Magnesium-doped sodium fluorinated iron phosphate materials with sodium phosphate and carbon layer double coating, their preparation methods and applications

Abstract

This invention discloses a magnesium-doped sodium fluorinated iron phosphate material with a double coating of sodium phosphate and carbon layer, its preparation method, and its application. The magnesium-doped sodium fluorinated iron phosphate material uses Na₂Fe₂O₃ as the substrate. x Mg y The core is composed of PO4F, coated with a sodium phosphate layer, and an outermost conductive carbon layer. Fe has a +2 valence, 0.05 ≤ y ≤ 0.10, and x + y = 1. This invention achieves surface phosphate enrichment through a single ball milling process by adding excess phosphorus and sodium sources. Molten sodium phosphate is then pre-sintered to coat the pre-sintered particles, improving the material's cycle performance. High-temperature sintering achieves carbon coating, enhancing electronic conductivity. This single ball milling and sintering process forms a uniform sodium phosphate and carbon double coating on the material surface, reducing interfacial side reactions between the material and the electrolyte and improving problems such as low discharge capacity and poor rate performance caused by the material's low intrinsic conductivity.

Description

Technical Field

[0001] This invention belongs to the technical field of sodium-ion battery cathode materials, and relates to a sodium fluorophosphate material and its preparation method, specifically to a magnesium-doped sodium fluorophosphate material with sodium phosphate and carbon layer double coating, its preparation method, and its application in sodium-ion batteries. Background Technology

[0002] With the depletion of traditional fossil fuels, various new renewable energy sources have broad application prospects. Lithium-ion batteries, due to their high specific energy and long cycle life, have been widely used in production and daily life; however, lithium resources suffer from uneven distribution and mining difficulties. Sodium-ion batteries, as an emerging rechargeable battery, possess advantages such as abundant resources, low price, and good safety. Their key technologies are not significantly different from lithium-ion batteries, providing a solid technological foundation. Furthermore, sodium-ion batteries can achieve wide-temperature-range discharge and rapid charge-discharge; therefore, they are becoming a primary choice for large-scale energy storage power stations with relatively lower energy density requirements.

[0003] Currently, common sodium-ion battery cathode materials still face many challenges, such as low energy density and poor cycle performance. Layered iron-based fluorophosphate (Na2FePO4F) is a potential candidate cathode material with a two-dimensional sodium-ion transport channel, exhibiting a narrow and stable charge-discharge curve, lower battery management difficulty, and suitability for large-scale grid energy storage. However, it still suffers from low intrinsic conductivity, poor rate capability, low energy density, and poor cycle stability, limiting its large-scale production and commercial application. Doping with weakly electronegative magnesium can induce charge redistribution, adjust the electronic structure, and increase conductivity. Carbon coating can improve the electronic conductivity and rate performance of the material, but simple carbon coating provides insufficient protection and cannot prevent side reactions between the matrix material and the electrolyte. Multilayer coating can achieve multiple effects, leading to significant performance improvements. Common cathode coating materials include oxides, fluorides, phosphates, and elemental metals. Sodium phosphate, as a good cathode coating layer, can significantly improve material performance. Summary of the Invention

[0004] To address the issues of poor rate performance and insufficient cycle life of current sodium fluorophosphate materials, this invention provides a magnesium-doped sodium fluorophosphate material with a double coating of sodium phosphate and a carbon layer, along with its preparation method and applications. This invention prepares a magnesium-doped sodium fluorophosphate core with a Na₂Fe core. x Mg yPO4F, where Fe has a +2 valence, 0.05≤y≤0.10, and x+y=1, adjusts the electronic structure, lowers the sodium ion migration barrier, and improves the intrinsic conductivity of the material. The spontaneously formed sodium phosphate coating on the surface helps to isolate the electrolyte, improving ionic conductivity and cycle performance. The outermost carbon layer is beneficial to improving the electronic conductivity of the material and improving its rate performance. The combination of these three elements, achieved through a one-step ball milling and sintering process, is simple and has broad application prospects.

[0005] The objective of this invention is achieved through the following technical solution:

[0006] A magnesium-doped sodium fluorinated iron phosphate material with a double coating of sodium phosphate and carbon layer, using Na2Fe x Mg y PO4F forms the core, which is covered by a sodium phosphate layer. The outermost layer is a conductive carbon layer, in which Fe is in the +2 valence, 0.05≤y≤0.10, and x+y=1.

[0007] A method for preparing the above-mentioned magnesium-doped sodium fluorophosphate material with a double coating of sodium phosphate and carbon layer, which achieves the preparation of the double-coated magnesium-doped sodium fluorophosphate material in one step, specifically includes the following steps:

[0008] Step 1: Mixing materials

[0009] Sodium, iron, magnesium, phosphorus, fluorine, and carbon sources are mixed in a certain proportion to obtain a solid-phase raw material mixture, wherein:

[0010] The element ratio in the solid raw material mixture is Na:Fe:Mg:P:F = 2-2.5: 0.85-0.99: 0.01-0.15: 1-1.5: 1-1.5, and the carbon source accounts for 10-20% of the total mass of the sodium, iron, magnesium, phosphorus and fluorine sources.

[0011] The sodium source is at least one of sodium fluoride, sodium acetate, sodium carbonate, sodium bicarbonate, sodium citrate, sodium oxalate, sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium isopropoxide, sodium carboxymethyl cellulose, sodium oleate, sodium ascorbate, and sodium alginate.

[0012] The iron source is at least one of ferrous oxalate, ferrous sulfate, ferrous chloride, ferric phosphate, ferric oleate, ferric citrate, ferric tartrate, and sodium ferric ethylenediaminetetraacetate.

[0013] The magnesium source is at least one of magnesium oxide, magnesium carbonate, magnesium chloride, magnesium phosphate, magnesium oxalate, magnesium gluconate, magnesium acetate, magnesium citrate, and magnesium methoxide.

[0014] The phosphorus source is at least one of phosphoric acid, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate.

[0015] The fluorine source is at least one of sodium fluoride and hydrofluoric acid;

[0016] The carbon source is at least one of glucose, sucrose, ascorbic acid, graphite, carbon nanotubes, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, sodium carboxymethyl cellulose, phytic acid, stearic acid, and urea.

[0017] Step 2, Mechanical ball milling:

[0018] The solid raw material mixture from step one is added to a ball mill along with milling beads for ball milling, and the milled product is then vacuum dried, wherein:

[0019] The mass ratio of the milling beads to the solid raw material mixture is 8 to 10:1;

[0020] The ball milling process may or may not involve the addition of a liquid medium, and the medium may include one or more of water, ethanol, oleic acid, and N-methylpyrrolidone.

[0021] The ball mill operates at a speed of 300–500 rpm for 6–8 hours, and the material temperature is controlled to be below 40°C.

[0022] The vacuum drying temperature is 60-100℃, and the time is 8-14 hours. The drying time is adjusted according to the amount of material. The target material is required to be dried and appear white or light yellow overall.

[0023] Step 3, Sintering:

[0024] Under inert gas protection, the material undergoes low-temperature pre-sintering, followed by secondary sintering in the same furnace at a higher temperature. This process ultimately yields a magnesium-doped sodium fluorinated phosphate material with a double coating of carbon and sodium phosphate layers.

[0025] The low-temperature pre-sintering process is as follows: first, a protective gas is pre-circulated at room temperature for 0.5 to 2 hours, then the temperature is raised to 350 to 450°C, and calcined at this temperature for 2 to 6 hours, with the heating rate controlled at 3 to 5°C / min;

[0026] The calcination temperature for the secondary sintering is 550–650℃, the calcination time is 8–12 h, and the heating rate is controlled at 3–5℃ / min.

[0027] The entire sintering process, including the low-temperature pre-sintering and secondary sintering, is carried out under a protective atmosphere.

[0028] The inert gas is one or more of argon, hydrogen, nitrogen, and helium.

[0029] Compared with the prior art, the present invention has the following advantages:

[0030] 1. This invention improves the electronic conductivity of phosphate materials by adding magnesium doping and sodium phosphate coating to sodium fluorophosphate (Na2FePO4F) materials, while also enhancing the structural stability of the materials. This results in a cathode active material with both high energy density and cycle stability. Specifically, magnesium doping effectively increases the structural strength of the material, modulates the ion transport barrier, and can anchor between crystal lattice layers to widen the lattice size, thereby increasing the sodium ion transport rate. The sodium phosphate coating layer helps suppress surface precipitation, enhances ionic conductivity, and improves the cycle stability of the material.

[0031] 2. This invention achieves surface phosphate enrichment through a single ball milling process by adding excessive amounts of phosphorus and sodium sources. Molten sodium phosphate is then generated by pre-sintering and coated onto the surface of the pre-sintered particles, improving the material's cycle performance. High-temperature sintering achieves carbon coating, which enhances electronic conductivity. This single ball milling and sintering process forms a uniform sodium phosphate and carbon double coating layer on the material surface, reducing interfacial side reactions between the material and the electrolyte and improving problems such as low discharge capacity and poor rate performance caused by the material's low intrinsic conductivity.

[0032] 3. This invention achieves double-layer coating in one step by adjusting the composition and calcination process during ball milling. The coating is denser and works synergistically with magnesium doping, improving the stability of the cathode structure. The preparation process is simple and suitable for mass production. Attached Figure Description

[0033] Figure 1 This is a schematic diagram of the magnesium-doped sodium fluoride iron phosphate structure with sodium phosphate and carbon layer double coating prepared in Example 1.

[0034] Figure 2 SEM image of magnesium-doped sodium fluorinated iron phosphate with sodium phosphate and carbon layer double coating prepared in Example 1;

[0035] Figure 3 The graph shows a comparison of the cycling curves of the magnesium-doped sodium fluorophosphate material with sodium phosphate and carbon layer double coating prepared in Example 1, and the undoped sodium fluorophosphate and uncoated sodium fluorophosphate.

[0036] Figure 4 This is a comparison of the charge-discharge curves of magnesium-doped sodium fluorinated iron phosphate material with sodium phosphate and carbon layer double coating prepared in Example 1 and undoped and uncoated sodium fluorinated iron phosphate. Detailed Implementation

[0037] The technical solution of the present invention will be further described below with reference to the embodiments, but it is not limited thereto. Any modifications or equivalent substitutions to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention should be covered within the protection scope of the present invention.

[0038] Example 1:

[0039] This embodiment provides a magnesium-doped sodium fluorinated iron phosphate material with a double coating of sodium phosphate and a carbon layer, such as... Figure 1 As shown, the material consists of three layers, with the inner layer being Na2Fe. 0.95 Mg 0.05 PO4F consists of an intermediate sodium phosphate layer and a conductive carbon layer on the surface, where Fe has a +2 valence, 0.05 ≤ y ≤ 0.10, and x + y = 1. The preparation method includes the following steps:

[0040] 1) Mix sodium oleate, ferrous oxalate, magnesium citrate, diammonium hydrogen phosphate and sodium fluoride in a ratio of Na:Fe:Mg:P:F = 2.3:0.95:0.05:1.1:1; weigh the mixture and add 5% by mass of ascorbic acid and 5% by mass of stearic acid.

[0041] 2) The mixture was ball-milled in N-methylpyrrolidone medium at a speed of 450 rpm for 8 h, and then vacuum-dried at 90 °C for 12 h.

[0042] 3) After vacuum drying, stepwise sintering is carried out under the protection of hydrogen and argon atmosphere: first, the protective gas is pre-purified at room temperature for 1 hour, then the temperature is raised to 350℃ at a heating rate of 5℃ / min, and calcined at this temperature for 3 hours, then the temperature is raised to 650℃ at a heating rate of 5℃ / min, and calcined at this temperature for 12 hours, finally obtaining a Mg-doped sodium fluorinated phosphate material with uniform carbon coating and carbon double coating.

[0043] Example 2:

[0044] This embodiment provides a magnesium-doped sodium fluorinated iron phosphate material with a double coating of sodium phosphate and a carbon layer, such as... Figure 1 As shown, the material consists of three layers, with the inner layer being Na2Fe. 0.98 Mg 0.02 PO4F consists of an intermediate sodium phosphate layer and a conductive carbon layer on the surface, where Fe has a +2 valence, 0.05 ≤ y ≤ 0.10, and x + y = 1. The preparation method includes the following steps:

[0045] 1) Mix sodium citrate, ferric phosphate, magnesium acetate, diammonium hydrogen phosphate and sodium fluoride in a ratio of Na:Fe:Mg:P:F = 2.12:0.98:0.02:1.04:1; weigh the mixture and add 5% by mass of ascorbic acid and 10% by mass of phytic acid.

[0046] 2) The mixture was ball-milled in N-methylpyrrolidone medium at a speed of 450 rpm for 8 h, and then vacuum-dried at 90 °C for 12 h.

[0047] 3) After vacuum drying, stepwise sintering is carried out under the protection of argon atmosphere: first, the protective gas is pre-purified at room temperature for 1 hour, then the temperature is raised to 400℃ at a heating rate of 3℃ / min, and calcined at this temperature for 3 hours, then the temperature is raised to 600℃ at a heating rate of 3℃ / min, and calcined at this temperature for 12 hours, finally obtaining a Mg-doped sodium fluorinated phosphate material with uniform carbon coating and carbon double coating.

[0048] Example 3:

[0049] This embodiment provides a magnesium-doped sodium fluorinated iron phosphate material with a double coating of sodium phosphate and a carbon layer, such as... Figure 1 As shown, the material consists of three layers, with the inner layer being Na2Fe. 0.98 Mg 0.02 PO4F consists of an intermediate sodium phosphate layer and a conductive carbon layer on the surface, where Fe has a +2 valence, 0.05 ≤ y ≤ 0.10, and x + y = 1. The preparation method includes the following steps:

[0050] 1) Mix sodium phosphate, ferric tartrate, magnesium gluconate, diammonium hydrogen phosphate and sodium fluoride in a ratio of Na:Fe:Mg:P:F = 2.12:0.98:0.02:1.04:1; weigh the mixture and add 5% by mass of ascorbic acid and 10% by mass of polyvinyl alcohol.

[0051] 2) The mixture was ball-milled in N-methylpyrrolidone medium at a speed of 450 rpm for 8 h, and then vacuum-dried at 90 °C for 12 h.

[0052] 3) After vacuum drying, stepwise sintering is carried out under nitrogen atmosphere protection: first, protective gas is pre-purified at room temperature for 1 hour, then the temperature is raised to 450℃ at a heating rate of 5℃ / min, and calcined at this temperature for 3 hours, then the temperature is raised to 650℃ at a heating rate of 5℃ / min, and calcined at this temperature for 12 hours, finally obtaining a Mg-doped sodium fluorinated phosphate material with uniform carbon coating and carbon double coating.

[0053] Comparative Example 1:

[0054] The difference between this comparative example and Example 1 is that magnesium doping was not performed.

[0055] Comparative Example 2:

[0056] The difference between this comparative example and Example 1 is that the phosphorus and sodium in the raw materials are in the normal stoichiometric ratio, that is, sodium phosphate coating is not performed.

[0057] Comparative Example 3:

[0058] The difference between this comparative example and Example 1 is that the phosphorus and sodium in the raw materials are in the normal stoichiometric ratio, that is, no sodium phosphate coating is performed, and no magnesium is added.

[0059] Analysis was conducted through testing and analysis of the product's morphology and battery cycle performance.

[0060] (1) SEM images of magnesium-doped sodium fluorinated iron phosphate material with carbon and sodium phosphate double coating.

[0061] SEM is the most fundamental testing method for observing the microstructure of materials. This invention uses a ZEISS GEMINI 500 scanning electron microscope (Germany) to characterize the surface morphology of the sample prepared in Example 1. To improve the material's electrical conductivity and obtain clearer scanning images, the material needs to be sputter-coated with gold before testing. The SEM images of the material are shown below. Figure 2 As shown, by Figure 2 It can be seen that the obtained material is a spherical particle with a diameter of 2-3 μm and a highly fluffy special morphology.

[0062] (2) Electrochemical performance testing of magnesium-doped sodium fluorinated iron phosphate material with carbon and sodium phosphate double coating

[0063] To test the electrochemical performance of the material, a half-cell was assembled using this material as the positive electrode. First, 200 mg of the material prepared in Example 1 and 25 mg of SuperP were weighed and ground in an agate mortar for approximately 20 minutes until thoroughly mixed. The mixture was then transferred to a paste bottle, and 500 mg of a pre-prepared 5 wt% PVDF NMP solution was added, followed by approximately 800 μL of NMP solution. The mixture was magnetically stirred for approximately 12 hours until a slightly viscous paste-like liquid was formed. Subsequently, a 120 μm spatula was used to evenly coat the paste onto an aluminum foil, and the mixture was vacuum dried at 120 °C for 12 hours. The resulting positive electrode was then formed into a 12 mm diameter positive electrode using a die-cutting machine. Using this electrode as the positive electrode, a sodium sheet as the negative electrode, and glass fiber as the separator, a 1.0 M NaClO4 / EC-PC electrolyte (volume ratio 1:1, 5.0 Vol% FEC) was selected. A CR2032 coin cell was assembled in an argon-filled glove box. The coin cell battery was subjected to constant current charge-discharge at 0.2C (1C = 120 mAh / g), with a voltage range of 2.0–4.0V. A comparison of its cycling curves with those of undoped sodium fluorophosphate and uncoated sodium fluorophosphate is shown in the figure below. Figure 3 As shown in the figure, the charge-discharge curves of sodium fluorophosphate are compared with those of undoped and uncoated sodium fluorophosphate. Figure 4 As shown. By Figure 3 It can be concluded that both magnesium doping and sodium phosphate coating are beneficial to improving cycle performance. Figure 4 It can be concluded that magnesium doping and sodium phosphate coating are beneficial to the significant improvement of the material's specific capacity.

[0064] Table 1. Results of the charge-discharge experiment

[0065]

[0066] As shown in Table 1, compared to no magnesium doping (Comparative Example 1), no sodium phosphate coating (Comparative Example 2), and both no magnesium doping and sodium phosphate coating (Comparative Example 3), this invention improves the electronic conductivity of the phosphate material by adding magnesium doping to the sodium fluorophosphate material, while also enhancing the structural stability of the material. This results in a cathode active material with both high energy density and cycle stability. The excess phosphate achieves sodium phosphate coating in one step during synthesis, forming a stable interface layer on the material surface, reducing interfacial side reactions and improving cycle stability. The second step of carbon coating increases surface electronic conductivity, thereby improving the rate performance of the material.

Claims

1. A magnesium-doped sodium fluorinated iron phosphate material with a double coating of sodium phosphate and a carbon layer, characterized in that... The magnesium-doped sodium fluoride phosphate material uses Na2Fe x Mg y PO4F forms the core, which is covered by a sodium phosphate layer. The outermost layer is a conductive carbon layer, in which Fe is in the +2 valence, 0.05≤y≤0.10, and x+y=1.

2. A method for preparing the magnesium-doped sodium fluorinated phosphate material with a double coating of sodium phosphate and carbon layer as described in claim 1, characterized in that... The method includes the following steps: Step 1: Mixing materials Sodium, iron, magnesium, phosphorus, fluorine, and carbon sources are mixed in a certain proportion to obtain a solid raw material mixture, wherein: the element ratio in the solid raw material mixture is Na:Fe:Mg:P:F = 2~2.5:0.85~0.99:0.01~0.15:1~1.5:1~1.5, and the carbon source accounts for 10~20% of the total mass of sodium, iron, magnesium, phosphorus, and fluorine sources; Step 2, Mechanical ball milling: The solid raw material mixture from step one is added to a ball mill along with the ball milling beads for ball milling, and the ball milling product is then vacuum dried. Step 3, Sintering: Under inert gas protection, the material is first pre-sintered at low temperature, and then sintered again in the same furnace at a higher temperature to finally obtain magnesium-doped sodium fluorinated phosphate material with a double coating of carbon layer and sodium phosphate layer.

3. The preparation method of magnesium-doped sodium fluorinated phosphate material with sodium phosphate and carbon layer double coating according to claim 2, characterized in that... The sodium source is at least one of sodium fluoride, sodium acetate, sodium carbonate, sodium bicarbonate, sodium citrate, sodium oxalate, sodium phosphate, sodium dihydrogen phosphate, disodium hydrogen phosphate, sodium isopropoxide, sodium carboxymethyl cellulose, sodium oleate, sodium ascorbate, and sodium alginate; the iron source is at least one of ferrous oxalate, ferrous sulfate, ferrous chloride, ferric phosphate, ferric oleate, ferric citrate, ferric tartrate, and sodium ferric ethylenediaminetetraacetate; the magnesium source is at least one of magnesium oxide, magnesium carbonate, magnesium chloride, magnesium phosphate, magnesium oxalate, magnesium gluconate, magnesium acetate, magnesium citrate, and magnesium methoxide; the phosphorus source is at least one of phosphoric acid, sodium phosphate, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, sodium dihydrogen phosphate, and disodium hydrogen phosphate; the fluorine source is at least one of sodium fluoride and hydrofluoric acid; and the carbon source is at least one of glucose, sucrose, ascorbic acid, graphite, carbon nanotubes, polyethylene glycol, polyvinyl alcohol, polyvinylpyrrolidone, sodium carboxymethyl cellulose, phytic acid, stearic acid, and urea.

4. The preparation method of magnesium-doped sodium fluorinated iron phosphate material with double coating of sodium phosphate and carbon layer according to claim 2, characterized in that... The mass ratio of the grinding balls to the solid raw material mixture is 8 to 10:

1.

5. The preparation method of magnesium-doped sodium fluorinated iron phosphate material with sodium phosphate and carbon layer double coating according to claim 2, characterized in that... During ball milling, a liquid medium is added, which is one or more of water, ethanol, oleic acid, and N-methylpyrrolidone.

6. The preparation method of magnesium-doped sodium fluorinated iron phosphate material with double coating of sodium phosphate and carbon layer according to claim 2, characterized in that... The ball mill operates at a speed of 300–500 rpm for 6–8 hours.

7. The preparation method of magnesium-doped sodium fluorinated iron phosphate material with double coating of sodium phosphate and carbon layer according to claim 2, characterized in that... The vacuum drying temperature is 60–100°C, and the time is 8–14 hours.

8. The preparation method of magnesium-doped sodium fluorinated phosphate material with double coating of sodium phosphate and carbon layer according to claim 2, characterized in that... The low-temperature pre-sintering process is as follows: first, a protective gas is pre-circulated at room temperature for 0.5 to 2 hours, then the temperature is raised to 350 to 450°C, and calcined at this temperature for 2 to 6 hours, with the heating rate controlled at 3 to 5°C / min; the calcination temperature for the secondary sintering is 550 to 650°C, the calcination time is 8 to 12 hours, and the heating rate is controlled at 3 to 5°C / min.

9. The preparation method of magnesium-doped sodium fluorinated phosphate material with double coating of sodium phosphate and carbon layer according to claim 2, characterized in that... The inert gas is one or more of argon, hydrogen, nitrogen, and helium.

10. The application of the magnesium-doped sodium fluorinated iron phosphate material with sodium phosphate and carbon layer double coating as described in claim 1 in sodium-ion batteries.

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