Carbon-coated multi-element doped sodium iron pyrophosphate and a preparation method thereof

Abstract

The application provides a carbon-coated multi-element doped sodium pyrophosphate ferric phosphate and a preparation method thereof, and comprises the following steps: S1. A pH regulator is added dropwise into a mixed solution of ferrous sulfate, sulfuric acid, a phosphorus source and a metal element additive, and a suspension obtained by reaction is filtered, rinsed and pressed to obtain a multi-element doped ferrous phosphate octahydrate filter cake; S2. A sodium source, a phosphorus source and an organic solvent are added into the ferrous phosphate octahydrate filter cake to perform slurry, and grinding is performed to obtain a grinding slurry; S3. The grinding slurry is subjected to freeze-drying, and a solid mixture is separated out; S4. A carbon source and pure water are added into the solid mixture, and stirring is uniformly performed to obtain a mixed slurry; S5. The mixed slurry is subjected to spray drying to obtain hollow spherical solid particles; and S6. The hollow spherical solid particles are subjected to high-temperature calcination to obtain the carbon-coated multi-element doped sodium pyrophosphate ferric phosphate. The method has low raw material cost, can realize in-situ doping, has small product particle size, and has good electrical performance and rate performance.

Description

Technical Field

[0001] This invention relates to the field of sodium ion cathode material preparation technology, specifically to a carbon-coated multi-element doped sodium iron pyrophosphate and its preparation method. Background Technology

[0002] Currently, the sodium-ion battery industry is in the early stage of achieving breakthroughs in both technology and industrialization. As a component of the battery, the mainstream sodium-ion battery cathode material technologies mainly include layered oxide systems, polyanionic systems, and Prussian systems. However, the Prussian system faces difficulties in overcoming the crystal water problem, making mass production challenging. Layered oxide systems suffer from poor consistency, poor cycle life, and high residual alkali content, hindering the commercialization of these materials. Polyanionic compounds, in particular, have attracted widespread attention due to their high theoretical specific capacity, good cycle life, and abundant raw material reserves, especially the iron-based mixed polyanionic compound Na4Fe3(PO4)2P2O7.

[0003] Currently, there are two methods for preparing Na4Fe3(PO4)2P2O7: liquid-phase and solid-phase. The liquid-phase method uses soluble raw materials that are mixed evenly and then spray-dried and calcined to prepare Na4Fe3(PO4)2P2O7. This method produces products with many impurities and poor electrical properties. The solid-phase method uses non-soluble raw materials that are ground, dried, and calcined. Chinese patent application CN 116487545A discloses a method for preparing a carbon-composite pyrophosphate sodium iron phosphate composite material. This method uses solid raw materials to prepare Na4Fe3(PO4)2P2O7 through grinding and calcination. The raw materials used in this process often contain sulfate, carbonate, ammonium, oxalate, acetate, and other anions and cations. During calcination, it easily generates waste gas containing elements such as sulfur and nitrogen, causing serious environmental pollution. Application CN 116621148... Chinese patent A discloses a method for preparing metal-based phosphate cathode materials for sodium-ion batteries. It uses non-water-soluble metal salts as iron sources, including Fe2O3, FePO4, FeC2O4, and / or Fe(OH)3. The metal-based phosphate cathode material is obtained through grinding, drying, and calcination. However, these raw materials are mostly dried or calcined at high temperatures, resulting in high raw material costs. Furthermore, the primary particles of the calcined raw materials are often molten, making them difficult to break down effectively during grinding. This leads to excessively long grinding times, high energy consumption, and poor grinding results, affecting the final performance of Na4Fe3(PO4)2P2O7. Chinese patent application CN 114613998 A discloses a carbon-coated sodium-ion battery cathode material, Na4Fe 3-x M x(PO4)2P2O7 / C and its preparation method: This method directly obtains transition metal element M-doped composite sodium iron phosphate by solid-state sintering. However, this method struggles to achieve uniform dispersion of the M source, resulting in the product Na4Fe. 3-x M x (PO4)2P2O7 / C has poor electrical properties. Summary of the Invention

[0004] To address the problems existing in the background technology, the present invention provides a carbon-coated multi-element doped sodium iron pyrophosphate and its preparation method. This method has the advantages of low raw material cost, in-situ doping capability, small product particle size, good electrical and rate performance, low energy consumption and no waste gas emission during the preparation process.

[0005] The technical solution of the present invention to solve the above-mentioned technical problems is as follows:

[0006] In a first aspect, the present invention provides a method for preparing carbon-coated multi-element doped sodium iron pyrophosphate, comprising the following steps:

[0007] S1. Precursor preparation: A pH adjuster is added dropwise to a mixed solution of ferrous sulfate, sulfuric acid, phosphorus source and metal element additives. The resulting suspension is filtered, washed and pressed to obtain a multi-element doped ferrous phosphate octahydrate filter cake.

[0008] S2. Grinding: Under a protective atmosphere, sodium source, phosphorus source and organic solvent are added to the ferrous phosphate octahydrate filter cake obtained in step S1 for slurrying. After stirring evenly, the mixture is ground to obtain a grinding slurry.

[0009] S3. Primary drying: The grinding slurry obtained in step S2 is freeze-dried to separate the solid mixture;

[0010] S4. Mixing: Add carbon source and pure water to the solid mixture and stir until homogeneous to obtain a mixed slurry;

[0011] S5. Secondary drying: The mixed slurry is spray-dried to obtain hollow spherical solid particles;

[0012] S6. Calcination: The hollow spherical solid particles are calcined at high temperature to obtain carbon-coated multi-element doped sodium iron pyrophosphate.

[0013] Furthermore, the multi-element doped ferrous phosphate octahydrate mentioned in step S1 is titanium, magnesium, and manganese doped ferrous phosphate octahydrate.

[0014] In step S1, titanium, magnesium and manganese elements are in situ doped into ferrous phosphate octahydrate by liquid-phase co-precipitation. In step S2, grinding is used to dehydrate the ferrous phosphate octahydrate doped with titanium, magnesium and manganese elements in situ and mix it evenly with sodium source, phosphorus source and organic solvent. After two-stage drying and coating with carbon source, carbon-coated multi-element doped iron pyrophosphate sodium is obtained.

[0015] In some specific embodiments, the ferrous sulfate is obtained by removing impurities from crude ferrous sulfate, a byproduct of titanium dioxide production, wherein the purity of the ferrous sulfate is ≥99%, the magnesium content is 1800-2200 ppm, the manganese content is 600-1000 ppm, and the metal element additive is a titanium element additive.

[0016] Furthermore, crude ferrous sulfate, a byproduct of titanium dioxide production, is obtained through steps such as dissolution, pH adjustment to remove impurities other than Mn and Mg, and filtration.

[0017] Because crude ferrous sulfate, a byproduct of titanium dioxide production, contains Ti and Fe... 3+ Al and a relatively high content of Mg 2+ and Mn 2+ Impurities such as Fe were removed after purification. 3+ And impurities such as insoluble Ti and Al, while retaining Mg. 2+ and Mn 2+ In the precursor preparation stage, Mg and Mn elements are co-precipitated with ferrous phosphate, which can avoid the use of additional magnesium and manganese additives and save costs.

[0018] Furthermore, the amount of titanium additive added is 0.1‰ to 2‰ of the carbon-coated multi-element doped sodium iron pyrophosphate product, more preferably 0.1‰ to 1‰.

[0019] Preferably, the titanium element additive is one or at least two of titanium oxysulfate, tetrabutyl titanate, and titanium sulfate.

[0020] In some other specific embodiments, the ferrous sulfate is of industrial grade purity, and the metal element additive is a mixture of titanium, magnesium and manganese element additives.

[0021] Preferably, the titanium element additive is one or at least two of titanium oxysulfate, tetrabutyl titanate, and titanium sulfate; the magnesium element additive is one or at least two of magnesium sulfate, magnesium nitrate, and magnesium chloride; and the manganese element additive is one or at least two of manganese sulfate, manganese nitrate, and manganese chloride.

[0022] According to the above scheme, the amount of titanium additive added is 0.1‰ to 2‰ of the carbon-coated multi-element doped sodium pyrophosphate, more preferably 0.1‰ to 1‰; the amount of magnesium additive added is 0.1‰ to 1‰ of the carbon-coated multi-element doped sodium pyrophosphate; and the amount of manganese additive added is 0.1‰ to 2‰ of the carbon-coated multi-element doped sodium pyrophosphate.

[0023] Further, the phosphorus source in step S1 is one or more of the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, phosphoric acid, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; and the pH adjuster is ammonia or sodium hydroxide. Preferably, when the cation in the phosphorus source is ammonium ion, the corresponding pH adjuster is ammonia; when the cation in the phosphorus source is sodium ion, the corresponding pH adjuster is sodium hydroxide.

[0024] According to the above scheme, the phosphorus source, pH adjuster and metal element additive in step S1 are all of industrial grade purity.

[0025] Furthermore, in step S1, the concentration of ferrous sulfate is 1-2 mol / L, the P:Fe molar ratio of the phosphorus source to ferrous sulfate is 2.9-3.1:2, more preferably 2.9-3.0:2, and a pH adjuster is added to maintain the pH value of the solution at 4-6, more preferably 4.5-5.5.

[0026] Specifically, in step S1, a phosphorus source and a metal element additive are added to the mixed solution of ferrous sulfate and sulfuric acid. The pH value of the mixed solution of ferrous sulfate and sulfuric acid is 0.8 to 1.2 to avoid oxidation of ferrous ions.

[0027] Optionally, the filtration method in step S1 is filter press filtration or centrifugal filtration.

[0028] Furthermore, the qualification standard for rinsing in step S1 is that the conductivity of the washing water is ≤500μs / cm.

[0029] Furthermore, the moisture content of the ferrous phosphate octahydrate filter cake in step S1 is ≤40%.

[0030] Furthermore, the sodium source in step S2 is one or more of sodium hydroxide, sodium monohydrogen phosphate, and sodium phosphate; the phosphorus source is one or more of phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; and the organic solvent is one or more of tert-butanol, glacial acetic acid, cyclohexane, and dimethyl sulfoxide.

[0031] Further, in step S2, the amount of sodium source added is such that the molar ratio of sodium to iron in ferrous phosphate octahydrate in the ground slurry is 4:2.9-3, the amount of phosphorus source added is such that the molar ratio of phosphorus to iron in ferrous phosphate octahydrate in the ground slurry is 2:3, the mass ratio of organic solvent to ferrous phosphate octahydrate is 1-2.5:1, and the pH value of the slurry is ≥11 after being stirred evenly.

[0032] Furthermore, in step S2, the output particle size D50 of the grinding process is 50-300 nm, and the grinding temperature is 60℃-80℃.

[0033] Furthermore, the carbon source mentioned in step S4 is one or more of glucose, sucrose, PEG2000, and white sugar, and the amount of carbon source added is 3% to 10% of the mass of the carbon-coated multi-element doped sodium iron pyrophosphate phosphate finished product.

[0034] Furthermore, in step S5, the inlet air temperature of the spray drying is 200-300°C, the outlet air temperature is 70-110°C, and the D50 particle size of the hollow spherical solid particles ranges from 1 to 20 μm, preferably from 1 to 5 μm.

[0035] Furthermore, the calcination conditions in step S6 are as follows: first, hold at 180–220℃ for 3–8 hours, then hold at 500–550℃ for 8–10 hours. Specifically, under a nitrogen atmosphere, sinter in a stepped manner, raising the temperature from room temperature to 200℃ at a rate not exceeding 5℃ / min and holding for 3–8 hours, then raising the temperature to 550℃ at a rate not exceeding 5℃ / min and holding for 8–10 hours, and finally cooling to room temperature at a rate not exceeding 5℃ / min.

[0036] Secondly, the present invention provides carbon-coated multi-element doped sodium ferric pyrophosphate prepared by the above preparation method.

[0037] The beneficial effects of this invention are:

[0038] 1) This invention achieves in-situ doping of multiple elements in the precursor octahydrate iron phosphate of composite sodium iron phosphate by liquid-phase co-precipitation, and the doping is uniform. Compared with direct mixing, the effect is better and it is beneficial to improve the conductivity of composite sodium iron phosphate.

[0039] 2) This invention prepares a composite sodium iron phosphate precursor by liquid-phase precipitation, using multi-element doped octahydrate ferrous phosphate as both a phosphorus and iron source. The bound water in the octahydrate ferrous phosphate is then removed by grinding. Compared to other iron sources such as ferric phosphate or ferric oxide, this method eliminates the need for pretreatment of the iron source (other iron sources require high-temperature drying and calcination to remove bound water), significantly reducing manufacturing costs. Furthermore, the precursor, without calcination, has smaller primary particles, fewer hard agglomerates, and better grinding performance. Since the composite sodium iron phosphate has low electronic conductivity, the smaller the grinding particle size, the better the rate performance and electrical properties of the material. Therefore, the composite sodium iron phosphate prepared by this method exhibits excellent electrical and rate performance.

[0040] 3) After the liquid-phase coprecipitation reaction of the present invention, easily decomposable anions and cations, such as ammonium ions and sulfate ions, are washed away by rinsing. No waste gas such as S / N is generated in the subsequent calcination process, and the preparation process is environmentally friendly and pollution-free.

[0041] 4) In the grinding stage, the present invention achieves the dehydration reaction of ferrous phosphate octahydrate through the dual action of mechanical energy activation and chemical reaction, thus achieving the dual effects of grinding and dehydration, reducing manufacturing energy consumption and manufacturing costs.

[0042] 5) The present invention can use ferrous phosphate, a by-product of titanium dioxide, as an iron source. Ferrous sulfate, a by-product of titanium dioxide, contains a high content of Mg and Mn elements. In the liquid phase synthesis stage, Mg and Mn elements co-precipitate with ferrous phosphate, forming a multi-element in-situ doping effect, reducing the addition of Mg and Mn additives. Attached Figure Description

[0043] Figure 1 This is a process flow diagram of the preparation method of the present invention;

[0044] Figure 2 This is a graph showing the change in slurry particle size over time during the grinding process in Embodiment 1 of the present invention.

[0045] Figure 3 This is a scanning electron microscope image of the hollow spherical particles prepared by spray drying in Example 1 of the present invention;

[0046] Figure 4 The image shows the 0.1C charge-discharge curve of the carbon-coated multi-element doped composite sodium iron phosphate prepared in Example 1 of this invention. Detailed Implementation

[0047] The principles and features of the present invention are described below with reference to the accompanying drawings and specific embodiments. The examples given are only for explaining the present invention and are not intended to limit the scope of the present invention.

[0048] To address the problems of high preparation cost, large particle size, and low electrical and rate performance of carbon-coated multi-element doped iron pyrophosphate, this invention provides a method for preparing carbon-coated multi-element doped iron pyrophosphate. The method involves reacting ferrous phosphate, a phosphorus source, and metal additives to obtain multi-element doped ferrous phosphate octahydrate. This multi-element doped ferrous phosphate octahydrate is then ground with a sodium source, a phosphorus source, and an organic solvent, followed by calcination with a carbon source to obtain carbon-coated multi-element doped iron pyrophosphate. Figure 1 As shown, it includes the following steps:

[0049] S1. Precursor preparation: A pH adjuster is added dropwise to a mixed solution of ferrous sulfate, sulfuric acid, phosphorus source and metal element additives. The resulting suspension is filtered, washed and pressed to obtain a multi-element doped ferrous phosphate octahydrate filter cake.

[0050] S2. Grinding: Under a protective atmosphere, sodium source, phosphorus source and organic solvent are added to the ferrous phosphate octahydrate filter cake obtained in step S1 for slurrying. After stirring evenly, the mixture is ground to obtain a grinding slurry.

[0051] S3. Primary drying: The grinding slurry obtained in step S2 is freeze-dried to separate the solid mixture;

[0052] S4. Mixing: Add carbon source and pure water to the solid mixture and stir until homogeneous to obtain a mixed slurry;

[0053] S5. Secondary drying: The mixed slurry is spray-dried to obtain hollow spherical solid particles;

[0054] S6. Calcination: The hollow spherical solid particles are calcined at high temperature to obtain carbon-coated multi-element doped sodium iron pyrophosphate.

[0055] Furthermore, the multi-element doped ferrous phosphate octahydrate mentioned in step S1 is a titanium, magnesium, and manganese doped ferrous phosphate octahydrate filter cake.

[0056] In some specific embodiments, the ferrous sulfate is obtained by removing impurities from crude ferrous sulfate, a byproduct of titanium dioxide production, wherein the purity of the ferrous sulfate is ≥99%, the magnesium content is 1800-2200 ppm, the manganese content is 600-1000 ppm, and the metal element additive is a titanium element additive.

[0057] The crude ferrous sulfate byproduct of titanium dioxide is obtained through steps such as dissolution, pH adjustment to remove impurities other than Mn and Mg, and filtration.

[0058] The specific steps for treating crude ferrous sulfate, a byproduct of titanium dioxide production, are as follows:

[0059] 1) Dissolve ferrous sulfate;

[0060] 2) Remove impurities with ferrous sulfate solution.

[0061] In this process, ferrous sulfate, a byproduct of titanium dioxide production via the sulfuric acid process, is used as the iron source. An appropriate amount of hot water (or steam reflux condensate) and solid ferrous sulfate are added to the ferrous sulfate dissolving tank. The temperature in the tank is maintained at approximately 55°C, and the mixture is stirred for 30 minutes until the density reaches 1.16–1.25 g / cm³. 3 When discharging, the density of the dissolved ferrous sulfate should be adjusted appropriately according to the seasonal temperature to ensure that the ferrous sulfate solution does not easily crystallize.

[0062] Solid ferrous sulfate often contains relatively high amounts of Ti and Fe. 3+ Impurity ions such as Al require impurity removal treatment. Ti mainly exists in the form of titanium oxysulfate. Dissolving solid ferrous sulfate can form insoluble metatitanic acid. Other cations are mainly removed by adjusting the pH of the solution to form hydroxide precipitates. Cations that cannot form precipitates are reduced through adsorption. In this process, the dissolved solid ferrous sulfate solution is heated to 80±5℃ and maintained at this temperature. Then, a 10% wt sodium hydroxide solution is slowly added dropwise to the dissolved ferrous sulfate solution while stirring to quickly disperse the sodium hydroxide solution and prevent excessively high local pH values ​​from forming ferrous hydroxide, thus avoiding iron loss and increased solid waste. The pH of the solution is monitored during the addition process. When the pH reaches 4.5±0.5, impurity removal is complete, and the solution is filtered to obtain a clear ferrous sulfate solution.

[0063] Preferably, the amount of titanium additive added is 0.1‰ to 2‰ of the carbon-coated multi-element doped sodium iron pyrophosphate product, and more preferably, it is 0.1‰ to 1‰.

[0064] Preferably, the titanium element additive is one or at least two of titanium oxysulfate, tetrabutyl titanate, and titanium sulfate.

[0065] In some other specific embodiments, the ferrous sulfate is of industrial grade purity, and the metal element additive is a mixture of titanium, magnesium and manganese element additives.

[0066] Preferably, the titanium element additive is one or at least two of titanium oxysulfate, tetrabutyl titanate, and titanium sulfate; the magnesium element additive is one or at least two of magnesium sulfate, magnesium nitrate, and magnesium chloride; and the manganese element additive is one or at least two of manganese sulfate, manganese nitrate, and manganese chloride.

[0067] Preferably, the amount of titanium additive added is 0.1‰ to 2‰ of the carbon-coated multi-element doped sodium iron pyrophosphate product, more preferably 0.1‰ to 1‰, the amount of magnesium additive added is 0.1‰ to 1‰ of the carbon-coated multi-element doped sodium iron pyrophosphate product, and the amount of manganese additive added is 0.1‰ to 2‰ of the carbon-coated multi-element doped sodium iron pyrophosphate product.

[0068] In some specific implementations, the phosphorus source in step S1 is one or more of the following: ammonium dihydrogen phosphate, diammonium hydrogen phosphate, ammonium phosphate, phosphoric acid, disodium hydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; and the pH adjuster is ammonia or sodium hydroxide. Preferably, when the cation in the phosphorus source is ammonium ion, the corresponding pH adjuster is ammonia; when the cation in the phosphorus source is sodium ion, the corresponding pH adjuster is sodium hydroxide.

[0069] Preferably, the phosphorus source, pH adjuster, and metal element additive in step S1 are all of industrial grade purity.

[0070] Furthermore, in step S1, the concentration of ferrous sulfate is 1-2 mol / L, the P:Fe molar ratio of the phosphorus source to ferrous sulfate is 2.9-3.1:2, the pH adjuster maintains the pH value of the solution at 4-6, more preferably 4.5-5.5, and the amount of titanium element additive added is 0.1‰-2‰ of the carbon-coated multi-element doped sodium ferric pyrophosphate product.

[0071] Specifically, in step S1, a phosphorus source and a metal element additive are added to the mixed solution of ferrous sulfate and sulfuric acid. The pH value of the mixed solution of ferrous sulfate and sulfuric acid is 0.8 to 1.2 to avoid oxidation of ferrous ions.

[0072] Optionally, the filtration method in step S1 is filter press filtration or centrifugal filtration.

[0073] Furthermore, the qualification standard for rinsing in step S1 is that the conductivity of the washing water is ≤500μs / cm.

[0074] Furthermore, the moisture content of the ferrous phosphate octahydrate filter cake in step S1 is ≤40%.

[0075] Furthermore, the sodium source in step S2 is one or more of sodium hydroxide, sodium monohydrogen phosphate, and sodium phosphate; the phosphorus source is one or more of phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; and the organic solvent is one or more of tert-butanol, glacial acetic acid, cyclohexane, and dimethyl sulfoxide.

[0076] Furthermore, in step S2, the amount of sodium source added is such that the molar ratio of sodium to iron in ferrous phosphate octahydrate in the ground slurry is 4:2.9-3, the amount of phosphorus source added is such that the molar ratio of phosphorus to iron in ferrous phosphate octahydrate in the ground slurry is 2:3, the mass ratio of organic solvent to ferrous phosphate octahydrate is 1-2.5:1, and the pH value of the slurry is ≥11 after being stirred evenly.

[0077] In some specific implementations, the particle size D50 of the output material in step S2 is 50-300 nm, and the grinding temperature is 60℃-80℃.

[0078] Furthermore, the carbon source mentioned in step S4 is one or more of glucose, sucrose, PEG2000, and white sugar, and the amount of carbon source added is 3% to 10% of the mass of the carbon-coated multi-element doped sodium iron pyrophosphate phosphate finished product.

[0079] In some specific implementations, the inlet air temperature of the spray drying in step S5 is 200-300°C, the outlet air temperature is 70-110°C, and the D50 particle size range of the hollow spherical solid particles is 1-20 μm, preferably 1-5 μm.

[0080] Furthermore, the calcination conditions in step S6 are as follows: first, maintain the temperature at 180–220℃ for 3–8 hours, and then maintain the temperature at 500–550℃ for 8–10 hours.

[0081] Specifically, under nitrogen atmosphere protection, a stepped sintering process is carried out, in which the temperature is raised from room temperature to 200℃ at a heating rate of no more than 5℃ / min and held for 3 to 8 hours, then raised to 550℃ at a heating rate of no more than 5℃ / min and held for 8 to 10 hours, and finally cooled to room temperature at a cooling rate of no more than 5℃ / min.

[0082] The present invention also provides carbon-coated multi-element doped sodium pyrophosphate prepared by the above preparation method.

[0083] The following are specific examples.

[0084] Example 1

[0085] In this embodiment, the ferrous sulfate is obtained from crude ferrous sulfate, a byproduct of titanium dioxide production, through dissolution, pH adjustment, and filtration. Specifically: 1. Take solid ferrous sulfate, a byproduct of titanium dioxide production, containing approximately 2000 ppm Mg and 800 ppm Mn. Dissolve it in an appropriate amount of hot water at a FeSO4 mass fraction of 20%–25%. Then, add 32% sodium hydroxide solution to adjust the pH to 2.5–3.0. After stirring for 0.5 hours, filter to obtain a clear ferrous sulfate solution. Add an appropriate amount of water and sulfuric acid to the above ferrous sulfate solution to make the Fe molar amount 1 mol / L and the pH = 1, thus obtaining the reaction base solution.

[0086] The preparation method of carbon-coated multi-element doped sodium iron pyrophosphate is as follows:

[0087] 1) Take the reaction base liquid prepared above, add sodium dihydrogen phosphate and titanium oxysulfate to the reaction base liquid according to the molar ratio of Fe:P = 1.48 and Ti:Fe = 0.002, stir evenly to dissolve and form a clear and transparent solution, then slowly add 16% sodium hydroxide solution to the solution to adjust the pH value of the solution to 4.8. After reacting for a period of time, a gray-blue titanium, magnesium and manganese multi-element doped octahydrate ferrous phosphate slurry is obtained.

[0088] 2) The above-mentioned multi-element doped ferrous phosphate octahydrate slurry was filtered by a filter press and rinsed. After washing until the conductivity of the effluent was ≤500μs / cm, it was pressed at a pressure of 2MPa.

[0089] 3) Under a protective atmosphere, sodium phosphate and tert-butanol were added to the multi-element doped ferrous phosphate octahydrate filter cake after pressure filtration for slurrying. The amount of sodium phosphate added was measured according to the molar ratio of Fe:P = 2:3, and the amount of tert-butanol added was the same as the mass of the filter cake. Then, 32% sodium hydroxide solution was added to adjust the pH to 11. After stirring evenly, the mixture was sand-milled at a grinding temperature of 80℃. The output particle size was ground to a D50 of 50-150nm.

[0090] 4) Freeze-dry the above-mentioned grinding slurry to separate the solid mixture;

[0091] 5) Add glucose, PEG2000 and pure water to the above solid mixture. The total weight of glucose and PEG is 3% of the weight of the solid mixture, and the weight ratio of glucose to PEG2000 is 5:1. The amount of water added is 2.5 times the weight of the solid mixture. Stir well.

[0092] 6) The slurry obtained in S above is spray-dried with an inlet air temperature of 250°C and an outlet air temperature of 80°C to obtain hollow spherical solid particles.

[0093] 7) The hollow spherical solid particles were heated from room temperature to 200℃ at a heating rate of 5℃ / min and held at that temperature for 5 hours. Then, the temperature was increased to 550℃ at a heating rate of 5℃ / min and held at that temperature for 10 hours. Finally, the temperature was lowered to obtain carbon-coated multi-element doped sodium iron pyrophosphate.

[0094] The grinding process curve in this embodiment is as follows: Figure 2 As shown, when the grinding time reaches 2 hours, the slurry particle size D50 reaches 150 nm, indicating that the grinding process is highly efficient.

[0095] The hollow spherical particles prepared by spraying in this embodiment are as follows: Figure 3As shown, the structure has a cavity inside and a relatively complete spherical structure on the outside. This unique structure gives it a large specific surface area and good fluidity and dispersibility, which can be uniformly dispersed during the preparation of battery slurry and improve the electrical performance of the battery. As can be seen from the figure, the hollow spherical particles prepared in this embodiment are about 3 μm in size.

[0096] The elemental contents of the carbon-coated multi-element doped sodium iron pyrophosphate prepared in this embodiment are shown in Table 1.

[0097] Table 1

[0098] 14.5％ 19.5％ 26.4％ 523ppm 478ppm 504ppm 1.8％

[0099] The carbon-coated multi-element doped sodium iron pyrophosphate prepared in this embodiment exhibits good electrical properties, high first-cycle coulombic efficiency, and excellent 0.1C charge-discharge performance. Figure 4 As shown, the specific capacity at 0.1C charging is 110.2 mAh / g, the specific capacity at 0.1C discharging is 108.8 mAh / g, and the coulombic efficiency is 98.7%. This is mainly because the material has a larger atomic radius difference after multi-element doping, which allows sodium ions to be inserted and extracted more easily. In addition, the grinding particle size is small, thereby improving the electrical performance and coulombic efficiency.

[0100] Example 2

[0101] The preparation method of carbon-coated multi-element doped sodium iron pyrophosphate in this embodiment differs from that in Example 1 only in that the Ti:Fe molar ratio in S2 is 0.003 and the grinding particle size D50 in S4 is 150-200 nm. All other steps are the same as in Example 1.

[0102] Example 3

[0103] The preparation method of carbon-coated multi-element doped sodium iron pyrophosphate in this embodiment differs from that in Example 1 only in that sodium hydroxide is added in S4 to adjust the pH of the solution to 12, while the other steps are the same as in Example 1.

[0104] Example 4

[0105] The preparation method of carbon-coated multi-element doped sodium iron pyrophosphate in this embodiment differs from that in Example 1 only in that the total weight of glucose and PEG2000 in S6 is 5% of the weight of the solid mixture, and the ratio of glucose to PEG2000 is 4:1. All other steps are the same as in Example 1.

[0106] The 1C and 10C charge / discharge data for Examples 1, 2, 3, and 4 are shown in Table 2.

[0107] Table 2

[0108] Example 1 103.4 99.6 96.3％ 93.8 89.4 95.3％ Example 2 100.7 96.8 96.1％ 88.9 82.6 92.9％ Example 3 104.2 100.8 96.7％ 94.6 90.3 95.5％ Example 4 108.3 104.4 96.4％ 96.3 92.2 95.7％

[0109] As can be seen from the data in the table, the carbon-coated multi-element doped sodium iron pyrophosphate prepared in the embodiments of the present invention has good electrical and rate performance. The 1C charging specific capacity is greater than 100 mAh / g, and the charge-discharge efficiency is greater than 96%; the 10C charging specific capacity is greater than 88 mAh / g, the discharge specific capacity is greater than 82 mAh / g, and the charge-discharge efficiency is greater than 92%. At the same time, the comparative embodiments show that the main factors affecting the electrical and rate performance are the grinding particle size and the content of coated carbon. The smaller the grinding particle size and the higher the content of coated carbon, the better the electrical and rate performance of the material.

[0110] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing carbon-coated multi-element doped sodium iron pyrophosphate, characterized in that, Includes the following steps: S1. Precursor preparation: A pH adjuster is added dropwise to a mixed solution of ferrous sulfate, sulfuric acid, phosphorus source and metal element additives. The resulting suspension is filtered, washed and pressed to obtain a multi-element doped ferrous phosphate octahydrate filter cake. S2. Grinding: Under a protective atmosphere, sodium source, phosphorus source and organic solvent are added to the ferrous phosphate octahydrate filter cake obtained in step S1 for slurrying. After stirring evenly, grinding is carried out to remove the bound water in the ferrous phosphate octahydrate and obtain the grinding slurry. S3. Primary drying: The grinding slurry obtained in step S2 is freeze-dried to separate the solid mixture; S4. Mixing: Add carbon source and pure water to the solid mixture and stir until homogeneous to obtain a mixed slurry; S5. Secondary drying: The mixed slurry is spray-dried to obtain hollow spherical solid particles; S6. Calcination: The hollow spherical solid particles above are calcined at high temperature to obtain carbon-coated multi-element doped sodium iron pyrophosphate. In step S2, the organic solvent is one or more of tert-butanol, glacial acetic acid, and dimethyl sulfoxide; the pH value of the slurry is ≥11 after it is stirred evenly.

2. The method for preparing carbon-coated multi-element doped sodium iron pyrophosphate according to claim 1, characterized in that, The multi-element doped ferrous phosphate octahydrate mentioned in step S1 is titanium, magnesium and manganese doped ferrous phosphate octahydrate.

3. The method for preparing carbon-coated multi-element doped sodium iron pyrophosphate according to claim 2, characterized in that, The ferrous sulfate is obtained by removing impurities from crude ferrous sulfate, a byproduct of titanium dioxide production. The purity of the ferrous sulfate is ≥99%, the magnesium content is 1800~2200ppm, the manganese content is 600~1000ppm, and the metal element additive is a titanium element additive.

4. The method for preparing carbon-coated multi-element doped sodium iron pyrophosphate according to claim 2, characterized in that, The ferrous sulfate is of industrial grade purity, and the metal element additive is a mixture of titanium, magnesium, and manganese element additives.

5. The method for preparing carbon-coated multi-element doped sodium iron pyrophosphate according to any one of claims 1 to 4, characterized in that, In step S1, the concentration of ferrous sulfate is 1~2 mol / L, the P:Fe molar ratio of the phosphorus source to ferrous sulfate is 2:2.9~3.1, and a pH adjuster is added to maintain the pH value of the solution at 4~6.

6. The method for preparing carbon-coated multi-element doped sodium iron pyrophosphate according to claim 4, characterized in that, The amounts of titanium, magnesium, and manganese added are 0.1‰~2‰, 0.1‰~1‰, and 0.1‰~2‰, respectively, of the carbon-coated multi-element doped sodium iron pyrophosphate phosphate product.

7. The method for preparing carbon-coated multi-element doped sodium iron pyrophosphate according to any one of claims 1 to 4, characterized in that, In step S2, the sodium source is one or more of sodium hydroxide, sodium monohydrogen phosphate, and sodium phosphate; the phosphorus source is one or more of phosphoric acid, sodium monohydrogen phosphate, sodium dihydrogen phosphate, and sodium phosphate; the amount of sodium source added is such that the molar ratio of sodium to iron in ferrous phosphate octahydrate in the ground slurry is 4:2.9~3; the amount of phosphorus source added is such that the molar ratio of phosphorus to iron in ferrous phosphate octahydrate in the ground slurry is 2:3; and the mass ratio of organic solvent to ferrous phosphate octahydrate is 1~2.5:

1.

8. The method for preparing carbon-coated multi-element doped sodium iron pyrophosphate according to any one of claims 1 to 4, characterized in that, The carbon source mentioned in step S4 is one or more of glucose, sucrose, and PEG2000, and the amount of carbon source added is 3% to 10% of the mass of the carbon-coated multi-element doped sodium iron pyrophosphate phosphate finished product.

9. The method for preparing carbon-coated multi-element doped sodium iron pyrophosphate according to any one of claims 1 to 4, characterized in that, The calcination conditions in step S6 are as follows: first, keep at 180℃~220℃ for 3~8 hours, and then keep at 500~550℃ for 8~10 hours.

10. A carbon-coated multi-element doped sodium iron pyrophosphate, characterized in that, It is prepared by the preparation method according to any one of claims 1 to 9.

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