Sulfate-based sodium electrode positive material, preparation method thereof and sodium ion battery

Abstract

The application discloses a sulfate-based sodium battery positive electrode material, a preparation method and a sodium ion battery. The sulfate-based sodium battery positive electrode material has a composite sandwich structure and has the following general formula: Na2Fe 1+x (SO4) 2+x ·(MPO4) y C z ; wherein x is a sulfate excess coefficient, the value range of x is 0<=x<=1; y is a phosphate metering coefficient, 0.01<=y<=0.3; z is the mass fraction of carbon material, 1<=z<=5; M is selected from one or more of Mg, Ca, Al, Mn, Fe, Co, Ni, Cu, Zn, Ti or Zr; [MPO4] forms the core of the composite sandwich structure, [Na2Fe 1+x (SO4) 2+x ] forms the filling layer of the composite sandwich structure, and [C z ] is distributed at least in the interior of the composite sandwich structure to form the inner layer conductive framework of the composite sandwich structure. The application can fundamentally inhibit the iron ion disproportionation side reaction and the generation of impurities, and guarantee the high purity of the sodium ferric sulfate main crystal phase at the source of the synthesis reaction.

Description

Technical Field

[0001] This application relates to the field of sodium-ion battery technology, specifically to a sulfate-based sodium-ion battery cathode material, a method for preparing the sulfate-based sodium-ion battery cathode material, and a sodium-ion battery. Background Technology

[0002] Sodium-ion batteries, with their abundant sodium resources, low manufacturing costs, and excellent safety performance, have become one of the preferred technologies for replacing lithium-ion batteries in large-scale energy storage. Sulfate-based cathode materials (mainly sodium iron sulfate, abbreviated as NFS, chemical formula Na2Fe(SO4)2) have become a key research and development direction for sodium-ion battery cathode materials due to their high theoretical capacity, stable voltage platform, wide availability of raw materials, and low price.

[0003] However, the preparation of sulfate-based sodium cathode materials still faces many key technical challenges, particularly the traditional water-soluble spray drying process, which severely restricts the improvement of their electrochemical performance and industrial application. Firstly, during aqueous solution synthesis and spray drying, ferrous ions readily oxidize upon contact with air to form ferric ions, leading to significant phase separation during subsequent sintering and a substantial reduction in NFS phase purity. Secondly, the precursor particles formed during spray drying have low density and a loose structure, resulting in poor conductivity and structural stability of the sintered material. Thirdly, the uneven distribution of the three-dimensional conductive network of carbon nanotubes, along with the presence of free entanglement, prevents the formation of continuous conductive channels, hindering electron and sodium ion transport and consequently reducing the material's discharge capacity and rate performance. Fourthly, high iron content or multi-component doping can provide more redox active sites for sulfate-based sodium cathode materials, improving their specific capacity. However, sintering sulfate-based cathode materials with high iron content or multiple doped elements is quite difficult. This is because excessively high iron content or too many doping elements will exceed the capacity limit of the material's crystal lattice structure, causing the doped ions to be unable to uniformly integrate into the main crystal lattice, thereby triggering phase separation in the material. Summary of the Invention

[0004] The present invention aims to provide a sulfate-based sodium-ion battery cathode material prepared by a complexation method using oxyacid anions (taking phosphate as an example), a preparation method thereof, and a sodium-ion battery. By introducing oxyacid anion (taking phosphate as an example) complexes as micro-cores into an aqueous precursor solution, a precursor is prepared using the oxyacid anion (taking phosphate as an example) complexes as micro-seedling cores. Combined with carbon materials as the conductive network and framework support, a process combining aqueous solution complexation, spray drying, and low-temperature sintering is employed. By precisely controlling the complexation process, optimizing the spray drying parameters, and optimizing the sintering process, a sulfate-based sodium-ion battery cathode material with a [MPO4] / NFS composite sandwich structure containing carbon materials as the conductive network and framework support is sintered.

[0005] In a first aspect, this application provides a sulfate-based sodium-ion battery cathode material, wherein the sulfate-based sodium-ion battery cathode material has a composite sandwich structure and has the following general formula: Na2Fe 1+x (SO4) 2+x (MPO4) y C z ; Where x is the sulfate excess coefficient, with a value range of 0 ≤ x ≤ 1; y is the stoichiometric coefficient of phosphate, 0.01 ≤ y ≤ 0.3; z is the mass fraction of carbon material, 1 ≤ z ≤ 5; M is selected from one or more of Mg, Ca, Al, Mn, Fe, Co, Ni, Cu, Zn, Ti, or Zr; [MPO4] forms the core of the composite sandwich structure, [Na2Fe 1+x (SO4) 2+x [C] forms the filling layer of the composite sandwich structure. z At least distributed within the composite sandwich structure to form the inner conductive framework of the composite sandwich structure.

[0006] Secondly, this application also provides a sulfate-based sodium-ion battery cathode material, wherein the sulfate-based sodium-ion battery cathode material has a composite sandwich structure and has the following general formula: Na2Fe 1+x (SO4) 2+x ·A y C z ; Where x is the sulfate excess coefficient, with a value range of 0 ≤ x ≤ 1; A is an oxyacid salt with a particle size less than or equal to 1000 nm; y is the stoichiometric coefficient of the oxyacid salt, 0.01 ≤ y ≤ 0.3; z is the mass fraction of carbon material, 1 ≤ z ≤ 5; M is selected from one or more of Mg, Ca, Al, Mn, Fe, Co, Ni, Cu, Zn, Ti, or Zr; A forms the core of the composite sandwich structure, [Na2Fe 1+x (SO4) 2+x [C] forms the filling layer of the composite sandwich structure. z At least distributed within the composite sandwich structure to form the inner conductive framework of the composite sandwich structure.

[0007] Thirdly, this application provides a method for preparing a sulfate-based sodium cathode material, specifically including the following steps: S1. A dispersion of carbon-containing materials is prepared using the mixed solution method; S2. An NFS mixed solution containing microcrystalline seed phosphate complexes was prepared using the water-soluble complexation method: Ferrous sulfate and sodium sulfate were added to deionized water and stirred to dissolve, resulting in an NFS precursor solution; then, water-soluble phosphate solution and water-soluble M salt solution were added sequentially, and a complexing agent was added dropwise to carry out the complexation reaction while adjusting the pH value of the solution. The solution was then dispersed evenly until a mixed solution containing suspended phosphate complex microcores was obtained. S3. Mix the solutions prepared in steps S1 and S2 evenly, and disperse the solutions evenly. S4. Spray drying: The mixed solution obtained in step S3 is stirred evenly and then spray dried to obtain precursor particles. S5. Sintering: The precursor particles are placed in an inert atmosphere and sintered by multi-stage heating to the target temperature and holding at that temperature. After cooling, the sulfate-based sodium cathode material is obtained.

[0008] Fourthly, this application provides a sodium-ion battery, including a positive electrode material, wherein the positive electrode material comprises a sulfate-based sodium-ion positive electrode material prepared by the phosphate complexation method as described above, or prepared by the sulfate-based sodium-ion positive electrode material preparation method described above.

[0009] This invention prepares a precursor by complexing with oxyacid anions (taking phosphate as an example), and then sintersects it to prepare a sulfate-based sodium-ion battery cathode material by complexing with oxyacid anions (taking phosphate as an example). This effectively solves the problems of low phase purity, loose particle structure, uneven carbon nanotube distribution, and poor cycle performance of traditional sulfate-based sodium-ion battery cathode materials. It successfully prepares a sulfate-based sodium-ion battery cathode material with a wide voltage range, high actual density, high specific capacity, high cycle stability, and excellent rate performance. This provides a brand-new solution for developing new types of low-cost, high-capacity, long-cycle sulfate-based polyanion sodium-ion battery materials.

[0010] Taking oxyacid anion complexes as phosphate complexes as an example, the specific technical advantages and mechanisms of action are as follows: 1. Using phosphate complexes as the core, phase purity is improved from the source to ensure high specific capacity. This invention introduces phosphate ions during the precursor aqueous solution preparation stage to form stable complexation centers for iron phosphate and M phosphate in situ with Fe and M ions in the NFS precursor solution, thereby creating stable complexation centers for iron phosphate and M phosphate. The general reaction formula can be described as follows: M n+ + x·L y- + z·PO4 3- → [ M (L) x (PO4) z ] (n-xy-3z)- ; That is, metal ion + complexing agent + phosphate ion → soluble metal-ligand-phosphate complex. Wherein: M is one or more of Mg, Ca, Al, Mn, Fe, Co, Ni, Cu, Zn, Ti or Zr; L is a complexing ligand.

[0011] In the preparation of sodium ferric sulfate materials, under low iron content (x≤0.5), this aqueous solution complexation process effectively inhibits the hydrolysis and oxidation of iron ions, preventing the premature formation of ineffective iron-containing impurities in the precursor stage. Under high iron content (x≥0.5) and multi-element doping, this aqueous solution complexation process effectively anchors and stabilizes excess iron ions or multi-element ions in advance, forming uniformly dispersed complex micro-cores. This fundamentally inhibits iron ion disproportionation side reactions and impurity phase formation, ensuring the high purity of the sodium ferric sulfate main crystalline phase at the source of the synthesis reaction. The improved phase purity directly reduces ineffective inert components, allowing the active material to fully participate in the sodium ion insertion / extraction reaction, thereby achieving a higher effective specific capacity and fundamentally solving the problems of low phase purity and insufficient capacity utilization in traditional sulfate-based cathodes.

[0012] 2. Based on phosphate complexes as the core, a dense particle precursor is constructed to improve compaction density. Based on the prepared phosphate complex microcore as the particle carrier in the spray drying process, this invention can obtain precursor particles with uniform particle size, dense interior, and regular morphology by precisely controlling key parameters such as spray drying temperature, feed rate, and atomization pressure. The dense particle structure significantly improves the material's packing density and structural strength, reduces the risk of volume deformation and structural collapse during charging and discharging, and enables the material to operate stably in a wider voltage and pressure range. It achieves excellent characteristics of wide pressure range and high compaction density, solving the pain points of loose particles and fragile structure in traditional materials. At the same time, it greatly increases the compaction density of the cathode material after sintering, laying the foundation for the subsequent preparation of high-compact battery electrodes.

[0013] 3. Uniform dispersion and optimized sintering achieve uniform dispersion of carbon nanotubes and construct a continuous conductive network. Based on the phosphate complex microcore, the phosphate complex serves as an effective seed and carrier during solution preparation and dispersion. Carbon nanotubes can be uniformly dispersed based on the phosphate complex microcore, avoiding agglomeration, free entanglement, and local enrichment. In the subsequent sintering stage, further temperature control, heat preservation, and atmosphere synergistic regulation enable carbon nanotubes to form a three-dimensional continuous conductive network penetrating the entire particle within the composite sandwich structure (also known as a shell-core structure). This continuous and efficient conductive pathway significantly shortens the electron and sodium ion transport path, significantly reduces interfacial impedance, and allows the material to complete charging and discharging quickly and stably under high current, thereby achieving excellent rate performance while effectively mitigating capacity decay caused by polarization.

[0014] 4. The dual-layer composite sandwich structure provides synergistic stability and significantly improves cycle life. In this invention, during the sintering process, the ligands of the phosphate complex undergo complete pyrolysis and ablation at higher temperatures, ultimately transforming the phosphate complex into [MPO4] cores. The resulting [MPO4] core-NFS shell composite sandwich structure (or a composite sandwich structure formed by multiple [MPO4] cores) stabilizes the crystal lattice, suppresses phase transitions and structural breakage during charge and discharge. Combined with the support and conductivity of the uniform carbon nanotube network, this significantly alleviates capacity decay during long-term cycling. The material maintains structural integrity and phase composition stability during long-term cycling, achieving ultra-high cycling stability and truly solving the industry problem of poor cycling performance and short lifespan of traditional sulfate-based cathodes.

[0015] 5. In-situ construction of phosphate complex microcores to achieve high iron content or multi-component doping. Preparing high-iron-content or multi-doped sulfate-based sodium-ion battery cathode materials without the involvement of phosphate complexes remains a significant challenge. Excessive iron content or the composite doping of multiple metal elements can easily lead to problems such as impurities, lattice distortion, and phase separation during subsequent heat treatment, making it difficult to obtain the target phase with high purity. By constructing phosphate complex microcores in situ in the precursor aqueous solution, and utilizing complexation to achieve uniform coordination and stable dispersion of metal ions, the formation process of the precursor can be controlled by using the phosphate complex as the structural center. This effectively anchors and stably binds excess iron ions and multiple doped metal ions. Finally, through a suitable sintering regime, crystal phase control and structural solidification are achieved, enabling the efficient preparation of high-iron-content or multi-doped sulfate-based sodium-ion battery cathode materials with high phase purity and uniform composition. Meanwhile, the introduction of multi-doped metal ions M with redox active sites, such as Cu, Ni, Co, Mn, Zn, etc., to form the [MPO4] core can also provide additional reversible Na ion insertion / extraction sites for sodium iron sulfate cathode materials, further improving the capacity of sodium iron sulfate cathode materials.

[0016] 6. Phosphate doping improves the air stability of cathode materials. In systems without phosphate complexes, the partial hydrolysis of iron ions can cause localized acidity on the sintered material surface, making the prepared NFS material highly susceptible to water absorption and affecting subsequent battery manufacturing processes. When the precursor is prepared by the complexation method, the complex structure of iron ions reduces the degree of hydrolysis, thereby reducing residual acid radicals on the surface of the cathode material, giving the material better air stability, and improving the water resistance stability of the cathode material during battery manufacturing.

[0017] The technical effects of this application are as follows: Compared to existing technologies, this application provides a sulfate-based sodium-ion battery cathode material prepared by a complexation method using oxyacid anions (taking phosphate as an example). This application, through the preparation of a precursor using a phosphate complexation method, combined with precise control of the complexation process, spray drying parameters, and sintering process, along with an optional multi-element synergistic complexation design, specifically addresses the core technical pain points of existing sulfate-based sodium-ion battery cathode materials, such as low phase purity, loose particle structure, uneven carbon nanotube distribution, and poor cycle performance. Ultimately, it achieves a significant improvement in the overall performance of the cathode material and the sodium-ion battery, while also considering technical feasibility and industrial application value. Specific technical effects are as follows: Firstly, it effectively improves the purity of the material phase, laying the foundation for high specific capacity. This application precisely controls the complexation process to form stable complexation centers between phosphate ions and iron ions (or multiple complexing elements), effectively inhibiting the hydrolysis and oxidation of iron ions and avoiding the formation of impurity phases. Simultaneously, it strictly controls the molar coefficient of the precursor complex iron phosphate or M-containing phosphate within a reasonable range to avoid phase separation problems, resulting in a significantly higher NFS phase purity in the prepared cathode material compared to existing conventional sulfate-based cathode materials. The pure active phase can fully utilize the sodium ion insertion / extraction capability, and combined with the synergistic effect of the [MPO4] core and NFS shell formed by subsequent sintering, the specific capacity of the material is greatly improved, completely solving the shortcomings of existing materials where impurity phases consume active sites and result in insufficient capacity utilization.

[0018] Secondly, the material's particle structure and carbon nanotube distribution are optimized to enhance structural stability and rate performance. Addressing the shortcomings of existing materials, such as loose particle size and free carbon nanotube entanglement, this application prepares precursor iron phosphate or phosphate M complexes to form microcrystalline seed distribution, suppressing free carbon nanotube entanglement and ensuring uniform dispersion of carbon nanotubes within the material, forming a continuous, interconnected three-dimensional conductive network. Furthermore, by optimizing spray drying parameters, precursor particles with uniform particle size and dense internal structure are prepared, effectively increasing the material's packing density and suppressing volume expansion during charge and discharge. This process, through the generation and regulation of precursor complexes, completely solves the problem of uneven carbon nanotube distribution and discontinuous conductive pathways caused by free entanglement. This not only significantly reduces electron and sodium ion transport resistance, greatly improving the material's retention rate at high rates, but also enhances the material's structural stability, broadens the applicable voltage range, and enables the material to operate stably within a wide voltage range of 2–4.3V. Meanwhile, based on the prepared phosphate complex as a micro-core, the resulting precursor has a higher packing density, which greatly increases the compaction density of the cathode material after sintering, laying the foundation for the subsequent preparation of high-compact battery electrodes.

[0019] Third, it significantly improves the cycle stability of the material and extends the battery life. The "[MPO4] core-NFS shell-carbon nanotube three-dimensional conductive network" composite structure formed in this application, combined with a dense particle morphology, can effectively suppress the peeling of the composite interlayer structure, the generation of lattice defects, and the loss of active components during charging and discharging. At the same time, the stable phosphate complex system improves the stability of the material phase composition from the source, avoiding performance degradation caused by the generation of impurity phases during cycling. Ultimately, this greatly extends the cycle life of the assembled sodium-ion battery, completely solving the industry pain point of poor cycle performance of existing sulfate-based cathode materials.

[0020] Fourth, the multi-element complex design enhances the flexibility and performance adjustability of the technology. This application allows for the flexible selection of using only iron as the sole complexing center, or the introduction of transition metals such as magnesium, copper, and aluminum to form a multi-element synergistic complexing center. The former can meet basic performance requirements while taking into account production costs, while the latter can further improve the stability of the complex. By controlling the electron cloud distribution of the material through multi-element doping, the overall performance of the material is synergistically optimized, adapting to the performance requirements of different scenarios, and improving the flexibility and applicability of the technical solution.

[0021] Fifth, this application balances low cost and industrial feasibility, promoting industrial application. The raw materials used in this application, such as ferric sulfate, sodium sulfate, water-soluble phosphates, and carbon nanotubes, are all low-cost industrial raw materials, requiring no rare or precious metals. The complexation, spray drying, and sintering processes employed in the preparation are all mature and easily scaled-up industrial processes, requiring no complex equipment or harsh reaction conditions, thus significantly reducing the preparation cost of the cathode material. Simultaneously, the cathode material and sodium-ion battery prepared in this application possess the core advantages of high capacity, high rate capability, long cycle life, and wide voltage range, providing a new path for developing new types of low-cost, high-capacity, long-cycle sulfate-based polyanion sodium-ion batteries, and promoting the industrial application of sulfate-based sodium battery materials.

[0022] To more clearly demonstrate the technical means of this application and to ensure the smooth implementation of the contents of the specification, some typical embodiments of this application will be listed below, along with accompanying drawings and detailed descriptions. Attached Figure Description

[0023] Figure 1 The XRD patterns of the sulfate-based sodium electrode materials prepared by the phosphate complexation method in Examples 1-8 of this application and the conventional sulfate-based sodium electrode materials prepared in Comparative Examples 1-2 are shown below. Figure 2 This is a SEM image of the sulfate-based sodium cathode material prepared by the phosphate complexation method in Example 1 of this application. Figure 3This is a schematic diagram comparing the first charge-discharge curves of sodium-ion half-cells assembled with cathode materials of Example 1 and Comparative Examples 1 and 2 of this application at 2.0~4.3V and 0.1C after activation. Figure 4 This is a schematic diagram showing the rate comparison of the sodium-ion half-cell assembled with the cathode material of Example 1 of this application at 2.0~4.3V, 0.1C, 0.2C, 0.5C, 1C, 2C and 5C. Figure 5 This is a schematic diagram of the sodium-ion half-cell assembled in Example 1 and Comparative Examples 1 and 2 of this application under long-term cycling at 2.0~4.3V and 0.2C. Detailed Implementation

[0024] To make the inventive objectives and technical solutions of this application clearer and easier to understand, the following detailed description of this application is provided in conjunction with the accompanying drawings and embodiments. However, it should be understood that the specific embodiments described herein are only for explaining this application and are not intended to limit this application. Modifications and substitutions made to the methods, preparation steps, or conditions of this application without departing from the spirit and substance of this application are all within the scope of this application.

[0025] Currently, the preparation of sulfate-based sodium cathode materials still faces many key technical challenges, especially the traditional water-soluble spray drying process, which severely restricts the improvement of their electrochemical performance and industrial application: First, during aqueous solution synthesis and spray drying, ferrous ions easily react with air to form ferric ions, leading to significant phase separation during subsequent sintering and a substantial reduction in NFS phase purity. Second, the precursor particles formed during spray drying have low density and loose structure, resulting in poor conductivity and structural stability of the sintered material. Third, the three-dimensional conductive network of carbon nanotubes is unevenly distributed, and there is free entanglement of carbon nanotubes, preventing the formation of continuous conductive channels, thus hindering electron and sodium ion transport and causing a decrease in discharge capacity and rate performance degradation. Fourth, sintering to prepare sulfate-based cathode materials with high iron content or multi-component doping is difficult and prone to phase separation. Therefore, this application prepared a precursor solution containing a complex core, (1) inhibiting the oxidation reaction of ferrous ions with air to generate ferric ions; (2) simultaneously increasing the microcrystal seeds in the precursor solution, uniformly distributing the three-dimensional conductive network of carbon nanotubes, and improving the agglomeration of particles; (3) finally preparing high-density, high-compact precursor particles by spray drying; (4) constructing phosphate complex microcores in situ, realizing the possibility of high iron content or multi-element doping; which is a better way to effectively improve the performance of sodium ferric sulfate cathode material.

[0026] Based on this, this application provides a sulfate-based sodium cathode material and its preparation method prepared by a complexation method using oxyacid anions (taking phosphate as an example). Taking the phosphate complexation method as an example, this application introduces phosphate ions to form in-situ complexes with Fe and M ions in the precursor stage, and further forms stable iron phosphate (M phosphate) as a microcrystalline seed with a complexation center, which enhances the spray drying effect. Combined with optional multi-element synergistic complexation design and segmented sintering, an innovative application of a composite sandwich structure is finally formed, which prepares a [MPO4] / NFS shell-core (filled layer) structure containing carbon materials as a conductive network and skeleton support. This effectively solves the problems of low phase purity, loose particle structure, uneven carbon nanotube distribution, and poor cycle performance of existing sulfate-based cathode materials, and successfully prepares a sulfate-based sodium cathode material with a wide voltage range, high actual density, high specific capacity, high cycle stability, and excellent rate performance.

[0027] First, this application provides a sulfate-based sodium cathode material prepared by a complexation method using oxyacid anions (taking phosphate as an example). The sulfate-based sodium cathode material prepared by the phosphate complexation method has a composite sandwich structure with a phosphate material as the core, a carbon material as the framework, and sodium ferrous sulfate as the filling layer. The sulfate-based sodium cathode material prepared by the phosphate complexation method has the following general formula: Na2Fe 1+x (SO4) 2+x (MPO4) y C z ; M is selected from one or more of Mg, Ca, Al, Mn, Fe, Co, Ni, Cu, Zn, Ti, or Zr.

[0028] Where x is the sulfate excess coefficient, with a value range of 0 ≤ x ≤ 1; y is the stoichiometric coefficient of phosphate, with a value of 0.01 ≤ y ≤ 0.3; and z is the mass fraction of carbon material, with a value of 1 ≤ z ≤ 5. For example: x is 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, etc.; y is 0.01, 0.02, 0.03, 0.05, 0.1, 0.15, 0.2, 0.3, etc.; z is 1, 2, 3, 4, 5, etc. Among them, [MPO4] forms the core of the composite sandwich structure, and [Na2Fe 1+x (SO4) 2+x The filling layer (which can also be considered as a wrapping layer) forms a composite sandwich structure. z At least distributed within the composite sandwich structure to form the inner conductive framework of the composite sandwich structure.

[0029] In some embodiments, in order to further control the content of complex microcrystal seeds in the precursor aqueous solution, and at the same time uniformly introduce an appropriate amount of carbon material as a conductive network and supporting framework, and suppress the possibility of phase separation of the material in the later stage, the above values ​​are 0.2≤x≤0.7, 0.01≤y≤0.2, 1≤z≤3, etc. For example: x is 0.2, 0.3, 0.4, 0.5, 0.6, etc.; y is 0.01, 0.02, 0.03, 0.05, 0.1, 0.15, 0.2, etc.; z is 1, 2, 3, etc.

[0030] The core reason for limiting the range of x and y is that if x and y are too large (x>1 or y>0.3), the generated [MPO4] will be excessive, which will disrupt the ratio balance between the [MPO4] core (taking FePO4 as an example) and the NFS shell, making it impossible to form a stable composite sandwich structure. This will easily lead to obvious phase separation during the material sintering process, thereby reducing the purity of the NFS phase and the overall electrochemical performance of the material. If y is too small (e.g., y<0.01), it means that too few water-soluble phosphate groups that can be complexed are introduced, which cannot play the role of complex formation. This will result in insufficient [MPO4] content, which will cause the core [MPO4] of the composite sandwich structure to lose its role in inhibiting iron ion hydrolysis and stabilizing the composite sandwich structure. This will also lead to phase separation and electrochemical performance degradation of the cathode material.

[0031] In some embodiments, the carbon material accounts for z% of the mass percentage of the sulfate-based sodium cathode material prepared by the phosphate complexation method; 1 ≤ z ≤ 3. By setting an appropriate range of carbon material values, the uniform distribution of carbon material within and on the surface of the main phase sodium ferric sulfate can be further optimized, thereby forming a continuous and interconnected three-dimensional conductive network, achieving the best conductivity, and improving the overall characterization performance of the material.

[0032] In some embodiments, the particle size of [MPO4] is 10 nm to 1000 nm.

[0033] For example, the particle size of [MPO4] can be 10nm, 20nm, 50nm, 80nm, 100nm, 200nm, 300nm, 400nm, 550nm, 600nm, 700nm, 850nm, 1000nm, etc. By controlling the particle size of [MPO4] to 10nm-1000nm, the anchoring and dispersion effect of iron and doped metal ions can be improved. This can suppress the hydrolysis, oxidation, disproportionation side reactions of iron and the formation of impurity phases from the reaction source, ensuring high purity of the main phase and reducing ineffective inert components in the system. At the same time, it can achieve atomic-level uniform dispersion of multi-element dopants, avoiding structural defects caused by doping segregation. The precisely controllable particle size of the complex can significantly shorten the diffusion path of sodium ions, maximize the utilization rate of active materials, fully release the theoretical specific capacity of the material, and simultaneously optimize the structural stability, cycle life and rate performance of the material; ultimately achieving synergistic optimization of the material's comprehensive performance and industrial adaptability.

[0034] In some embodiments, M is selected from two or more of the elements described above, so as to sufficiently form a robust complex center in the prepared precursor solution.

[0035] In the precursor preparation process of this invention, the introduction of multiple elements is one of the core optimization methods. Its core purpose is to further improve the stability of phosphate complexes, construct more robust complex centers, and simultaneously achieve a certain multi-element doping effect, laying the foundation for the improvement of the overall performance of the cathode material. Specifically, the introduction of multiple elements (preferably transition metal elements with similar coordination ability to iron ions, such as magnesium, copper, and aluminum) can form synergistic coordination with phosphate and iron ions, enhancing the strength of the complex bond through a multidentate coordination mode, effectively suppressing the dissociation of the complex system, and avoiding the problems of complex instability and easy hydrolysis that may occur when using single iron ion complexes, thereby forming a more stable and uniformly distributed complex center. At the same time, the introduction of a small amount of multiple elements can achieve mild doping modification of the complex system, regulate the electron cloud distribution of the complex, optimize the formation quality of the active phase during subsequent sintering, and indirectly improve the conductivity and structural stability of the cathode material.

[0036] It should be noted that this invention is not limited to the introduction of multiple elements. In practical applications, iron can also be used as the sole complexing center. When only iron is used, by precisely controlling the complexation process as described above (such as pH value, reaction temperature, and raw material ratio), iron ions and phosphate ions can form a stable single complexing center, which can meet the basic requirements for precursor preparation and thus produce a qualified sulfate-based sodium cathode material. This will not affect the implementation of the core technical solution of this invention, taking into account both the flexibility and practicality of the technology, and can be flexibly adjusted according to actual production costs and performance requirements.

[0037] Preferably, this invention employs the introduction of additional elements to form stable complex centers. Compared to single iron ion complexation, complex centers formed by the synergistic effect of multiple elements exhibit superior stability and uniformity, improving the uniformity of the precursor from the source and reducing the generation of impurity phases and particle agglomeration during subsequent spray drying and sintering processes. This further addresses the pain points of existing sulfate-based cathode materials, such as low phase purity and loose particle structure. Simultaneously, the electronic structure regulation effect brought about by multi-element doping can synergistically improve the specific capacity, cycle stability, and rate performance of the cathode material. Therefore, the scheme of introducing multiple elements to form stable complex centers is the optimal choice that balances performance and stability.

[0038] In some embodiments, the inner layer of the sandwich structure of the prepared phosphate-based cathode material can contain one or more cores. For example, a complex precursor can be prepared using FePO4 microcrystals as the core, followed by sintering of the cathode material; or a complex precursor can be prepared using FePO4[MPO4] (or [MPO4][MPO4]) microcrystals as the core, followed by sintering of the cathode material. Introducing additional elements, as mentioned above, is to form more stable complexing centers during the precursor preparation process. The synergistic complexation of multiple elements improves the stability and uniformity of the precursor solution, enhancing its uniformity from the source. This reduces the formation of impurities and particle agglomeration during subsequent spray drying and sintering, further addressing the shortcomings of existing sulfate-based cathode materials such as low phase purity and loose particle structure. Simultaneously, the electronic structure regulation effect brought about by multi-element doping can also synergistically improve the specific capacity, cycle stability, and rate performance of the cathode material.

[0039] In some embodiments, the composite sandwich structure can be a double-shell core sandwich structure supported by a carbon material skeleton; preferably, the composite sandwich structure can be a composite sandwich structure containing multiple M cores in a double-shell core structure; wherein, the microcrystal seed of the phosphate M complex forms the core, and the carbon material is uniformly distributed around the microcrystal seed of the complex inside and outside the sodium ferrous sulfate to form a conductive network and support structure as a skeleton; the microcrystal seed of the complex, the carbon material skeleton, and the sodium ferrous sulfate form the composite sandwich structure.

[0040] Specifically, the sulfate-based sodium cathode material prepared by the phosphate complexation method introduces carbon material to form a composite sandwich structure by uniformly distributing microcrystals around the complex. This divides the interior of the sodium ferric sulfate cathode material into regions and uses carbon material to wrap and support the sulfate crystal blocks. Thus, with the microcrystals as the core and the carbon material as the support frame, the stability of the sodium ferric sulfate crystals within the blocks is further strengthened, greatly increasing the overall cycle stability of the material.

[0041] In some embodiments, the carbon material serving as the conductive framework support structure can be one or more of nano- or submicron-sized carbon nanotubes, nano-conductive carbon black, carbon nanofibers, or nano-scale graphene oxide. The selection of these carbon materials is based on considerations of suitable cost-effectiveness and conductivity. Nano- or submicron-sized carbon nanotubes are preferred because they possess a large specific surface area and a good long-chain structure, effectively enabling the construction of the conductive network framework structure required for the cathode material. Simultaneously, nano- or submicron-sized carbon nanotubes ensure optimal conductivity of the individual carbon material; however, a combination of nano- or submicron-sized carbon nanotubes or nano-scale conductive carbon black can also be selected.

[0042] By selecting materials such as carbon nanotubes, conductive carbon black nanoparticles, carbon nanofibers, or nanoscale graphene oxide, the introduced carbon materials can be uniformly attached (or wrapped) to the surface of the phosphate complex microcore in the precursor aqueous solution. Simultaneously, after spray drying, the introduced carbon materials are more evenly distributed between the primary particles (within the sodium ferrous sulfate particles) and the secondary particles (clusters of small sodium ferrous sulfate particles) of the sodium ferrous sulfate cathode material. This regional segmentation of the sodium ferrous sulfate cathode material and the encapsulation and support of the sulfate crystal blocks using carbon materials are preferred. Therefore, the preferred nanoscale carbon materials should primarily be nano- or submicron-sized carbon nanotubes, so that the formed carbon nanotubes can more easily form a framework support, further strengthening the stability of the sodium ferrous sulfate crystals within the blocks and greatly increasing the overall cycle stability of the material.

[0043] For example, the carbon material can be pure carbon nanotube material (e.g., CNT carbon nanotubes); or it can be mainly carbon nanotube material, supplemented by any one or more of nano-conductive carbon black, nano-carbon fiber, or nano-scale graphene oxide, without any specific limitation.

[0044] In some embodiments, the main phase of the sulfate-based sodium cathode material prepared by the phosphate complexation method is sodium ferrous sulfate cathode material.

[0045] For example, the main phase of the sulfate-based sodium electrode positive electrode material prepared by the phosphate complexation method is sodium ferrous sulfate positive electrode material, and it may also contain a partial ferrous sulfate phase or a partial sodium sulfate phase or other mixed phases.

[0046] This application also provides a sulfate-based sodium-ion battery cathode material, which is prepared by a complexation method using oxyanions (taking phosphate as an example), and the sulfate-based sodium-ion battery cathode material has a composite sandwich structure. The sulfate-based sodium-ion battery cathode material has the following general formula: Na2Fe 1+x (SO4) 2+x ·A y C z ; Where x is the sulfate excess coefficient, with a value ranging from 0 to 1; A is an oxyacid salt with a particle size less than or equal to 1000 nm; y is the stoichiometric coefficient of the oxyacid salt, with a value of 0.01 to 0.3; z is the mass fraction of carbon material, with a value of 1 to 5; M is selected from one or more of Mg, Ca, Al, Mn, Fe, Co, Ni, Cu, Zn, Ti, or Zr; A forms the core of the composite sandwich structure, [Na2Fe 1+x (SO4) 2+x [C] forms a filling layer in the composite sandwich structure. z At least distributed within the composite sandwich structure to form the inner conductive framework of the composite sandwich structure.

[0047] Since A is an oxyacid salt with a particle size of less than or equal to 1000 nm, it means that A is in the form of a complex in aqueous solution. After sintering, A can serve as a micro-core uniformly dispersed by the complex in the composite sandwich structure. Therefore, it can also fundamentally suppress the iron ion disproportionation side reaction and the generation of impurity phases, and ensure the high purity of the sodium ferric sulfate main crystal phase at the source of the synthesis reaction.

[0048] In some embodiments, the oxyacid salts include one or more of phosphates, sulfates, nitrates, pyrophosphates, silicates, and carbonates.

[0049] For example, the oxyacid salt can be MPO4, MSO4, MNO3, MP2O7, MSiO3, or MCO3. The anion in MSO4 is completely homologous to the target material sodium ferric sulfate, without the introduction of heterogeneous anions, thus avoiding the generation of impurity phases from the source. It can participate in the synthesis of the main phase simultaneously, maximizing industrial compatibility.

[0050] It should be noted that pyrophosphate, silicate, and carbonate should be controlled in trace amounts so that the pyrophosphate complex, silicate complex, and carbonate complex are in suspension or visually clear (not in a precipitate state) during the preparation process. This is beneficial for the formation of uniformly dispersed complex micro-cores, which will eventually be converted into [MPO4].

[0051] Wherein, M is selected from one or more of Mg, Ca, Al, Mn, Fe, Co, Ni, Cu, Zn, Ti or Zr; preferably, M is selected from two or more of the above elements so as to fully form a sufficiently stable complex center in the prepared precursor solution.

[0052] The following describes a method for preparing a sulfate-based sodium electrode material using an oxyacid anion (phosphate anion as an example) complexation, taking the phosphate complexation method as the preparation route.

[0053] This invention provides a method for preparing sulfate-based sodium cathode materials using the phosphate complexation method. Specifically, this application uses ferrous sulfate, sodium sulfate, phosphate, and carbon materials as main raw materials, and prepares the material through a three-step core process of aqueous solution complexation, spray drying, and low-temperature sintering, including the following steps: S1. Prepare carbon-containing material solutions using a mixed solution method and ball milling (or mechanical method or ultrasonic vibration method); S2. Prepare an NFS mixed solution containing microcrystalline seed phosphate complexes using a water-soluble complexation method and ball milling (or mechanical or ultrasonic oscillation method): Add ferrous sulfate and sodium sulfate to deionized water, stir to dissolve and obtain an NFS precursor solution; gradually add appropriate proportions of water-soluble phosphate and water-soluble M salt solutions (such as a solution of water-soluble sulfate M), add a complexing agent dropwise to carry out the complexation reaction and adjust the pH value of the solution, and then disperse evenly by ultrasonic oscillation (or mechanical or ball milling method) until a mixed solution containing phosphate complex microcores is obtained; S3. Mix the solutions prepared in steps S1 and S2 evenly, and disperse them evenly by ultrasonic vibration (or by mechanical method or ball milling). S4. Spray drying: The mixed solution obtained in step S3 is stirred evenly and then spray dried to obtain dense precursor particles. S5. Sintering: The precursor particles are placed in an inert atmosphere (such as nitrogen or argon) and sintered by multi-stage heating to the target temperature and holding at the temperature. After cooling and pulverizing, the sulfate-based sodium cathode material prepared by the phosphate complexation method is obtained.

[0054] It should be noted that the phosphate complex in the mixed solution containing phosphate complex microcores in this application is in a suspended or visually clear state (not a precipitate state), which is conducive to the formation of uniformly dispersed complex microcores of phosphate complex, so as to obtain precursor particles with uniform particle size, dense interior and regular morphology in step S4.

[0055] The specific preparation details are as follows: (1) The dispersant and carbon material are added to water in a certain proportion under the first stirring condition and mixed. The mixture is then dispersed under the second stirring condition. The dispersion is carried out until the particle size distribution of the nano-carbon material is D. max ≤60um, then transfer to a sand mill for grinding under the first grinding conditions, grinding until the particle size distribution of the nano-carbon material is D max ≤20um, D 50 After reaching ≤5µm, a carbon nanomaterial dispersion is obtained; wherein, the carbon nanomaterial can be one or more of carbon nanotubes (CNT), conductive carbon black nanoparticles (CB), carbon nanofibers (CNF), or nanoscale graphene oxide, and carbon nanotubes (CNT) are used as an example in this case.

[0056] (2) Sodium sulfate and ferrous sulfate are dissolved in water according to the stoichiometric ratio and mixed evenly to obtain a sulfate aqueous solution. In the above sulfate aqueous solution, a solution made of water-soluble phosphate and water-soluble sulfate M salt in the corresponding proportion is gradually added to make the water-soluble phosphate fully dissociate into phosphate ions; then a complexing agent is added dropwise to carry out the complexation reaction and the pH value of the solution is adjusted, and the solution is dispersed evenly by ultrasonic vibration (or by mechanical method or ball milling) to further promote the phosphate ions to fully complex with divalent or trivalent metal ions M to form a stable phosphate complex until a mixed solution containing phosphate complex micro-cores is obtained; (3) The carbon nanomaterial dispersion obtained in step (1) and the mixed solution containing phosphate complex microcores obtained in step (2) are mixed under the third stirring condition. After mixing, the mixture is transferred to a sand mill and ground under the second grinding condition until the particle size distribution of the nano-carbon material is D. max ≤15um, D 50 After reaching ≤3µm, a sulfate aqueous solution containing phosphate complex microcores and uniformly distributed carbon material was obtained.

[0057] (4) Spray drying was carried out by spraying to obtain sodium ferrous sulfate precursor with carbon nanotubes distributed as the core of phosphate complex.

[0058] (5) The sodium ferrous sulfate precursor with carbon nanotube support structure distributed with phosphate complex as the core is first sintered under the first sintering condition, and then heated to the second sintering condition for sintering to obtain the sulfate-based sodium cathode material with phosphate as the core.

[0059] (6) Use air jet milling or mechanical milling to pulverize the obtained sulfate-based sodium electrode material to the required particle size range.

[0060] In some embodiments, the dispersant may be one or more combinations of sodium dodecyl sulfate, sodium dodecyl sulfonate, sodium dodecylbenzene sulfonate, ammonium polyacrylate, polyethylene glycol, polysorbate, sodium carboxymethyl cellulose, and polyvinyl alcohol. The complexing agent not only enables ferrous sulfate and sodium sulfate to undergo complexation reactions with water-soluble phosphate solutions and water-soluble sulfate M solutions, but also adjusts the pH of the solution. The complexing agent may be one or more combinations of ammonia, citric acid, sodium citrate, sodium gluconate, ethylenediaminetetraacetic acid, malic acid, and dilute phosphoric acid. The water-soluble phosphate may be one or more combinations of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate. The water, carbon, and dispersant ratio in the carbon nanomaterial dispersion is 1000:(20~80):(0~10); the first stirring conditions include a stirring speed of 500-1000 rpm and a stirring time of 30-90 min; the second stirring conditions include a stirring speed of 1000-2500 rpm and a stirring time of 30-90 min; the first grinding conditions include a grinding speed of 1000-2500 rpm, a zirconium bead particle size of 0.3-1 mm, and a grinding time of 100-200 min; the particle size requirement of the carbon nanomaterial dispersion is D. 90 ≤20um, D 50 ≤5um.

[0061] For example, the ratio of water, carbon, and dispersant in the carbon nanomaterial dispersion can be 1000:80:10, 1000:60:8, 1000:60:5, 1000:20:3, etc.; the carbon nanomaterial dispersion may also not contain a dispersant, wherein the ratio of water to carbon in the carbon nanomaterial dispersion can be 1000:50, 1000:80, 1000:30, 1000:20, etc.

[0062] In some embodiments, the carbon nanotubes have a diameter of 10-50 nm, a length of 2-20 μm, and the pH of the adjusted solution is 6-8.

[0063] In some embodiments, the ratio of sodium sulfate and ferrous sulfate to be dissolved and mixed is 1:(1~2); the solid-liquid ratio in the sulfate aqueous solution is 1:(3~4); the sodium sulfate can be anhydrous sodium sulfate or sodium sulfate decahydrate; the ferrous sulfate can be anhydrous ferrous sulfate, ferrous sulfate monohydrate or ferrous sulfate heptahydrate.

[0064] For example, the ratio of sodium sulfate and ferrous sulfate to be dissolved and mixed can be 1:, 1:1.2, 1:1.3, 1:1.4, 1:1.5, 1:1.8, or 1:2; the solid-liquid ratio in the sulfate aqueous solution can be 1:3, 1:4, etc.

[0065] In some embodiments, the molar ratio of sodium sulfate to phosphate is 1:(0.01~0.3); the mass of carbon material is 1-5% of the total mass of raw materials.

[0066] For example, the molar ratio of sodium sulfate to phosphate can be 1:0.01, 1:0.1, 1:0.15, 1:0.2, 1:0.25, 1:0.3, etc.; the mass of carbon material can be 1%, 2%, 3%, 3.5%, 5%, etc., of the total mass of sulfate-based sodium electrode materials.

[0067] In some embodiments, the molar ratio of carbon atoms in the carbon nanomaterial dispersion to sodium sulfate molecules in the sulfate aqueous solution is (2~5):3; the third stirring conditions include a stirring rate of 1000-2500 rpm and a stirring time of 30-90 min; the second grinding conditions include a grinding speed of 1000-2500 rpm, a zirconium bead particle size of 0.3-1 mm, and a grinding time of 150-200 min.

[0068] For example, the molar ratio of carbon atoms in the carbon nanomaterial dispersion to sodium sulfate molecules in the sulfate aqueous solution can be 2:3, 3:3, 4:3, or 5:3.

[0069] In some embodiments, the conditions corresponding to spray drying are: inlet air temperature 180-220℃, outlet air temperature 80-105℃, D 50 ≤45um; tap density ≥0.7g / cm³.

[0070] In some embodiments, the first sintering condition involves isothermal sintering at 180-320°C for 5-15 hours, followed by heating to the second sintering condition at a target temperature of 340-400°C for 4-48 hours. The target temperature for the second sintering condition is preferably 340-380°C, and the holding time is preferably 8-12 hours. The sintering atmosphere for both the first and second sintering conditions is a nitrogen atmosphere. This parameter range ensures complete conversion of the phosphate complex to [MPO4] and sufficient NFS phase formation; wherein the ligands of the phosphate complex undergo complete pyrolysis within the 350-400°C range, ultimately converting the phosphate complex to [MPO4].

[0071] This application provides a sodium-ion battery, including a positive electrode material, wherein the positive electrode material is a sulfate-based sodium-ion battery positive electrode material prepared by the phosphate complexation method as described in the above embodiments, or a sulfate-based sodium-ion battery positive electrode material prepared by the preparation method of the sulfate-based sodium-ion battery positive electrode material prepared by the phosphate complexation method as described in the above embodiments.

[0072] Specifically, the sulfate-based sodium-ion battery cathode material prepared by the phosphate complexation method is mixed with a conductive agent, binder, and solvent to form a cathode slurry. This slurry is then coated onto the surface of the cathode current collector, and subsequently dried, rolled, and cut into cathode sheets. The mass ratio of the sulfate-based sodium-ion battery cathode material, conductive agent, and binder prepared by the phosphate complexation method in the cathode sheet is 90:5:5. In preparing the cathode slurry, the solvent can be N-methylpyrrolidone (NMP), the cathode binder is selected from PVDF, and the cathode conductive additive is selected from carbon black, acetylene black, Super P, etc.

[0073] Based on the positive electrode sheet provided above, this invention further provides a method for preparing a sodium-ion battery. The sodium-ion battery includes a positive electrode sheet, a negative electrode sheet, a separator, and an electrolyte.

[0074] In some embodiments, the sodium-ion battery provided in this application uses metallic sodium as the negative electrode material and any one of the following electrolytes: 1M NaPF6 / (EC:DMC=1:1), 1M NaPF6 / PC, and 1M NaClO4 / (EC:PC=1:1), wherein EC represents ethylene carbonate, DMC represents dimethyl carbonate, PC represents propylene carbonate, EC:DMC=1:1 represents a solvent formed by ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1, and EC:PC represents a solvent formed by ethylene carbonate and propylene carbonate in a volume ratio of 1:1.

[0075] In some embodiments, the sodium-ion battery provided in this application uses a polyolefin microporous membrane such as polyethylene or polypropylene, or a Celgard membrane.

[0076] To more effectively illustrate the technical solution of the present invention, several specific embodiments are described below.

[0077] Example 1 A method for preparing a sulfate-based sodium cathode material using the phosphate complexation method includes the following steps: (1) Carbon nanotubes (CNTs) and pure water were added to a stirred tank at a molar ratio of 3:100. Then, sodium dodecyl sulfate was added at a carbon / dispersant weight ratio of 10:1. The mixture was stirred at 700 rpm for 1 h, and then stirred and dispersed at 1500 rpm for 30-90 min. The particle size distribution of the nano-carbon material in the dispersion was controlled to be D. 90 For particles ≤60µm, transfer them to a sand mill and grind them at 2000rpm, grinding zirconium beads to a size of 0.6mm for 100-200min; control the particle size of the carbon nanomaterials in the dispersion to D. 90 ≤20um, D 50 ≤5µm, prepare dispersions containing carbon nanotube materials; (2) Anhydrous sodium sulfate, ferrous sulfate and pure water were dissolved and mixed in a molar ratio of 1:1.7:100 to obtain a sulfate aqueous solution. A certain concentration of sodium dihydrogen phosphate solution and magnesium sulfate solution were gradually added to the above sulfate aqueous solution in a molar ratio, wherein the ratio of sodium dihydrogen phosphate: magnesium sulfate: sodium sulfate was 0.2:0.3:1. The pH of the solution was adjusted to about 6, and the solution was dispersed evenly by ball milling until a sulfate mixed aqueous solution containing phosphate complex microcores was obtained.

[0078] (3) Calculate the actual mass of solute in the solutions obtained in steps (1) and (2) above. According to the actual solute weight ratio in the solution = 2:100, mix the carbon material dispersion obtained in (1) with the sulfate mixed aqueous solution obtained in (2) at 1500 rpm for 60 min, and then transfer it to a sand mill at 2500 rpm for grinding. Grind the zirconium beads to 0.4 mm and grind for 100-200 min. Control the particle size of the carbon nanomaterial in the dispersion to D. 90 ≤15um, D 50 ≤3um; Obtain a sulfate aqueous solution containing phosphate complex microcores and uniformly distributed carbon material; (4) The carbon composite sulfate aqueous solution containing phosphate complex microcores prepared in step (3) is spray dried by spray drying. The inlet air temperature is controlled at 180-220℃ and the outlet air temperature is controlled at 80-105℃ to obtain sodium ferrous sulfate precursor with carbon nanotube support structure distributed with phosphate complex as core.

[0079] (5) The sodium ferrous sulfate precursor containing a carbon nanotube support structure with phosphate complex as the core was sintered at 180°C for 10 h under a nitrogen atmosphere, then heated to 350°C and sintered at that temperature for 20 h. After being removed from the furnace, it was pulverized to the required particle size to obtain the sulfate-based sodium cathode material prepared by the phosphate complexation method. The structural formula of the sulfate-based sodium cathode material prepared by the phosphate complexation method is Na2Fe 1.7 (SO4) 2.7 ·[Mg3(PO4)2] 0.1 C2. Its XRD pattern is as follows: Figure 1 As shown in Table 1, the compaction, capacity, and cycle performance of the material particles are as follows.

[0080] Example 2 A method for preparing a sulfate-based sodium cathode material using the phosphate complexation method includes the following steps: Carbon nanotubes (CNTs) and pure water were added to a stirred tank at a molar ratio of 3:100. Sodium dodecyl sulfate was then added at a carbon / dispersant weight ratio of 10:1. A carbon nanomaterial dispersion was prepared according to the stirring, dispersion, and grinding conditions described in Example 1. Anhydrous sodium sulfate, ferrous sulfate, and pure water were dissolved and mixed at a molar ratio of 1:1.6:100 to obtain a sulfate aqueous solution. The concentrations of sodium and iron ions in the sulfate aqueous solution were calculated. Sodium dihydrogen phosphate and calcium acetate solutions were gradually added to the sulfate aqueous solution at a molar ratio. In step (3) of Example 1, a sulfate aqueous solution containing phosphate complex microcores and uniformly distributed carbon material is prepared, wherein the weight ratio of the carbon material solution to the actual solute in the sulfate solution is 3:100. Following steps (4) to (5) of Example 1, the obtained carbon composite sulfate aqueous solution containing phosphate complex microcores is spray-dried to obtain a sodium ferrous sulfate precursor with a carbon nanotube support structure containing phosphate complex cores. After sintering, a sulfate-based sodium electrode positive electrode material prepared by the phosphate complexation method is obtained. The structural formula of the sulfate-based sodium electrode positive electrode material prepared by the phosphate complexation method is Na2Fe. 1.6 (SO4) 2.6 ·[Ca3(PO4)2] 0.03 C3. Its XRD pattern is as follows: Figure 1 As shown in Table 1, the compaction, capacity, and cycle performance of the material particles are as follows.

[0081] Example 3 A method for preparing a sulfate-based sodium cathode material using the phosphate complexation method includes the following steps: Carbon nanotubes (CNTs) and pure water were added to a stirred tank at a molar ratio of 3:100. Sodium dodecyl sulfate was then added at a carbon / dispersant weight ratio of 10:0.5. A carbon nanomaterial dispersion was prepared according to the stirring, dispersion, and grinding conditions described in Example 1. Anhydrous sodium sulfate, ferrous sulfate, and pure water were dissolved and mixed at a molar ratio of 1:1.4:100 to obtain the sulfate aqueous solution. The concentrations of sodium and iron ions in the sulfate aqueous solution were calculated. Sodium dihydrogen phosphate and copper sulfate solution were gradually added to the sulfate aqueous solution at a molar ratio. A sulfate aqueous solution containing phosphate complex microcores and uniformly distributed carbon material was prepared according to step (3) of Example 1, wherein the weight ratio of the carbon material solution to the actual solute in the sulfate solution was 2:100. Following steps (4) to (5) of Example 1, the obtained carbon composite sulfate aqueous solution containing phosphate complex microcores was spray-dried to obtain a sodium ferrous sulfate precursor with a carbon nanotube support structure containing phosphate complex cores. This precursor was then sintered to obtain a sulfate-based sodium electrode material prepared by the phosphate complexation method. The structural formula of the sulfate-based sodium electrode material prepared by the phosphate complexation method is Na2Fe. 1.4 (SO4)2.4 ·[Cu3(PO4)2] 0.05 C2. Its XRD pattern is as follows: Figure 1 As shown in Table 1, the compaction, capacity, and cycle performance of the material particles are as follows.

[0082] Example 4 A method for preparing a sulfate-based sodium cathode material using the phosphate complexation method includes the following steps: Carbon nanotubes (CNTs) and pure water were added to a stirred tank at a molar ratio of 3:100. Sodium dodecyl sulfate was then added at a carbon / dispersant weight ratio of 10:0.5. A carbon nanomaterial dispersion was prepared according to the stirring, dispersion, and grinding conditions of Example 1. Anhydrous sodium sulfate, ferrous sulfate, and pure water were dissolved and mixed at a molar ratio of 1:1.2:100 to obtain a sulfate aqueous solution. The concentrations of sodium and iron ions in the sulfate aqueous solution were calculated. Sodium dihydrogen phosphate and ferrous sulfate solutions were gradually added to the sulfate aqueous solution at a molar ratio. In step (3) of Example 1, a sulfate aqueous solution containing phosphate complex microcores and uniformly distributed carbon material is prepared, wherein the weight ratio of the actual solute in the carbon material solution to the sulfate solution is 1.5:100. Following steps (4) to (5) of Example 1, the obtained carbon composite sulfate aqueous solution containing phosphate complex microcores is spray-dried to obtain a sodium ferrous sulfate precursor with a carbon nanotube support structure containing phosphate complex cores. After sintering, a sulfate-based sodium electrode positive electrode material prepared by the phosphate complexation method is obtained. The structural formula of the sulfate-based sodium electrode positive electrode material prepared by the phosphate complexation method is Na2Fe. 1.2 (SO4) 2.2 ·[FePO4] 0.15 C 1.5 Its XRD pattern is as follows: Figure 1 As shown in Table 1, the compaction, capacity, and cycle performance of the material particles are as follows.

[0083] Example 5 A method for preparing a sulfate-based sodium cathode material using the phosphate complexation method includes the following steps: Carbon nanotubes (CNTs) and pure water were added to a stirred tank at a molar ratio of 3:100. Sodium dodecyl sulfate was then added at a carbon / dispersant weight ratio of 10:0.5. A carbon nanomaterial dispersion was prepared according to the stirring, dispersion, and grinding conditions of Example 1. Anhydrous sodium sulfate, ferrous sulfate, and pure water were dissolved and mixed at a molar ratio of 1:1.5:100 to obtain a sulfate aqueous solution. The concentrations of sodium and iron ions in the sulfate aqueous solution were calculated. Sodium dihydrogen phosphate, ferrous sulfate, and zinc sulfate solutions were gradually added to the above sulfate aqueous solution at a molar ratio. A sulfate aqueous solution containing phosphate complex microcores and uniformly distributed carbon material was prepared according to step (3) of Example 1, wherein the weight ratio of the carbon material solution to the actual solute in the sulfate solution was 1.5:100. Following steps (4) to (5) of Example 1, the obtained carbon composite sulfate aqueous solution containing phosphate complex microcores was spray-dried to obtain a sodium ferrous sulfate precursor with a carbon nanotube support structure containing phosphate complex cores. This precursor was then sintered to obtain a sulfate-based sodium electrode material prepared by the phosphate complexation method. The structural formula of the sulfate-based sodium electrode material prepared by the phosphate complexation method is Na2Fe. 1.5 (SO4) 2.5 ·[FePO4] 0.1 [Zn3(PO4)2] 0.05 C 1.5 Its XRD pattern is as follows: Figure 1 As shown in Table 1, the compaction, capacity, and cycle performance of the material particles are as follows.

[0084] Example 6 A method for preparing a sulfate-based sodium cathode material using the phosphate complexation method includes the following steps: Carbon nanotubes (CNTs) and pure water were added to a stirred tank at a molar ratio of 3:100. Sodium dodecyl sulfate was then added at a carbon / dispersant weight ratio of 10:0.5. A carbon nanomaterial dispersion was prepared according to the stirring, dispersion, and grinding conditions of Example 1. Anhydrous sodium sulfate, ferrous sulfate, and pure water were dissolved and mixed at a molar ratio of 1:1.5:100 to obtain a sulfate aqueous solution. The concentrations of sodium and iron ions in the sulfate aqueous solution were calculated. Sodium dihydrogen phosphate, titanium sulfate, and manganese sulfate were gradually added to the above sulfate aqueous solution at a molar ratio. A sulfate aqueous solution containing phosphate complex microcores and uniformly distributed carbon material was prepared according to step (3) of Example 1, wherein the weight ratio of the carbon material solution to the actual solute in the sulfate solution was 1:100. Following steps (4) to (5) of Example 1, the obtained carbon composite sulfate aqueous solution containing phosphate complex microcores was spray-dried to obtain a sodium ferrous sulfate precursor with a carbon nanotube support structure containing phosphate complex cores. This precursor was then sintered to obtain a sulfate-based sodium electrode material prepared by the phosphate complexation method. The structural formula of the sulfate-based sodium electrode material prepared by the phosphate complexation method is Na2Fe. 1.5 (SO4) 2.5 ·[Ti3(PO4)4] 0.025 [Mn3(PO4)2] 0.05 C1. Its XRD pattern is as follows: Figure 1 As shown in Table 1, the compaction, capacity, and cycle performance of the material particles are as follows.

[0085] Example 7 A method for preparing a sulfate-based sodium cathode material using the phosphate complexation method includes the following steps: Carbon nanotubes (CNTs) and pure water were added to a stirred tank at a molar ratio of 3:100. Sodium dodecyl sulfate was then added at a carbon / dispersant weight ratio of 10:0.5. A carbon nanomaterial dispersion was prepared according to the stirring, dispersion, and grinding conditions of Example 1. Anhydrous sodium sulfate, ferrous sulfate, and pure water were dissolved and mixed at a molar ratio of 1:1.5:100 to obtain a sulfate aqueous solution. The concentrations of sodium and iron ions in the sulfate aqueous solution were calculated. Sodium dihydrogen phosphate, zirconium sulfate, and nickel sulfate solutions were gradually added to the above sulfate aqueous solution at a molar ratio. A sulfate aqueous solution containing phosphate complex microcores and uniformly distributed carbon material was prepared according to step (3) of Example 1, wherein the weight ratio of the carbon material solution to the actual solute in the sulfate solution was 1:100. Following steps (4) to (5) of Example 1, the obtained carbon composite sulfate aqueous solution containing phosphate complex microcores was spray-dried to obtain a sodium ferrous sulfate precursor with a carbon nanotube support structure containing phosphate complex cores. This precursor was then sintered to obtain a sulfate-based sodium electrode material prepared by the phosphate complexation method. The structural formula of the sulfate-based sodium electrode material prepared by the phosphate complexation method is Na2Fe.1.5 (SO4) 2.5 ·[Zr3(PO4)4] 0.025 [Ni3(PO4)2] 0.05 C1. Its XRD pattern is as follows: Figure 1 As shown in Table 1, the compaction, capacity, and cycle performance of the material particles are as follows.

[0086] Example 8 A method for preparing a sulfate-based sodium cathode material using the phosphate complexation method includes the following steps: Carbon nanotubes (CNTs) and pure water were added to a stirred tank at a molar ratio of 3:100. Sodium dodecyl sulfate was then added at a carbon / dispersant weight ratio of 10:0.5. A carbon nanomaterial dispersion was prepared according to the stirring, dispersion, and grinding conditions of Example 1. Anhydrous sodium sulfate, ferrous sulfate, and pure water were dissolved and mixed at a molar ratio of 1:1.5:100 to obtain a sulfate aqueous solution. The concentrations of sodium and iron ions in the sulfate aqueous solution were calculated. Sodium dihydrogen phosphate, aluminum sulfate, and cobalt sulfate solutions were gradually added to the above sulfate aqueous solution at a molar ratio. A sulfate aqueous solution containing phosphate complex microcores and uniformly distributed carbon material was prepared according to step (3) of Example 1, wherein the weight ratio of the carbon material solution to the actual solute in the sulfate solution was 1:100. Following steps (4) to (5) of Example 1, the obtained carbon composite sulfate aqueous solution containing phosphate complex microcores was spray-dried to obtain a sodium ferrous sulfate precursor with a carbon nanotube support structure containing phosphate complex cores. This precursor was then sintered to obtain a sulfate-based sodium electrode material prepared by the phosphate complexation method. The structural formula of the sulfate-based sodium electrode material prepared by the phosphate complexation method is Na2Fe. 1.5 (SO4) 2.5 ·[AlPO4] 0.1 [CoPO4] 0.1 C1. Its XRD pattern is as follows: Figure 1 As shown in Table 1, the compaction, capacity, and cycle performance of the material particles are as follows.

[0087] Comparative Example 1 A traditional method for preparing a sulfate-based sodium cathode material with high iron content, but not prepared using phosphate complexes as a core (and without MPO4), includes the following steps: Carbon nanotubes (CNTs) and pure water were added to a stirred tank at a molar ratio of 3:100. Sodium dodecyl sulfate was then added at a carbon / dispersant weight ratio of 10:0.5. A carbon nanomaterial dispersion was prepared according to the stirring, dispersion, and grinding conditions of Example 1. Anhydrous sodium sulfate, ferrous sulfate, and pure water were dissolved and mixed at a molar ratio of 1:1.7:100 to obtain a sulfate aqueous solution. A sulfate aqueous solution without phosphate complex microcores but with uniformly distributed carbon materials was prepared according to step (3) of Example 1, wherein the weight ratio of the carbon material solution to the actual solute in the sulfate solution was 2:100. The obtained carbon-containing composite sulfate aqueous solution was spray-dried according to steps (4) to (5) of Example 1 to obtain a sodium ferrous sulfate precursor with a carbon tube-supported structure. After sintering, a sulfate-based sodium electrode material prepared by the phosphate complex method was obtained. The obtained sulfate-based sodium electrode material has the structural formula Na2Fe 1.7 (SO4) 2.7 C2. Its XRD pattern is as follows: Figure 1 As shown in Table 1, the compaction, capacity, and cycle performance of the material particles are as follows.

[0088] Comparative Example 2 A method for preparing a sulfate-based sodium cathode material by excess preparation of phosphate complex (y=0.4) includes the following steps: Carbon nanotubes (CNTs) and pure water were added to a stirred tank at a molar ratio of 3:100. Sodium dodecyl sulfate was then added at a carbon / dispersant weight ratio of 10:0.5. A carbon nanomaterial dispersion was prepared according to the stirring, dispersion, and grinding conditions of Example 1. Anhydrous sodium sulfate, ferrous sulfate, and pure water were dissolved and mixed at a molar ratio of 1:1.7:100 to obtain a sulfate aqueous solution. The concentrations of sodium and iron ions in the sulfate aqueous solution were calculated. Sodium dihydrogen phosphate and magnesium sulfate were gradually added to the above sulfate aqueous solution at a molar ratio, the difference being that the ratio of sodium dihydrogen phosphate to magnesium sulfate to the above sulfur... Sodium sulfate = 0.8:1.2:1; Prepare a sulfate aqueous solution containing phosphate complex microcores and uniformly distributed carbon material according to step (3) of Example 1, wherein the weight ratio of the carbon material solution to the actual solute in the sulfate solution is 2:100; Spray dry the obtained carbon composite sulfate aqueous solution containing phosphate complex microcores to obtain a sodium ferrous sulfate precursor with carbon nanotube support structure containing phosphate complex cores, and then sinter it to obtain a sulfate-based sodium electrode positive electrode material prepared by phosphate complexation method. The structural formula of the sulfate-based sodium electrode positive electrode material prepared by phosphate complexation method is Na2Fe 1.7 (SO4) 2.7 ·[Mg3(PO4)2] 0.4 C2. Its XRD pattern is as follows: Figure 1As shown in Table 1, the compaction, capacity, and cycle performance of the material particles are as follows.

[0089] Application examples A sodium-ion battery using a sulfate-based sodium cathode material prepared by the phosphate complexation method, the preparation method of which includes the following steps: (1) The samples obtained in Examples 1-8 and Comparative Examples 1-2 were used as positive electrode active materials. They were mixed with conductive carbon black and polyvinylidene fluoride (PVDF) binder in a mass ratio of 9:0.5:0.5 and then dissolved in N-methylpyrrolidone (NMP) solvent to prepare positive electrode slurry. The slurry was coated on aluminum foil, dried and cut to obtain positive electrode sheet.

[0090] (2) Assemble the positive electrode obtained in step (1) with metallic sodium and Celgard membrane to form a sodium-ion battery. The electrolyte of the sodium-ion battery is a 1M NaPF6 / (EC:DMC=1:1) electrolyte mixture (by volume EC:DMC=1:1). After assembly, let it stand for 24 hours, and then perform charge-discharge activation test.

[0091] Specifically, the test method was as follows: three charge and discharge cycles were performed at a current density of 0.1 C (1C=100 mA / g) within a voltage range of 2.0-4.3V; then, the cycle performance was tested at 0.2C (1C=100 mA / g). The results are shown in Table 1.

[0092] Figure 1 The main diffraction peaks of the samples in Examples 1-8 show that they are sulfate-based sodium electrode materials with complete crystal structure and low impurity content, proving that the obtained material is a sulfate-based sodium electrode cathode material prepared by the phosphate complexation method. Na2Fe 1.7 (SO4) 2.7 ·[Mg3(PO4)2] 0.1 C2 (Example 1); Na2Fe 1.6 (SO4) 2.6 ·[Ca3(PO4)2] 0.03 C3 (Example 2); Na2Fe 1.4 (SO4) 2.4 ·[Cu3(PO4)2] 0.05 C2 (Example 3); Na2Fe 1.2 (SO4) 2.2 ·[FePO4] 0.15 C 1.5 (Example 4); Na2Fe 1.5 (SO4)2.5 ·[FePO4] 0.1 [Zn3(PO4)2] 0.05 C 1.5 (Example 5); Na2Fe 1.5 (SO4) 2.5 ·[Ti3(PO4)4] 0.025 [Mn3(PO4)2] 0.05 C1 (Example 6); Na2Fe 1.5 (SO4) 2.5 ·[Zr3(PO4)4] 0.025 [Ni3(PO4)2] 0.05 C1 (Example 7); Na2Fe 1.5 (SO4) 2.5 ·[AlPO4] 0.1 [CoPO4] 0.1 C1 (Example 8); However, as a comparative case, the sample of Comparative Example 1 was prepared without the addition of a phosphate complex as the core, and also had a high iron content (x=0.7); the sample of Comparative Example 2 had a phosphate complex introduced during preparation, but the complex was in excess (y=0.4, exceeding the limit of y≤0.3). The main diffraction peaks of the samples of Comparative Examples 1 and 2 showed that in addition to the sulfate-based sodium battery material, there were also phase separation or impurity peaks (marked with asterisks). Among them, Comparative Example 1 showed differential phases, with the sulfate-based sodium battery material showing both partial sodium iron sulfate Na2Fe1(SO4)2 phase (212 phase) and partial sodium iron sulfate Na2Fe2(SO4)3 phase (223 phase); Comparative Example 2 showed impurity phases in addition to the main phase of the sulfate-based sodium battery material. The crystal phase separation and increased impurity content in samples 1 and 2, as observed in Comparative Examples 1 and 2, indirectly confirm the important promoting effect of introducing phosphate complexes on the preparation of sulfate-based cathode materials with high iron content (the introduction of phosphate complexes can effectively anchor and stabilize excess iron ions). This also indicates that the amount of phosphate introduced into sulfate-based sodium cathode materials prepared by the phosphate complexation method has a certain threshold; exceeding this threshold will result in the formation of excessive phosphate complex micro-cores, leading to significant phase separation during subsequent sintering.

[0093] from Figure 2 SEM images show that the sulfate-based sodium-ion cathode material prepared by the phosphate complexation method in Example 1 exhibits dense particles with a uniform particle size distribution. Some carbon nanotube structures appear as tiny tubular structures, interpenetrating and entwining within the surface and interior of the sulfate-based sodium-ion crystals. Furthermore, the carbon material is uniformly distributed throughout the overall structure, forming a complete conductive support network. It should be noted that... Figure 2 The dimension in the lower right corner is marked as 10µm.

[0094] The sulfate-based sodium cathode material Na2Fe prepared by the phosphate complexation method in Example 1 was used. 1.6 (SO4) 2.6 ·[Mg3(PO4)2] 0.1 C3, along with Comparative Examples 1 and 2, was used as the positive electrode material to assemble sodium-ion batteries with sodium metal negative electrodes, and the electrochemical performance was tested. The results are as follows: Figure 3 As shown, the sodium-ion battery assembled in Example 1, when cycled at a current density of 0.1C (10mA / g) within a voltage range of 2.0~4.3V, had an initial capacity of approximately 101 mAh / g after activation. In contrast, the sodium-ion battery assembled in Comparative Example 1, under the same conditions, had an initial capacity of only approximately 85 mAh / g after activation, and the sodium-ion battery assembled in Comparative Example 2, under the same conditions, had an initial capacity of only approximately 74 mAh / g after activation. The reason is that the sample in Example 1 adopted a preparation scheme with phosphate complex as the core, which suppressed the oxidation of iron ions during solution and spray preparation, and made the introduced carbon nanotubes more evenly distributed and the prepared material more compact, thus showing a higher specific capacity. As for Comparative Example 1, without the preparation scheme with phosphate complex as the core, the high iron content caused differential phases (partial sodium ferric sulfate 212 phase, partial sodium ferric sulfate 223 phase) after sintering, which destroyed the integrity of the crystal structure and ultimately reduced the electrochemical performance of the material, resulting in a lower overall capacity. As for Comparative Example 2, because an excessive amount of phosphate complex was used, the material also caused phase separation and generated impurity phases during the later sintering process, thus showing the lowest capacity.

[0095] In subsequent multiplier tests, such as Figure 4As shown, the sodium-ion batteries assembled with the materials of Example 1 were tested under cycling conditions at current densities of 0.1C (10 mA / g), 0.2C (20 mA / g), 0.5C (50 mA / g), 1C (100 mA / g), 2C (200 mA / g), and 5C (500 mA / g). The discharge capacities of Example 1 at different rates were approximately 101 mAh / g (0.1C), 100 mAh / g (0.2C), 96 mAh / g (0.5C), 93 mAh / g (1C), 89 mAh / g (2C), and 76 mAh / g (5C), demonstrating the good rate performance of the materials. Therefore, it can be seen that the sulfate-based sodium cathode material prepared by the phosphate complexation method in this application fully utilizes the conductive carbon nanotube network as a supporting framework. While improving the compaction density and capacity of the material by using phosphate complexes, the material can still maintain good rate performance. This is an effective way to simultaneously improve the specific capacity of the material and overcome the insufficient rate performance of traditional sulfate-based polyanionic sodium cathode materials.

[0096] In subsequent long-cycle stability tests, such as Figure 5 As shown, the sulfate-based sodium cathode material Na2Fe prepared by the phosphate complexation method in Example 1 is used. 1.6 (SO4) 2.6 ·[Mg3(PO4)2] 0.1 C3, along with the materials from Comparative Examples 1 and 2, were used as positive electrode materials to assemble sodium-ion batteries with sodium metal negative electrodes, and 0.2C (20 mA / g) cycle tests were conducted. The material in Example 1 retained a discharge specific capacity of approximately 100 mAh / g, and after 100 long cycles, the capacity retention rate was still approximately 99%, demonstrating excellent cycle stability. In contrast, the samples from Comparative Examples 1 and 2 showed capacity retention rates of only approximately 85% and 65%, respectively, after 100 long cycles, indicating poor cycle stability. This is because the sample from Comparative Example 1 had a high iron content and did not employ a preparation scheme based on phosphate complexes; while Comparative Example 2 used an excessive amount of phosphate complexes, causing phase separation during the later sintering process, thus exhibiting poor cycle performance.

[0097] In addition, Table 1 presents the test results of the materials in Examples 1-8 and Comparative Examples 1-2 in terms of specific capacity, cycle performance, and powder compaction density. The results show that the sulfate-based sodium cathode materials prepared by the phosphate complexation method in Examples 1-8 all exhibited high specific capacity (97-106 mAh / g at 0.1C) and good cycle performance (≥90% retention rate after 100 cycles). Among them, the low-iron-content sulfate-based sodium cathode materials prepared by the phosphate complexation method (Examples 3-4) demonstrated a high specific capacity of ≈100 mAh / g, indicating that introducing phosphate complexes as micro-core precursors and then further preparing sulfate-based cathode materials can significantly improve the specific capacity of sulfate-based sodium cathode materials even with low iron content. Furthermore, the high-iron-content sulfate-based sodium cathode materials prepared by the phosphate complexation method (Example 1) or the multi-doped (Examples 5-8) exhibited even higher specific capacity (≥100 mAh / g) and higher compaction density (≈2.2 g / cm³). 3 Compared with Comparative Example 1 (traditional sulfate-based material) and Comparative Example 2 (sulfate-based material with excessive phosphate complex), the materials exhibit significant advantages in terms of specific capacity, cycle performance, and compaction density. Furthermore, the water settling experiment shows that the sulfate-based cathode materials prepared in Examples 1-8 and Comparative Example 2 demonstrate superior water resistance. This is because the preparation scheme using phosphate complex as the core reduces the degree of hydrolysis of iron ions through the complex precursor, thereby reducing residual acid radicals on the cathode material surface and resulting in better air stability. Additionally, the sulfate-based material prepared using phosphate complex has a higher compaction density and is more densely packed, thus exhibiting better hydrophobicity.

[0098] As can be seen from the above, introducing a phosphate complex ligand scheme into the traditional preparation process of sulfate-based sodium electrode materials, ultimately forming a sulfate-based sodium electrode material prepared by the phosphate complexation method, can effectively solve the problems of low compaction density, inability to further uniformly distribute carbon nanotubes, loose precursor / finished product particle structure, low material capacity, and poor cycle performance of existing sulfate-based cathode materials. The reasons are as follows: First, using the phosphate complex as the core, precise coordination and uniform dispersion of metal ions and phosphate ions are achieved from the reaction source, effectively improving the phase purity of the material while inhibiting the oxidation of iron, ensuring the cathode material has a high specific capacity. Second, based on the phosphate complex, a dense particle precursor is constructed, optimizing particle morphology and packing characteristics, and improving electrode compaction density. Simultaneously, through ion-level uniform dispersion and sintering optimization, uniform distribution of carbon nanotubes in the material system is achieved, constructing a continuous and efficient conductive network. Furthermore, relying on the synergistic stabilizing effect of the double-layer composite sandwich structure, the structural stability of the material is enhanced, suppressing structural decay during cycling and significantly improving long cycle life. In summary, the sulfate-based sodium-ion battery cathode material prepared by the phosphate complexation method in this invention effectively solves the problems of low phase purity, loose particle structure, uneven carbon nanotube distribution, and poor cycle performance of existing sulfate-based cathode materials. It successfully prepares a sulfate-based sodium-ion battery cathode material with a wide voltage range, high actual density, high specific capacity, high cycle stability, and excellent rate performance, providing a brand-new solution for developing new types of low-cost, high-capacity, long-cycle sulfate-based polyanion sodium-ion battery materials.

[0099] Table 1

[0100] It should be understood that the terminology used in this application specification is for the purpose of describing particular embodiments only and is not intended to limit the application.

[0101] It should also be understood that the term “and / or” as used in this application specification and the appended claims means any combination of one or more of the associated listed items and all possible combinations, and includes such combinations.

[0102] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any person skilled in the art can easily conceive of various equivalent modifications or substitutions within the technical scope disclosed in this application, and such modifications or substitutions should all be covered within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

Claims

1. A sulfate-based sodium cathode material, characterized in that, The sulfate-based sodium-ion battery cathode material has a composite sandwich structure and the following general formula: Na2Fe 1+x (SO4) 2+x ·(MPO4) y C z ; Where x is the sulfate excess coefficient, with a value range of 0 ≤ x ≤ 1; y is the stoichiometric coefficient of phosphate, 0.01 ≤ y ≤ 0.3; z is the mass fraction of carbon material, 1 ≤ z ≤ 5; M is selected from one or more of Mg, Ca, Al, Mn, Fe, Co, Ni, Cu, Zn, Ti, or Zr; [MPO4] forms the core of the composite sandwich structure, [Na2Fe 1+x (SO4) 2+x [C] forms the filling layer of the composite sandwich structure. z At least distributed within the composite sandwich structure to form the inner conductive framework of the composite sandwich structure.

2. The sulfate-based sodium cathode material as described in claim 1, characterized in that, 0.2≤x≤0.7、0.01≤y≤0.2、1≤z≤3.

3. The sulfate-based sodium cathode material as described in claim 1, characterized in that, The particle size of [MPO4] is 10nm-1000nm.

4. The sulfate-based sodium cathode material as described in claim 1, characterized in that, The carbon material contains at least nano or submicron-sized carbon nanotubes.

5. The sulfate-based sodium cathode material as described in claim 1, characterized in that, The general formula of the sulfate-based sodium cathode material includes: Na2Fe 1.7 (SO4) 2.7 ·[Mg3(PO4)2] 0.1 C2; Na2Fe 1.6 (SO4) 2.6 ·[Ca3(PO4)2] 0.03 C3; Na2Fe 1.4 (SO4) 2.4 ·[Cu3(PO4)2] 0.05 C2; Na2Fe 1.2 (SO4) 2.2 ·[FePO4] 0.15 C 1.5 ; Na2Fe 1.5 (SO4) 2.5 ·[FePO4] 0.1 [Zn3(PO4)2] 0.05 C 1.5 ; Na2Fe 1.5 (SO4) 2.5 ·[Ti3(PO4)4] 0.025 [Mn3(PO4)2] 0.05 C1; Na2Fe 1.5 (SO4) 2.5 ·[Zr3(PO4)4] 0.025 [Ni3(PO4)2] 0.05 C1; Na2Fe 1.5 (SO4) 2.5 ·[AlPO4] 0.1 [CoPO4] 0.1 C1。 6. A sulfate-based sodium cathode material, characterized in that, The sodium-ion cathode material has a composite sandwich structure and has the following general formula: Na2Fe 1+x (SO4) 2+x ·A y C z ; Where x is the sulfate excess coefficient, with a value range of 0 ≤ x ≤ 1; A is an oxyacid salt with a particle size less than or equal to 1000 nm; y is the stoichiometric coefficient of the oxyacid salt, 0.01 ≤ y ≤ 0.3; z is the mass fraction of carbon material, 1 ≤ z ≤ 5; M is selected from one or more of Mg, Ca, Al, Mn, Fe, Co, Ni, Cu, Zn, Ti, or Zr; A forms the core of the composite sandwich structure, [Na2Fe 1+x (SO4) 2+x [C] forms the filling layer of the composite sandwich structure. z At least distributed within the composite sandwich structure to form the inner conductive framework of the composite sandwich structure.

7. The sulfate-based sodium cathode material as described in claim 6, characterized in that, The oxyacid salts include at least one of phosphates, sulfates, nitrates, pyrophosphates, silicates, and carbonates.

8. A method for preparing a sulfate-based sodium cathode material as described in any one of claims 1 to 7, characterized in that, Specifically, the following steps are included: S1. A dispersion of carbon-containing materials is prepared using the mixed solution method; S2. An NFS mixed solution containing microcrystalline seed phosphate complex was prepared by water-soluble complexation: ferrous sulfate and sodium sulfate were added to deionized water and stirred to dissolve to obtain an NFS precursor solution. Then, water-soluble phosphate solution and water-soluble M salt solution are added sequentially, a complexing agent is added dropwise to carry out the complexation reaction and the pH value of the solution is adjusted, and the above solution is dispersed evenly until a mixed solution of suspended phosphate complex micro-cores is obtained. S3. Mix the solutions prepared in steps S1 and S2 evenly, and disperse the solutions evenly. S4. Spray drying: The mixed solution obtained in step S3 is stirred evenly and then spray dried to obtain precursor particles. S5. Sintering: The precursor particles are placed in an inert atmosphere and sintered by multi-stage heating to the target temperature and holding at that temperature. After cooling, the sulfate-based sodium cathode material is obtained.

9. The preparation method according to claim 8, characterized in that, The molar ratio of sodium sulfate to ferrous sulfate is 1:(1~2), and the molar ratio of sodium sulfate to phosphate is 1:(0.01~0.3); the mass of carbon material is 1-5% of the total mass of raw materials.

10. The preparation method according to claim 8, characterized in that, Step S2 is as follows: Ferrous sulfate and sodium sulfate were added to deionized water and stirred to dissolve, thus obtaining an NFS precursor solution. Sulfate M and water-soluble phosphate were separately added to deionized water to prepare solutions of a certain concentration. The temperature of the phosphate solution was adjusted and stirred until the raw materials were completely dissolved. The above solution was added to the NFS precursor solution, the pH value of the solution was adjusted, and the solution was stirred at a constant temperature until a mixed solution containing phosphate complex microcores was obtained.

11. The preparation method according to claim 8, characterized in that, In step S2, depending on the required acidity or alkalinity, the complexing agent includes at least one of ammonia, citric acid, sodium citrate, sodium gluconate, ethylenediaminetetraacetic acid, malic acid, and dilute phosphoric acid. The water-soluble phosphate includes at least one of sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate.

12. The preparation method according to claim 8, characterized in that, In step S3, the stirring temperature of the NFS precursor solution is 10-80℃, and the stirring time is 20-120min; and in step S4, the conditions corresponding to the spray drying are: inlet air temperature 180-220℃, outlet air temperature 80-105℃, precursor particle D50≤45um, and tap density≥0.7g / cm³.

13. The preparation method according to claim 8, characterized in that, In step S5, the inert atmosphere is nitrogen or argon; the sintering conditions are as follows: first, sinter at a target sintering temperature of 180-320℃ for 5-15 hours, then raise the temperature to a target sintering temperature of 340-400℃ for 4-48 hours. The target sintering temperature for the second stage is preferably 340-380℃, and the holding time is preferably 8-12 hours.

14. A sodium-ion battery, comprising a positive electrode material, characterized in that, The positive electrode material comprises the sulfate-based sodium electrode material as described in any one of claims 1 to 7, or the sulfate-based sodium electrode material prepared by the method for preparing the sulfate-based sodium electrode material as described in any one of claims 8 to 13.

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