Magnesium ion doped sodium ferric phosphate pyrophosphate material as well as preparation method and application thereof

Abstract

The invention relates to a magnesium ion doped sodium ferric phosphate pyrophosphate material as well as a preparation method and application thereof, belongs to the technical field of sodium ion batteries, and solves the problems of low density and low capacity of the existing sodium ferric phosphate pyrophosphate. Comprising the following steps: mixing liquid paraffin with Span 80, heating and stirring to obtain an oil phase; the preparation method comprises the following steps: adding part of a Na source, part of a P source and a Fe < + > source into water to prepare slurry, and adjusting pH to obtain a water phase A; dropwise adding the water phase A into the oil phase to form a water-in-oil emulsion; dispersing a Mg source, a Fe < 2 + > source, the residual Na source and the residual P source in water, adding polyvinylpyrrolidone, and carrying out ball milling to obtain a water phase B; dropwise adding the water phase B into the water-in-oil emulsion, stirring, adding glutaraldehyde, and preserving heat to form a precursor; and demulsifying the precursor, washing, drying, placing in an inert atmosphere, heating to 300-350 DEG C, preserving heat, heating to 750-800 DEG C, preserving heat, and cooling to obtain the magnesium ion doped sodium ferric phosphate pyrophosphate. The magnesium ion doped sodium ferric phosphate pyrophosphate disclosed by the invention is high in density and excellent in electrochemical performance.

Description

Technical Field

[0001] This invention relates to the field of sodium-ion battery technology, and in particular to a magnesium ion-doped sodium iron pyrophosphate material, its preparation method, and its application. Background Technology

[0002] Sodium iron pyrophosphate (NFPP) has become a research hotspot for sodium-ion battery cathode materials due to its high theoretical capacity and stable structure. However, existing NFPP material preparation methods have the following problems: (1) The material particles prepared by existing methods (such as spray drying and sol-gel method) are mostly hollow or porous, with loose packing density usually ≤1.2g / cm³ and tapped density ≤2.3g / cm³, resulting in low coating density of battery electrode and difficulty in meeting commercial demand for volumetric energy density; (2) Weak particle interface bonding: Some existing methods fill the gaps by physically mixing particles of different sizes, but the small particles and large particles are only in physical contact, and the interface bonding force is weak, which easily generates gaps in charge-discharge cycles, leading to capacity decay; (3) Poor doping uniformity: Magnesium ion doping can improve the conductivity and structural stability of materials, but existing doping methods (such as precursor mixing and solution blending) easily lead to magnesium ion agglomeration, with a distribution deviation of ≥10%, and local generation of impurity phases such as MgO, which affects electrochemical performance.

[0003] Therefore, how to achieve high packing / tap density, integrated particle bonding, and uniform magnesium ion doping in NFPP materials has become an urgent problem to be solved. Summary of the Invention

[0004] In view of the above analysis, the present invention aims to provide a magnesium ion-doped sodium iron pyrophosphate material, its preparation method and application, to at least solve one of the following problems: in the prior art, sodium iron pyrophosphate has low loose packing density, low tap density, poor doping uniformity and low capacity.

[0005] This invention provides a method for preparing magnesium ion-doped sodium iron pyrophosphate, comprising the following steps: S1. Preparation of the oil phase system: Liquid paraffin is mixed with Span 80, heated and stirred to obtain the oil phase; S2. Preparation of aqueous phase A: Partial Na source, partial P source, Fe³⁺ + The source is added to water to prepare a slurry, and the pH is adjusted to acidity to obtain aqueous phase A; S3. Preparation of water-in-oil emulsion: Add aqueous phase A dropwise to the oil phase and stir to form a water-in-oil emulsion; S4. Preparation of aqueous phase B: Mg source, Fe... 2+ The source, remaining Na source, and remaining P source are dispersed in water, polyvinylpyrrolidone is added, and the mixture is ball-milled to obtain aqueous phase B; S5. Interfacial polymerization filling: Aqueous phase B is dropped into water-in-oil emulsion, stirred, glutaraldehyde is added, and the mixture is kept at 55~65℃ to form a precursor. S6. Post-treatment and sintering: After demulsifying, washing and drying the precursor, place it in an inert atmosphere, heat it to 300~350℃ with v1 and hold it, then heat it to 750~800℃ with v2 and hold it, and then cool it naturally to room temperature to obtain magnesium ion-doped iron pyrophosphate sodium phosphate.

[0006] Furthermore, in S1, the mass ratio of Span 80 to the volume ratio of liquid paraffin in the oil phase system is 0.3~0.7g:100mL.

[0007] Furthermore, in S1, the mixture is heated to 55~65℃ and stirred.

[0008] Furthermore, in S2, the pH is adjusted to 2.5~3.5.

[0009] Furthermore, in S3, the volume ratio of the oil phase to the water phase A is controlled to be 1:2 to 1:4.

[0010] Furthermore, in S3, the droplet diameter of the water-in-oil emulsion is controlled to be 15~20μm.

[0011] Furthermore, in S4, the particles are ball-milled to a particle size of 0.8~1.2μm to obtain aqueous phase B.

[0012] Furthermore, in S5, the volume ratio of aqueous phase A to aqueous phase B is controlled to be 2:1 to 3:1.

[0013] This invention also provides a magnesium ion-doped iron pyrophosphate sodium, which is prepared by the above-described preparation method. The chemical formula of the magnesium ion-doped iron pyrophosphate sodium is Na₄Fe₂O₃. 3-x Mg x (PO4)2P2O7, where x = 0.06~0.12.

[0014] The present invention also provides a sodium-ion battery, which includes a positive electrode comprising the above-mentioned magnesium ion-doped sodium iron pyrophosphate.

[0015] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects: The preparation method of this invention involves first preparing an oil-in-water emulsion to form a large-particle shell template, then filling the shell with magnesium-doped small particles to form a "core-shell" precursor structure. Glutaraldehyde initiates an interfacial polymerization reaction, chemically bonding the shell to the internal small particles, avoiding interfacial gaps caused by subsequent physical mixing. Combined with gradient temperature-controlled sintering: the low-temperature stage removes organic components, while the high-temperature stage promotes crystal growth, ensuring material densification while preventing excessive particle agglomeration. This method eliminates the need for subsequent physical mixing; the small particles and large-particle framework form a solid structure through chemical bonding, with a solidity rate ≥90%. It exhibits high loose and tap densities, for example, loose density ≥1.45 g / cm³ and tap density ≥2.7 g / cm³. Magnesium ions are uniformly dispersed within the material through the small particle filling, with a distribution deviation ≤5%. The near-spherical particles have good flowability and strong process compatibility. This magnesium-doped sodium iron pyrophosphate material lays the foundation for high volumetric energy density batteries.

[0016] The sodium-ion battery prepared from the magnesium-doped sodium iron pyrophosphate material of the present invention exhibits stable electrochemical performance. For example, the initial charge specific capacity at 0.1C rate is above 116 mAh / g, for example, 116.1~118 mAh / g; the initial discharge specific capacity is above 106 mAh / g, for example, 106~108 mAh / g; the initial charge-discharge efficiency is above 90%, for example, 90.34%~93%; and the capacity retention rate after 500 cycles is ≥93%, for example, 94.5%~95.5%, meeting the long-cycle requirements of commercial sodium-ion batteries.

[0017] In this invention, the above-described technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of this invention will be set forth in the following description, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of this invention can be realized and obtained by means of what is particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0018] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts. Figure 1 This is an electron microscope image of magnesium ion-doped sodium iron pyrophosphate from Example 1; Figure 2 This is an electron microscope image of magnesium ion-doped sodium iron pyrophosphate after it has been broken up in Example 1; Figure 3 This is the ICP-OES result diagram of magnesium ion-doped sodium iron pyrophosphate in Example 1; Figure 4 The charge-discharge curve of the button cell battery in Example 1; Figure 5 The charge-discharge curve of the button cell battery in Example 2; Figure 6 The charge-discharge curve of the button cell in Example 3; Figure 7 The charge-discharge curve of the button cell battery in Example 4; Figure 8 The charge-discharge curve of the button cell battery in Example 5; Figure 9 The charge-discharge curves are for the button cell battery in Comparative Example 1. Detailed Implementation

[0019] The preferred embodiments of the present invention are described below in detail to illustrate the principles of the present invention, and are not intended to limit the scope of the present invention.

[0020] This invention provides a method for preparing magnesium ion-doped sodium iron pyrophosphate, wherein the chemical formula of the magnesium ion-doped sodium iron pyrophosphate is Na₄Fe₂O₃. 3-x Mg x (PO4)2P2O7, where x = 0.06~0.12; the preparation method includes the following steps: S1. Preparation of oil phase system: Liquid paraffin is mixed with Span 80, heated and stirred to obtain a homogeneous oil phase; S2. Preparation of aqueous phase A: Partial Na source, partial P source, Fe³⁺ + The source is added to deionized water to prepare a slurry with a solid content of 25% to 35%, and the pH is adjusted to 2.5 to 3.5 to obtain aqueous phase A; S3. Preparation of water-in-oil emulsion: Add aqueous phase A dropwise to the oil phase and stir to form a water-in-oil emulsion; S4. Preparation of aqueous phase B: Mg source, Fe... 2+ The source, residual Na source, and residual P source are dispersed in deionized water, polyvinylpyrrolidone is added, and the mixture is ball-milled to a particle size of 0.8~1.2μm to obtain aqueous phase B; S5. Interfacial polymerization filling: Add aqueous phase B dropwise into water-in-oil emulsion, stir for 30-40 min, add glutaraldehyde, keep warm at 55-65℃ for 1.5-2.5 h to allow the aqueous phase component to polymerize and solidify, forming a precursor. S6. Post-treatment and sintering: After demulsifying, washing and drying the precursor, place it in an inert atmosphere, heat it to 300~350℃ at v1 and hold it for 2~3 hours, then heat it to 750~800℃ at v2 and hold it for 4~6 hours, and then cool it naturally to room temperature to obtain spherical magnesium ion-doped iron pyrophosphate sodium.

[0021] Specifically, in S1 above, Span 80 acts as an emulsifier, adsorbing at the interface between the aqueous phase (aqueous phase A / B) and liquid paraffin to reduce interfacial tension and prevent droplet aggregation. If the mass of Span 80 is too small, the emulsifier concentration is insufficient, failing to completely cover the surface of the aqueous droplets. This leads to easy demulsification of the water-in-oil emulsion, preventing the formation of a stable outer shell template. Ultimately, this results in uneven particle size and irregular morphology, directly affecting the loose / tap density of the material. Therefore, the mass ratio of Span 80 to liquid paraffin in the oil phase system is controlled to be 0.3~0.7g:100mL, for example, 0.3g:100mL, 0.4g:100mL, 0.5g:100mL, 0.6g:100mL, and 0.7g:100mL.

[0022] Specifically, in S1 above, 55~65℃ is the optimal active temperature range for Span 80. In order to maximize the emulsification effect, it is heated to 55~65℃ and stirred, for example, 55℃, 60℃, 65℃.

[0023] Specifically, in S1 above, considering that too low a stirring rate will cause Span 80 to not dissolve and disperse sufficiently, resulting in uneven mixing of the oil phase and poor heat transfer, thus affecting the subsequent emulsification effect; while too high a rate will entrain too much air and generate bubbles that are difficult to eliminate, interfering with subsequent processes, and will also increase unnecessary energy consumption and equipment wear; therefore, the stirring rate is controlled at 500~700 r / min, for example 500 r / min, 600 r / min, 700 r / min.

[0024] Specifically, in S2 above, the molar percentage of P provided by a portion of the P sources to the total P provided by all P sources is 60%~70%; Fe³ + Fe³ provided by the source + The molar percentage of total Fe provided by all Fe sources is 60%~70%. This ratio precisely matches the structural design requirement of "large particle framework - small particle filling", ensuring that aqueous phase A can form a 15~20μm water-in-oil emulsion shell, while reserving sufficient filling space for aqueous phase B. The molar percentage of Na provided by part of the Na source is 75%~80% of the total Na provided by all Na sources. This is because the Na element not only needs to meet the charge balance requirements of the framework and small particles, but also needs to reserve 3%~5% redundancy to compensate for slight volatilization loss during high-temperature sintering. At the same time, priority is given to ensuring sufficient Na element in the shell framework of aqueous phase A, avoiding structural defects or incomplete molding due to local Na deficiency, and ultimately ensuring the crystal structure integrity and compositional uniformity of the target product.

[0025] Specifically, in S2 above, the Na source is at least one of sodium nitrate, sodium acetate, sodium carbonate, and sodium citrate; Fe³ +The source is at least one of ferric nitrate, ferric chloride, and ferric phosphate; the P source is at least one of phosphoric acid and sodium dihydrogen phosphate.

[0026] Specifically, in the above S2, improper pH control can lead to impure phases, for example, pH > 3.5: Fe³ + The Fe(OH)3 generated by hydrolysis is transformed into the Fe2O3 impurity phase after sintering; pH < 2.5: The strongly acidic environment will promote the polymerization of P source to generate an excess of pyrophosphate, which may form byproducts such as Na3PO4 and disrupt the stoichiometry of the target product; therefore, the pH should be controlled and adjusted to 2.5~3.5, for example 2.5, 3, 3.5.

[0027] Specifically, in step S3 above, the water-in-oil emulsion serves as a template for the large particle shell in subsequent steps. The oil phase is the "dispersion medium" of the emulsion, and the aqueous phase A is the "dispersed phase." The volume ratio of the oil phase to the aqueous phase A directly determines the droplet size. If the oil phase proportion is too low, it cannot fully encapsulate the aqueous droplets, causing them to easily collide and merge, ultimately forming large droplets >25μm, which cannot form a uniform shell template. If the oil phase proportion is too high, the aqueous phase is over-diluted, and the droplet size will shrink to <10μm. Furthermore, the amount of oil phase used is wasted, increasing the cost of subsequent demulsification and separation, and also resulting in an excessively thin large particle shell that is prone to cracking after sintering. Therefore, the volume ratio of the oil phase to the aqueous phase A is controlled to be 1:2 to 1:4, for example, 1:2, 1:3, or 1:4.

[0028] Specifically, in S3 above, in order to ensure the emulsification effect, the temperature is maintained at 55~65℃ and the stirring speed is maintained at 500~700r / min.

[0029] Specifically, in step S3, the droplet diameter of the water-in-oil emulsion is controlled to be 15-20 μm; in step S4, the particles are ball-milled to a particle size of 0.8-1.2 μm to obtain aqueous phase B. In subsequent steps, the water-in-oil emulsion serves as a template for the large particle shell, while aqueous phase B functions as a "magnesium-doped small particle filler." The added Fe, Na, and P sources, along with the Mg source, collectively form small particles of 0.8-1.2 μm, which fill the shell formed by the water-in-oil emulsion. Further research revealed that controlling the particle size ratio of large particles (shell) to small particles (filler) to approximately 15-25:1 allows the small particles to tightly fill the voids within the shell, reducing voids and ultimately achieving a high solidity. Therefore, controlling the particle size of aqueous phase B to be 0.8-1.2 μm and the droplet diameter of the water-in-oil emulsion to be 15-20 μm ensures the formation of reasonable internal voids. If the droplet diameter of the water-in-oil emulsion is <15μm, the internal space of the shell is too small, and small particles of 0.8~1.2μm cannot be fully filled, resulting in a decrease in the overall solidity. If the droplet diameter is >20μm, the internal voids of the shell are too large, and there is still a lot of remaining space after the small particles are filled, which also leads to a decrease in solidity. Moreover, if the shell is too thick, it is easy to crack during sintering. Taking all factors into consideration, the droplet diameter of the water-in-oil emulsion should be controlled at 15~20μm.

[0030] Specifically, in S4 above, Fe 2+ Fe provided by the source 2+ The molar percentage of total Fe ions is 30%–40%. It should be noted that magnesium ions are required to prepare aqueous phase B, and Fe²⁺ ions are also present. + The ionic radius (0.074 nm) is slightly larger than that of Fe³⁺. + (0.0645 nm), the coordination with PVP (polyvinylpyrrolidone) in aqueous solution is milder, and the PVP molecular chains can be more uniformly adsorbed on Fe²⁺. + The composite particles formed with P and Mg form a stable "steric hindrance film" on their surface, preventing particle agglomeration (if Fe³⁺ is used). + Source, Fe³ + It has a higher charge density and stronger interionic attraction, making it more readily associated with PO4. 3- A dense FePO4 precipitate forms rapidly, but it is difficult to refine to 0.8~1.2 μm during ball milling. The sintering process under an inert atmosphere... + It will be slowly oxidized to Fe³ + The small amount of energy released during oxidation can promote the Fe³⁺ bonding between small particles and the outer shell. + The P source undergoes a "metallurgical bond," which allows the large outer shell and small particles to fit together better, increasing the solidity rate.

[0031] Specifically, in S4 above, the Mg source is at least one of magnesium chloride and magnesium nitrate; Fe² + The source is at least one of ferrous sulfate and ferrous chloride.

[0032] Specifically, in S4 above, the mass percentage of polyvinylpyrrolidone is 0.8% to 1.2%, for example, 0.8%, 0.9%, 1.0%, 1.1%, and 1.2%.

[0033] Specifically, the goal of S5 is to ensure that the small particles of aqueous phase B perfectly fill the internal voids of the outer shell of aqueous phase A, without any gaps or overflow, while maintaining a suitable ratio between the outer shell and the small particles. This is because the large particles form the supporting framework, the core of the structure, while the small particles are merely fillers. Only with a higher proportion of the framework can structural support, high filling efficiency, and stable performance be simultaneously achieved. If the proportion of large particles is too small, the outer shell wall will be too thin, causing it to break during mixing and filling, shrink and collapse during drying, and crack during sintering, leading to the overflow of small particles and the direct collapse of the "shell-filler" structure. Therefore, to ensure sufficient strength to withstand process impacts without excessive thickness resulting in large internal voids and a reduced solidity, the volume ratio of aqueous phase A to aqueous phase B is controlled at 2:1 to 3:1. This ensures a tightly fitted "shell-small particle" structure; the higher the fit, the higher the overall solidity.

[0034] Specifically, in S5 above, the mass ratio of glutaraldehyde to the sum of the masses of aqueous phase A and aqueous phase B is controlled to be 0.6%~1.1%. This ratio is designed to ensure stable crosslinking efficiency of glutaraldehyde and polyvinylpyrrolidone (PVP) and to meet the requirements of precursor molding. Preferably, the mass ratio of glutaraldehyde to the sum of the masses of aqueous phase A and aqueous phase B is 0.8%~1.0%.

[0035] Specifically, in S5 above, 55~65℃ is the optimal activation temperature for Span 80 emulsifier. This temperature avoids excessively low temperatures leading to increased oil phase viscosity and emulsion separation / demulsification, or excessively high temperatures causing emulsifier failure. It ensures that the small particles of aqueous phase B are uniformly dispersed within each aqueous phase A shell. It also promotes glutaraldehyde interfacial polymerization: glutaraldehyde, as a crosslinking agent, can rapidly initiate crosslinking reactions between components in aqueous phase A (skeleton) and aqueous phase B (small particles) at 55~65℃, forming a chemically bonded solidified structure. If the temperature is too low (<55℃), the polymerization rate is slow, and complete solidification cannot occur within 2.5 hours, making the precursor prone to loosening. If the temperature is too high (>65℃), the polymerization reaction is too vigorous, causing the shell to shrink too quickly, squeezing the internal small particles and forming cracks. Therefore, maintaining the temperature at 55~65℃ for 1.5~2.5 hours is recommended.

[0036] Specifically, in S6 above, the 300~350℃ holding time for 2~3 hours is a low-temperature pre-firing section, the function of which is to remove organic components such as PVP and glutaraldehyde from the precursor. If the temperature is directly raised to 800℃ at high speed, the organic components will decompose rapidly and produce gases such as CO2 and N2. The instantaneous overflow of gas will break through the "shell-small particle" structure, forming a large number of pores and reducing the solidity. Therefore, 300~350℃ (the suitable temperature for the decomposition of organic components), combined with a heating rate of 3~5℃ / min, allows the organic components to decompose and volatilize slowly and fully, avoiding gas impact, and ensuring that the "shell-small particle" maintains its structural integrity in a state without organic residue. The heating rate is reduced to 2-3℃ / min during the 750-800℃ holding period for 4-6 hours (high-temperature densification stage) because: it avoids cracking caused by inconsistent thermal expansion rates between the outer shell and small particles at high temperatures, and allows Fe, P, Mg ions to diffuse fully between the two phases, achieving the bonding of the "outer shell and small particles"; the 4-6 hour holding period allows magnesium ions to be more uniformly doped into the Fe site lattice (avoiding local agglomeration), while promoting full crystal growth and improving structural stability.

[0037] Specifically, in S6 above, v1 is greater than v2, for example, v1 is 3~5℃ / min, such as 3℃ / min, 4℃ / min, 5℃ / min. v2 is 2~3℃ / min, such as 2℃ / min, 2.5℃ / min, 3℃ / min.

[0038] Specifically, the chemical formula of the spherical magnesium ion-doped sodium iron pyrophosphate prepared by S6 can be Na4Fe 2.91 Mg 0.09 (PO4)2P2O7, Na4Fe 2.94 Mg 0.06 (PO4)2P2O7 or Na4Fe 2.88 Mg 0.12 (PO4)2P2O7.

[0039] Specifically, the spherical magnesium ion-doped iron pyrophosphate sodium obtained by S6 above has a core-shell structure with small particles encapsulated in the outer shell.

[0040] Specifically, the spherical magnesium ion-doped iron pyrophosphate sodium obtained by S6 above has a loose packing density ≥1.45 g / cm³ (e.g., 1.47~1.57 g / cm³), a tap density ≥2.7 g / cm³ (e.g., 2.72~2.92 g / cm³), a particle size of 16~22 μm, a solidity ≥90%, a sphericity ≥0.85, and magnesium ions are uniformly dispersed inside the material through small particles with a distribution deviation ≤5%, e.g., 2.9%~4.5%.

[0041] The present invention also provides an application of magnesium ion-doped sodium iron pyrophosphate, which can be used as a positive electrode material for sodium-ion batteries.

[0042] The present invention also provides a sodium-ion battery, wherein the sodium-ion battery includes the above-mentioned positive electrode material.

[0043] The sodium-ion battery prepared from the magnesium-doped sodium iron pyrophosphate material of the present invention exhibits stable electrochemical performance. For example, the initial charge capacity at 0.1C rate is above 116 mAh / g, such as 116.1~118 mAh / g; the initial discharge capacity is above 106 mAh / g, such as 106~108 mAh / g; the initial charge-discharge efficiency is above 90%, such as 90.34%~93%; and the capacity retention rate after 500 cycles is ≥93%, such as 94.5%~95.5%, meeting the long-cycle requirements of commercial sodium-ion batteries.

[0044] The preparation method of this invention involves first preparing an oil-in-water emulsion to form a large-particle shell template, then filling the shell with magnesium-doped small particles to form a "core-shell" precursor structure. Glutaraldehyde initiates an interfacial polymerization reaction, chemically bonding the shell to the internal small particles, avoiding interfacial gaps caused by subsequent physical mixing. Combined with gradient temperature-controlled sintering: the low-temperature stage removes organic components, while the high-temperature stage promotes crystal growth, ensuring material densification while preventing excessive particle agglomeration. This method eliminates the need for subsequent physical mixing; the small particles and large-particle framework form a solid structure through chemical bonding, with a solidity rate ≥90%. It exhibits high loose and tap densities, for example, loose density ≥1.45 g / cm³ and tap density ≥2.7 g / cm³. Magnesium ions are uniformly dispersed within the material through the small particle filling, with a distribution deviation ≤5%. The near-spherical particles have good flowability and strong process compatibility. This magnesium-doped sodium iron pyrophosphate material lays the foundation for high volumetric energy density batteries.

[0045] The sodium-ion battery prepared from the magnesium-doped sodium iron pyrophosphate material of the present invention exhibits stable electrochemical performance. For example, the initial charge specific capacity at 0.1C rate is above 116 mAh / g, for example, 116.1~118 mAh / g; the initial discharge specific capacity is above 106 mAh / g, for example, 106~108 mAh / g; the initial charge-discharge efficiency is above 90%, for example, 90.34%~93%; and the capacity retention rate after 500 cycles is ≥93%, for example, 94.5%~95.5%, meeting the long-cycle requirements of commercial sodium-ion batteries.

[0046] Example 1 This embodiment provides a magnesium ion-doped sodium iron pyrophosphate and its preparation method, the preparation method including: Preparation of oil phase system: Take 100 mL of liquid paraffin, add 0.5 g of Span 80, and stir at 600 r / min for 18 min at 60℃ to obtain a homogeneous and transparent oil phase. The oil phase is clear and without stratification. Preparation of aqueous phase A: Weigh 2.02g sodium nitrate, 4.04g ferric nitrate nonahydrate, and 1.56g food-grade phosphoric acid (phosphoric acid mass concentration of 85wt%), dissolve them in 30mL deionized water to prepare a slurry with a solid content of approximately 28%; adjust the pH to 3.0 with 1.5mol / L dilute nitric acid while stirring at 350r / min, and continue stirring for 35min until completely dissolved to obtain clear and transparent aqueous phase A; Preparation of water-in-oil emulsion: Aqueous phase A was slowly added dropwise to the oil phase at an oil-to-water volume ratio of 1:3 over a period of 25 min. The temperature was maintained at 60℃ and the stirring rate at 600 r / min. Stirring was continued for another 12 min to form a water-in-oil emulsion. The droplet diameter of the emulsion was measured by a laser particle size analyzer. The droplet diameter was 17-19 μm, and the particle size distribution span (D90-D10) / D50 = 0.25. The droplets were uniformly dispersed without agglomeration. Preparation of aqueous phase B: Weigh 0.08 g magnesium chloride hexahydrate, 1.39 g ferrous sulfate heptahydrate, and 0.85 g anhydrous sodium dihydrogen phosphate, disperse them in 20 mL of deionized water, and add 0.3 g polyvinylpyrrolidone; wet ball milling was performed using an agate ball mill with a grinding ball diameter of 0.8 mm, a ball-to-material ratio of 10:1, a rotation speed of 300 r / min, and a 5 min pause every 20 min, for a total ball milling time of 1 h; dynamic light scattering analysis showed that the particle size in aqueous phase B was 0.9~1.1 μm, with a uniform particle size distribution; Interfacial polymerization filling: Aqueous phase B was added dropwise to water-in-oil emulsion at a volume ratio of aqueous phase A to aqueous phase B of 2.5:1 over a period of 20 min. After stirring for 35 min, 0.5 g of 50 wt% analytical grade glutaraldehyde (approximately 0.82% of the total mass of the aqueous phase) was added. The mixture was kept at 60 °C for 2 h, with stirring for 10 min every 30 min during the incubation period, to form a pale yellow, granular, large-particle precursor. Observation under an optical microscope showed that the precursor particle size was 18-20 μm, and the particles were spherical. Post-treatment and sintering: The precursor was washed with anhydrous ethanol to remove the upper oil phase until no oil film remained on the precursor after washing. The washed precursor was vacuum dried for 10 h to obtain a dried precursor. The dried precursor was placed in an inert atmosphere furnace and nitrogen gas was introduced at a flow rate of 250 mL / min. The temperature was increased to 320 °C at a rate of 4 °C / min and held for 2.5 h. Then the temperature was increased to 780 °C at a rate of 2 °C / min and held for 5 h. The temperature was then allowed to cool naturally to room temperature to obtain spherical magnesium ion-doped sodium iron pyrophosphate.

[0047] The magnesium ion-doped sodium iron pyrophosphate of this embodiment was detected by ICP-OES, and the detection method is as follows: (1) Take 0.1g of solid sample, digest it and then make up to 100mL; (2) First step of dilution: Take 1 mL of the above mother liquor and dilute it to 200 mL with deionized water to obtain the intermediate dilution; (3) Second step of dilution: Take 1 mL of the above intermediate diluent and dilute it to 100 mL with deionized water. The total dilution factor after the two dilutions are combined is 200 × 100 = 2000 times.

[0048] The ICP-OES results for magnesium ion-doped sodium iron pyrophosphate in this embodiment are as follows: Figure 3 As shown. The chemical formula of the magnesium ion-doped sodium iron pyrophosphate in this embodiment is Na₄Fe₂O₃. 2.91 Mg 0.09 Electron micrograph of (PO4)2P2O7, magnesium ion-doped sodium iron pyrophosphate, is shown below. Figure 1 As shown in the figure, magnesium ion-doped sodium iron pyrophosphate is spherical. The electron micrograph of the broken magnesium ion-doped sodium iron pyrophosphate is shown in the figure below. Figure 2 As shown, the magnesium ion-doped sodium iron pyrophosphate of the present invention has a core-shell structure in which small particles are encapsulated in a shell.

[0049] The obtained magnesium ion-doped sodium iron phosphate pyrophosphate was tested, and the results are as follows: The loose density was 1.53 g / cm³, and the tapped density was 2.82 g / cm³; the Mg element distribution deviation was 3.2%, with no local agglomeration. The above magnesium ion-doped sodium iron phosphate pyrophosphate material was used as the positive electrode material for sodium-ion batteries, assembled into button cells, and the battery performance was tested. The button cell manufacturing method is as follows: The positive electrode material (magnesium ion-doped sodium iron phosphate pyrophosphate in this embodiment), conductive agent (SuperP), and binder (polyvinylidene fluoride, PVDF) are mixed in a mass ratio of 9:0.5:0.5. An appropriate amount of N-methylpyrrolidone (NMP) solvent is added, and the mixture is stirred in a planetary pulper at a speed of 3000 r / min for 20 min to form a uniformly dispersed electrode slurry without agglomeration. Coating and drying: The above electrode paste was uniformly coated onto a 15μm thick aluminum foil current collector using a doctor blade coating method, with the coating density controlled at 10~12mg / cm². The coated aluminum foil was pre-dried in a 60℃ forced-air drying oven for 4h to remove most of the solvent, and then transferred to a 120℃ vacuum drying oven for 12h to completely remove residual NMP. Cutting and compaction: The dried electrode sheets are cut into circular positive electrode sheets with a diameter of 12mm using a punching machine. The sheets are then compacted using a roller press under a pressure of 5MPa. The compacted positive electrode sheets are placed in a vacuum drying oven and kept at 80℃ for 2 hours for later use.

[0050] Assembly of button batteries: Assembly was carried out in an argon-filled glove box (water and oxygen content ≤0.1ppm), using CR2032 button battery casings, gaskets, springs, and other accessories. The electrodes are stacked in the following order: negative electrode shell → sodium sheet (14mm diameter, 0.3mm thickness, sodium purity ≥99.9%) → electrolyte → separator → electrolyte → positive electrode sheet → spring sheet → gasket → positive electrode shell. The electrolyte is a 1 mol / L NaPF6 mixed solution of ethylene carbonate (EC) / dimethyl carbonate (DMC) (volume ratio 1:1). Each battery is injected with 50 μL of electrolyte to ensure complete wetting of the separator. The batteries were sealed using a button cell sealing machine under 10MPa pressure. After sealing, the batteries were checked for leakage and bulging. They were then transferred to a battery testing chamber and left to stand for 4 hours before testing.

[0051] Figure 4 The charge-discharge curves of the button cell assembled in Example 1 are shown. The electrochemical performance is as follows: at 25°C and a voltage range of 2.0~4.0V, the initial charge capacity at a 0.1C rate (1C=129mA / g) is 116.10mAh / g, the initial discharge capacity is 106.39mAh / g, the initial charge-discharge efficiency is 91.63%, and the capacity retention after 500 cycles is 95.2%.

[0052] Example 2 This embodiment provides a magnesium ion-doped sodium iron pyrophosphate and its preparation method, the preparation method including: Preparation of oil phase system: Take 100 mL of liquid paraffin, add 0.4 g of Span 80, and stir at 550 r / min for 16 min at 58 °C to obtain a homogeneous oil phase; Preparation of aqueous phase A: Weigh 1.64g sodium acetate, 3.25g ferric chloride hexahydrate, and 1.56g 85wt% phosphoric acid, dissolve them in 28mL deionized water to prepare a slurry with a solid content of approximately 30%; adjust the pH to 2.8 with 1.2mol / L dilute phosphoric acid while stirring at 320r / min, and continue stirring for 32min to obtain aqueous phase A (clear and without precipitate). Preparation of water-in-oil emulsion: Aqueous phase A was added dropwise to the oil phase at an oil-water volume ratio of 1:2.5 over a period of 22 min. The temperature was 58℃ and the stirring rate was 550 r / min. After stirring for 10 min, an emulsion was formed. Laser particle size analyzer showed that the droplet diameter was 16~18 μm and the span was 0.28. Preparation of aqueous phase B: Weigh 0.08 g magnesium chloride hexahydrate, 1.39 g ferrous sulfate heptahydrate, and 0.82 g anhydrous sodium dihydrogen phosphate, disperse them in 18 mL of deionized water, and add 0.25 g polyvinylpyrrolidone; wet ball mill (ball-to-material ratio 9:1, speed 280 r / min, time 0.8 h), particle size 0.8~1.0 μm; Interfacial polymerization filling: The volume ratio of aqueous phase A to aqueous phase B is 2.2:1, the dropping time is 18 min, and after stirring for 32 min, 0.45 g of glutaraldehyde (about 0.81% of the total mass of the aqueous phase) is added. The mixture is kept at 58℃ for 2.2 h to form a precursor with a particle size of 17~19 μm. Post-treatment and sintering: The precursor was washed with anhydrous ethanol to remove the upper oil phase until no oil film remained on the precursor after washing. It was then vacuum dried at 80℃ for 8 hours. Under nitrogen atmosphere (220 mL / min), the temperature was increased to 330℃ at 4℃ / min and held for 2 hours. The temperature was then increased to 790℃ at 2.5℃ / min and held for 4.5 hours. The temperature was then allowed to cool naturally to room temperature to obtain spherical magnesium ion-doped iron pyrophosphate sodium.

[0053] The magnesium ion-doped sodium iron pyrophosphate in this embodiment has the chemical formula Na₄Fe₂O₃. 2.91 Mg 0.09 The magnesium ion-doped sodium iron pyrophosphate obtained from (PO4)2P2O7 was analyzed, and the results are as follows: Loose bulk density: 1.51 g / cm³; tapped density: 2.78 g / cm³; Mg element distribution deviation: 3.8%; The aforementioned magnesium ion-doped sodium iron phosphate pyrophosphate material was used as the positive electrode material for sodium-ion batteries, assembled into button cells, and the battery performance was tested. The button cell manufacturing method is the same as in Example 1, and will not be repeated here.

[0054] Figure 5 The charge-discharge curves of the button cell assembled in this embodiment are shown. The electrochemical performance is as follows: the initial charge capacity at 0.1C is 117.34 mAh / g, the initial discharge capacity is 106.01 mAh / g, the initial charge-discharge efficiency is 90.34%, and the retention rate after 500 cycles is 94.5%.

[0055] Example 3 This embodiment provides a magnesium ion-doped sodium iron pyrophosphate and its preparation method, the preparation method including: Preparation of oil phase system: Take 100mL of liquid paraffin, add 0.3g Span 80, stir at 55℃ and 500r / min for 15min until the oil phase is homogeneous; Preparation of aqueous phase A: Weigh 1.06g sodium carbonate (99.0% purity), 3.02g ferric phosphate (98.0% purity), and 1.20g 85wt% phosphoric acid according to the stoichiometric ratio, dissolve them in 25mL deionized water to a concentration of 32%; adjust the pH to 2.5 with 1.0mol / L dilute phosphoric acid, stir for 40min, and aqueous phase A will be a light brown clear liquid; Preparation of water-in-oil emulsion: oil-water volume ratio 1:2, dropping time 20 min, stirring at 55℃ and 500 r / min for 15 min, emulsion droplet diameter 15~17 μm, span 0.26; Preparation of aqueous phase B: Weigh 0.05 g magnesium nitrate hexahydrate (purity 98.0%), 1.42 g ferrous chloride tetrahydrate (purity 98.0%), and 0.78 g anhydrous sodium dihydrogen phosphate, disperse them in 15 mL of deionized water, and add 0.2 g polyvinylpyrrolidone; ball mill (ball-to-material ratio 8:1, speed 250 r / min, 1.5 h), particle size 1.0~1.2 μm; Interfacial polymerization filling: Aqueous phase A to aqueous phase B volume ratio 3:1, dropwise addition time 25 min, stirring for 30 min, then add 0.4 g glutaraldehyde (approximately 0.83% of the total mass of the aqueous phase), keep warm at 55℃ for 2.5 h, precursor particle size 16~18 μm; Post-treatment and sintering: The precursor was washed with anhydrous ethanol to remove the upper oil phase until no oil film remained on the precursor after washing. It was then vacuum dried at 60℃ for 12h. Under argon atmosphere (200mL / min), the temperature was increased to 300℃ at 3℃ / min and held for 3h. The temperature was then increased to 750℃ at 2℃ / min and held for 6h. The temperature was then naturally cooled to room temperature to obtain spherical magnesium ion-doped iron pyrophosphate sodium.

[0056] The magnesium ion-doped sodium iron pyrophosphate in this embodiment has the chemical formula Na₄Fe₂O₃. 2.94 Mg 0.06 The magnesium ion-doped sodium iron pyrophosphate obtained from (PO4)2P2O7 was analyzed, and the results are as follows: Loose bulk density: 1.47 g / cm³; tapped density: 2.72 g / cm³; Mg element distribution deviation: 2.9%; The aforementioned magnesium ion-doped sodium iron phosphate pyrophosphate material was used as the positive electrode material for sodium-ion batteries, assembled into button cells, and the battery performance was tested. The button cell manufacturing method is the same as in Example 1, and will not be repeated here.

[0057] Figure 6 The charge-discharge curves and electrochemical performance of the button cell assembled in this embodiment are as follows: the initial charge capacity at 0.1C is 116.57 mAh / g, the initial discharge capacity is 107.30 mAh / g, the initial charge-discharge efficiency is 92.05%, and the retention rate after 500 cycles is 95.0%.

[0058] Example 4 This embodiment provides a magnesium ion-doped sodium iron pyrophosphate and its preparation method, the preparation method including: Preparation of the oil phase system: Take 100 mL of liquid paraffin, add 0.7 g of Span 80, stir at 65℃ and 700 r / min for 15 min, and the oil phase is transparent; Preparation of aqueous phase A: Weigh 2.94g sodium citrate, 4.04g ferric nitrate nonahydrate, and 1.60g 85wt% phosphoric acid, dissolve them in 32mL deionized water to a concentration of 25%; adjust the pH to 3.5 with 2.0mol / L dilute nitric acid, stir for 30min, and aqueous phase A becomes clear; Preparation of water-in-oil emulsion: oil-water volume ratio 1:4, dropping time 30 min, stirring at 65℃ and 700 r / min for 10 min, emulsion droplet diameter 18~20 μm, span 0.29; Preparation of aqueous phase B: Weigh 0.11 g magnesium chloride hexahydrate, 1.36 g ferrous sulfate heptahydrate, and 0.88 g anhydrous sodium dihydrogen phosphate, disperse them in 20 mL of deionized water, add 0.35 g polyvinylpyrrolidone; ball mill to a particle size of 0.8~1.0 μm; Interfacial polymerization filling: Aqueous phase A and aqueous phase B are added in a volume ratio of 2:1 over 15 min. After stirring for 40 min, 0.6 g of glutaraldehyde (approximately 0.94% of the total mass of the aqueous phase) is added and the mixture is kept at 65℃ for 1.5 h to form a precursor. The precursor particle size is 19-22 μm. Post-treatment and sintering: The precursor was washed with anhydrous ethanol to remove the upper oil phase until no oil film remained on the precursor after washing. It was then vacuum dried at 75℃ for 9 hours. Nitrogen gas (300 mL / min) was applied, and the temperature was increased to 350℃ at 5℃ / min and held for 2 hours. The temperature was then increased to 800℃ at 3℃ / min and held for 4 hours. The temperature was then allowed to cool naturally to room temperature to obtain spherical magnesium ion-doped iron pyrophosphate sodium.

[0059] The magnesium ion-doped sodium iron pyrophosphate in this embodiment has the chemical formula Na₄Fe₂O₃. 2.88 Mg 0.12 The magnesium ion-doped sodium iron pyrophosphate obtained from (PO4)2P2O7 was analyzed, and the results are as follows: Loose bulk density: 1.57 g / cm³; tapped density: 2.92 g / cm³; Mg element distribution deviation: 4.5%; The aforementioned magnesium ion-doped sodium iron phosphate pyrophosphate material was used as the positive electrode material for sodium-ion batteries, assembled into button cells, and the battery performance was tested. The button cell manufacturing method is the same as in Example 1, and will not be repeated here.

[0060] Figure 7 The charge-discharge curves and electrochemical performance of the button cell assembled in this embodiment are as follows: the initial charge capacity at 0.1C is 117.57 mAh / g, the initial discharge capacity is 107.21 mAh / g, the initial charge-discharge efficiency is 91.19%, and the retention rate after 500 cycles is 94.8%.

[0061] Example 5 This embodiment provides a magnesium ion-doped sodium iron pyrophosphate and its preparation method, the preparation method including: Preparation of the oil phase system: 100 mL of liquid paraffin was mixed with 0.6 g of Span 80 and stirred at 62 °C and 650 r / min for 17 min until the oil phase was homogeneous; Preparation of aqueous phase A: Weigh 1.01g sodium nitrate, 0.82g sodium acetate, 2.02g ferric nitrate, 1.62g ferric chloride, and 1.56g phosphoric acid, dissolve them in 29mL deionized water to a mass concentration of 29%; adjust the pH to 3.2 with dilute nitric acid, stir for 33min, and aqueous phase A becomes clear; Preparation of water-in-oil emulsion: oil-water ratio 1:3.5, dropwise addition for 28 min, stirring at 62℃ and 650 r / min for 11 min, emulsion droplet diameter 17~19 μm, span 0.27; Preparation of aqueous phase B: 0.08 g magnesium chloride, 0.69 g ferrous sulfate + 0.71 g ferrous chloride, 0.85 g sodium dihydrogen phosphate, 19 mL water, 0.32 g polyvinylpyrrolidone; ball milling (ball-to-material ratio 11:1, rotation speed 320 r / min, total ball milling time 0.9 h), particle size 0.9~1.1 μm; Interfacial polymerization filling: Aqueous phase A and aqueous phase B were added dropwise for 19 min, stirred for 34 min, and 0.52 g of glutaraldehyde (approximately 0.89% of the total mass of the aqueous phase) was added. The mixture was kept at 62℃ for 2.1 h to form a precursor with a particle size of 18~21 μm. Post-treatment and sintering: The precursor was washed with anhydrous ethanol to remove the upper oil phase until no oil film remained on the precursor after washing. It was then vacuum dried at 72℃ for 10h. Nitrogen gas (260mL / min) was applied, and the temperature was increased to 340℃ at 4.5℃ / min and held for 2.2h. The temperature was then increased to 770℃ at 2.2℃ / min and held for 5.5h. The temperature was then allowed to cool naturally to room temperature to obtain spherical magnesium ion-doped iron pyrophosphate sodium.

[0062] The magnesium ion-doped sodium iron pyrophosphate in this embodiment has the chemical formula Na₄Fe₂O₃. 2.91 Mg 0.09 The magnesium ion-doped sodium iron pyrophosphate obtained from (PO4)2P2O7 was analyzed, and the results are as follows: Loose bulk density: 1.54 g / cm³; tapped density: 2.85 g / cm³; Mg distribution deviation: 3.6%; The aforementioned magnesium ion-doped sodium iron phosphate pyrophosphate material was used as the positive electrode material for sodium-ion batteries, assembled into button cells, and the battery performance was tested. The button cell manufacturing method is the same as in Example 1, and will not be repeated here.

[0063] Figure 8The charge-discharge curves and electrochemical performance of the button cell assembled in this embodiment are as follows: the initial charge capacity at 0.1C is 118.27 mAh / g, the initial discharge capacity is 107.34 mAh / g, the initial charge-discharge efficiency is 90.76%, and the retention rate after 500 cycles is 95.1%.

[0064] The suboptimal solution is now presented as a comparative example.

[0065] Comparative Example 1 A certain amount of iron, magnesium, sodium, and carbon sources were weighed as raw materials, and 3.0 g of polyvinylpyrrolidone was added as a dispersant. The mixture was then mixed with an appropriate amount of deionized water to obtain a slurry with a solid content of 35 w%–45 w%. The slurry was transferred to a grinding medium and milled at 2500–3000 r / min for 6–8 hours. After milling, the slurry was transferred to a spray drying device for spray granulation. The inlet temperature was maintained at 200–230℃, the outlet temperature at 95–105℃, and the feed rate at 250–300 ml / h to obtain a precursor. The precursor was then sintered in a tube furnace with nitrogen or argon as the protective gas for 16–22 hours to obtain the sintered finished product, which was then tested.

[0066] Test data: Loose bulk density 1.04 g / cm³, tapped density 1.55 g / cm³; Mg distribution deviation 13%; The aforementioned magnesium ion-doped sodium iron phosphate pyrophosphate material was used as the positive electrode material for sodium-ion batteries, assembled into button cells, and the battery performance was tested. The button cell manufacturing method is the same as in Example 1, and will not be repeated here.

[0067] Figure 9 The charge-discharge curves and electrochemical performance of the button cell assembled for this comparative example are as follows: initial charge specific capacity is 111.04 mAh / g, initial discharge specific capacity is 95.38 mAh / g, initial charge-discharge efficiency is 85.89%, and retention rate after 500 cycles is 80.75%.

[0068] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a magnesium ion-doped pyrophosphoryl ferric sodium phosphate, characterized by, The preparation method comprises the following steps: S1, preparing an oil phase system: mixing liquid paraffin and Span 80, heating and stirring to obtain an oil phase; S2, preparing water phase A: adding part of Na source, part of P source and Fe3+ + source into water, preparing slurry, adjusting pH to acidity to obtain water phase A; S3, preparing a water-in-oil emulsion: dropping the water phase A into the oil phase and stirring to form a water-in-oil emulsion; S4, Preparation of aqueous phase B: dispersing Mg source, Fe source, remaining Na source, remaining P source in water, adding polyvinylpyrrolidone, ball milling to obtain aqueous phase B; 2+ S4, Preparation of aqueous phase B: dispersing Mg source, Fe source, remaining Na source, remaining P source in water, adding polyvinylpyrrolidone, ball milling to obtain aqueous phase B; S5, interfacial polymerization filling: dropping the water phase B into the water-in-oil emulsion, stirring, adding glutaraldehyde, and incubating at 55-65°C to form a precursor; S6, post-treatment and sintering: after the precursor is demulsified, washed, and dried, it is placed in an inert atmosphere, incubated at 300-350°C, then incubated at 750-800°C, and naturally cooled to room temperature to obtain magnesium ion-doped pyrophosphate sodium ferrite.

2. The production method according to claim 1, characterized by, In S1, in the oil phase system, the mass ratio of Span 80 to the volume of liquid paraffin is 0.3-0.7 g:100 mL.

3. The preparation method according to claim 1, characterized in that, In S1, heating to 55-65°C and stirring.

4. The method of claim 1, wherein, In S2, adjusting the pH to 2.5-3.

5.

5. The preparation method according to claim 1, characterized in that, In S3, the volume ratio of the oil phase to the water phase A is controlled to be 1:2-1:

4.

6. The method of claim 1, wherein, In S3, the droplet diameter of the water-in-oil emulsion is controlled to be 15-20 μm.

7. The preparation method according to claim 1, characterized in that, In S4, ball milling to a particle size of 0.8-1.2 μm to obtain the water phase B.

8. The method of claim 1, wherein, In S5, the volume ratio of the water phase A to the water phase B is controlled to be 2:1-3:

1.

9. A magnesium ion-doped pyrophosphorylated sodium ferric phosphate characterized in that, The magnesium ion doped sodium pyrophosphate ferric phosphate is prepared by the preparation method in any one of claims 1 to 8, and the chemical formula of the magnesium ion doped sodium pyrophosphate ferric phosphate is Na4Fe 3-x Mg x (PO4)2P2O7, wherein x = 0.06 ~ 0.

12.

10. A sodium-ion battery, characterized in that, The sodium ion battery comprises a positive electrode, and the positive electrode comprises the magnesium ion-doped pyrophosphate sodium ferrite prepared by the preparation method of any one of claims 1-8 or the magnesium ion-doped pyrophosphate sodium ferrite of claim 9.

Images