical formula of the sodium iron phosphate cathode material obtained in this comparative example can be represented as: Na4Fe7(PO4)6@Na4Fe 5.95 Mn 1.50 (PO4)6@C.

[0085] Comparative Example 5 This embodiment provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method. The difference from Embodiment 1 is that in S2, the ratio of core precursor material: Mn-doped precursor material: Mg-doped precursor material is 60:10:30.

[0086] Comparative Example 6 This embodiment provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method. The difference from Embodiment 1 is that in S2, the ratio of core precursor material: Mn-doped precursor material: Mg-doped precursor material is 60:35:10.

[0087] Comparative Example 7 This embodiment provides a sodium iron phosphate cathode material for sodium-ion batteries and its preparation method. The difference from Embodiment 1 is that in S1, sodium, iron, and phosphorus sources are prepared according to the stoichiometric ratio of Na2FeP2O7 material to prepare the core precursor material; the compositions of the other two precursor materials remain unchanged. The chemical formula of the sodium iron phosphate cathode material obtained in this comparative example can be represented as: Na2FeP2O7@Na4Fe 5.95 Mn 1.50 (PO4)6@Na4Fe 6.44 Mg 0.56 (PO4)6@C.

[0088] The positive electrode materials prepared in Examples 1-5 and Comparative Examples 1-7 were respectively fabricated into positive electrode sheets, with metallic sodium as the negative electrode, and then assembled into cylindrical batteries for charge-discharge testing at 0.1C rate under 1.5-4V. The test parameters are shown in Table 1 below. Table 1 Electrode Test Parameters

[0089] As can be seen from the content of Example 1, the cathode material of the present invention has relatively high first discharge capacity, first cycle coulombic efficiency (i.e., first efficiency), and still has excellent cycle retention rate (i.e., cycle capacity retention rate) at high rates. It can adapt to long-term, fast charge and discharge environments and has a long service life.

[0090] Compared to Example 1, Example 2, by using milder sintering conditions and extending the holding time during gradient structure formation, resulted in more uniform gradient structure diffusion, improved interfacial bonding and stability, leading to increased capacity and a certain degree of extended lifespan. Example 3, due to the doping of more Mn, directly increased the average operating voltage of the cathode material, thus significantly increasing its initial discharge capacity. However, the higher Mn doping also introduced some lattice distortion, resulting in a decrease in initial efficiency and capacity retention. Example 4, due to the increased Mg doping amount, provided a thicker magnesium-rich surface layer with stronger interfacial physical isolation, achieving the highest initial efficiency and excellent cycle stability, but also leading to a certain decrease in the cathode material's capacity.

[0091] Compared to Example 1, Comparative Example 1, due to excessive Mn doping, has Mn doping that dominates the doped layer. Even with the constraints of the core layer and Mg doped layer, it cannot maintain stability, resulting in severe lattice distortion and structural defects. This leads to overall structural instability of the cathode material, severely hindering ion conduction and causing a significant decrease in capacity, initial efficiency, and especially cycle retention. Comparative Example 2, lacking a holding period at the diffusion-active temperature stage, prevents the Mn / Mg elements from forming a smooth concentration gradient through solid-phase diffusion. This results in sharp interfaces between layers with high internal stress, making the cathode material prone to failure during long-term cycling, failing to effectively improve initial capacity, and causing a sharp drop in cycle retention. Comparative Example 3, by eliminating the core layer used to provide the overall structure and stable capacity of the cathode material and replacing it with a Mn-doped bilayer structure (core and Mn layer combined), achieves good capacity. However, due to the lack of constraint on the perturbation of the manganese-doped core structure and its own poor stability, the internal structure of the cathode material is significantly distorted, failing to fully utilize its capacity, resulting in poor actual capacity and cycle stability. Comparative Example 4, due to the removal of the Mg doping layer, has its outermost cathode material protected only by a carbon layer. The highly reactive Mn readily undergoes violent side reactions with the electrolyte, leading to a significant reduction in initial efficiency. Furthermore, the interface continues to degrade rapidly during long-term cycling, resulting in rapid capacity decay and low cycle retention. Comparative Example 5, due to the use of an excessively thick Mg doping layer, suffers from excessive inertness in its cathode material, severely reducing ion and electron conduction and significantly decreasing capacity. The excessively thick Mg doping layer is also prone to stress problems during long-term use, resulting in a low long-term cycle retention. Comparative Example 6, due to its excessively thick Mn doping layer, experiences excessively high internal stress in its cathode material, making it prone to interfacial conflicts and hindering effective capacity utilization. Moreover, during long-term use, its particle structure is prone to distortion or cracking, leading to poor cycle stability. Comparative Example 7 uses sodium iron pyrophosphate as the base material for the core layer. Although it is a thermodynamically stable material that can be prepared by sintering, the strain during use is relatively large (generally greater than 2%). As a core layer material, it is prone to problems such as interface mismatch and stress mismatch, and cannot provide stable support. This leads to the cathode material being prone to disintegration or cracking during long-term use, and it cannot maintain a high cycle retention rate. Furthermore, its structure has few sites for iron bonding, making it difficult to form an effective solid solution structure. It is also difficult to form an effective gradient doping structure with Mn doping during sintering, and it cannot efficiently conduct ions, resulting in a significant decrease in both capacity and initial efficiency.

[0092] In summary, the present invention can solve the technical problems of poor thermodynamic stability, short cycle life, and low capacity and charge / discharge rate of sodium iron phosphate cathode materials in the prior art.

[0093] 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 sodium iron phosphate cathode material for sodium-ion batteries, characterized in that, From the inside out, the structure comprises a Na4Fe7(PO4)6 core, a Mn-doped layer, a Mg-doped layer, and a conductive carbon coating layer. Its general structural formula can be simplified as Na4Fe7(PO4)6@Mn@Mg@C; wherein the chemical formula of the Mn-doped layer is Na4Fe 7-x Mn x (PO4)6; the general chemical formula of the Mg-doped layer is Na4Fe. 7-y Mg y (PO4)6; x and y are both stoichiometric coefficients that satisfy charge balance, both x and y are greater than 0 and x does not exceed 2.1 and y does not exceed 1.

75.

2. The sodium iron phosphate cathode material for sodium-ion batteries as described in claim 1, characterized in that, Based on stoichiometry, the molar ratio of Fe to Mn in the Mn-doped layer is Fe:Mn = 80-90:10-20; and the molar ratio of Fe to Mg in the Mg-doped layer is Fe:Mg = 85-95:5-15.

3. The sodium iron phosphate cathode material for sodium-ion batteries as described in claim 1 or 2, characterized in that, The particle size of the sodium iron phosphate cathode material does not exceed 20 micrometers; the mass ratio of the Na4Fe7(PO4)6 core, Mn doped layer, Mg doped layer and conductive carbon coating layer is 55-70:13-25:10-20:2-10.

4. A method for preparing sodium iron phosphate cathode material for sodium-ion batteries as described in claims 1-3, characterized in that, Includes the following steps: S1: Provide core precursor materials; weigh sodium source, iron source, manganese source and phosphorus source, mix and spray granulate to obtain Mn-doped precursor materials; weigh sodium source, iron source, magnesium source and phosphorus source, mix and spray granulate to obtain Mg-doped precursor materials; S2: Mix the core precursor material with the Mn-doped precursor material and granulate to obtain primary particles; mix the primary particles with the Mg-doped precursor material and granulate to obtain secondary particles; The secondary particles are mixed with a carbon source and granulated to obtain a gradient composite precursor material; S3: Sinter the gradient composite precursor material to crystallize the core precursor material, Mn-doped precursor material and Mg-doped layer precursor material, and carbonize the carbon source to form a conductive carbon coating layer, thus obtaining sodium iron phosphate cathode material.

5. The method for preparing sodium iron phosphate cathode material for sodium-ion batteries as described in claim 4, characterized in that, In S1, the particle size of the core precursor material is 1-13 micrometers; the particle size of the Mn-doped precursor material is 0.1-4 micrometers; and the particle size of the Mg-doped precursor material is 0.01-3 micrometers.

6. The method for preparing sodium iron phosphate cathode material for sodium-ion batteries as described in claim 4, characterized in that, In S1, the sodium source includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, sodium pyrophosphate, sodium carbonate, sodium nitrate, sodium oxalate, sodium sulfate, sodium sulfite, sodium acetate, sodium citrate, and sodium hydroxide. The iron source is at least one of ferrous sulfate, ferric sulfate, ferric phosphate, ferrous phosphate, ferrous oxalate, ferrous acetate, ferrous oxide, ferric oxide, iron tetroxide, and ferrous carbonate. The phosphorus source is at least one of the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, phosphoric acid, ferric phosphate, triammonium phosphate, pyrophosphate, or sodium pyrophosphate. The carbon source is at least one of sucrose, glucose, polyacrylic acid, malic acid, oxalic acid, citric acid, and ascorbic acid. The manganese source includes at least one of manganese sulfate, manganese acetate, manganese carbonate, and manganese dioxide; The magnesium source includes at least one of magnesium sulfate, magnesium acetate, magnesium oxide, and basic magnesium carbonate. The adhesive is at least one of polymethyl methacrylate, polyvinylpyrrolidone, polyethylene glycol, polyacrylonitrile, polyacrylic acid, polystyrene, hydroxypropyl methylcellulose, methylcellulose, asphalt, dextrin, and sucrose.

7. The method for preparing sodium iron phosphate cathode material for sodium-ion batteries as described in claim 4, characterized in that, In S2, the core precursor material and the Mn-doped precursor material are mixed, and a binder of 1%-2% of their total weight is added. After high-speed shearing and granulation, the mixture is dried to obtain the primary particles. The primary particles are mixed with the Mg-doped precursor material, and then 1%-2% of a binder is added according to their total weight. After high-speed shearing and granulation, the mixture is dried to obtain the secondary particles. The secondary particles are mixed with a solution containing a carbon source and then spray-granulated to obtain a gradient composite precursor material.

8. The method for preparing sodium iron phosphate cathode material for sodium-ion batteries as described in claim 4, characterized in that, In S2, the ratio of the core precursor material: Mn-doped precursor material: Mg-doped precursor material by mass is 60-70:15-25:10-20.

9. The method for preparing sodium iron phosphate cathode material for sodium-ion batteries as described in claim 4, characterized in that, In S2, the weight of the carbon source is 5%-20% of the total weight of the core precursor material, the Mn-doped precursor material, and the Mg-doped layer precursor material; the weight of the binder is 1%-6% of the total weight of the core precursor material, the Mn-doped precursor material, and the Mg-doped layer precursor material.

10. The method for preparing sodium iron phosphate cathode material for sodium-ion batteries as described in claim 4, characterized in that, In S3, the sintering process includes the following steps: S31: Under an inert atmosphere, the temperature of the environment in which the gradient composite precursor material is located is raised to 250-350℃ at a heating rate not exceeding 3℃ / min for the first stage of heat preservation. S32: Raise the temperature of the environment in which the gradient composite precursor material is located to 500-550℃ at a heating rate of 3-6℃ / min, and carry out the second stage of heat preservation. S33: The temperature of the environment in which the gradient composite precursor material is located is increased to 600-700℃ at a heating rate not exceeding 3℃ / min, and the third stage of heat preservation is carried out to obtain the sodium iron phosphate cathode material.

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