Preparation method for iron phosphate with core-shell structure and use thereof

Abstract

A preparation method for iron phosphate with a core-shell structure and the use thereof. The method comprises: mixing a surfactant, an organic iron source, an organic phosphorus source, a waste oil coagulant and liquid oil at 60-90°C, adding the obtained mixed liquid to water to form an emulsion, separately adding a trivalent iron salt solution and a phosphate solution to the emulsion for a reaction, naturally cooling and filtering the reaction solution, and subjecting the obtained wet solid material to microwave heating in an oxygen atmosphere, so as to obtain iron phosphate with a core-shell structure.

Description

Preparation method and application of core-shell structured iron phosphate Technical Field

[0001] The present disclosure belongs to the technical field of lithium-ion batteries, and particularly relates to a preparation method of core-shell iron phosphate and its application. Background Art

[0002] In recent years, with the rapid development of the new energy industry and the continuous improvement of people's quality of life, new energy vehicles have entered the public eye, and demand for them is increasing. Lithium iron phosphate (LiFePO4), as a cathode material for lithium-ion batteries, is considered one of the most ideal battery cathode materials for new energy vehicles due to its low cost, environmental friendliness, high safety, and excellent electrochemical performance.

[0003] Lithium iron phosphate (LIFP) is the most popular cathode material for lithium-ion batteries in recent years. It has an extremely stable olivine structure. Compared to lithium-ion batteries with other cathode materials, batteries made with LFP have excellent safety and longer cycle life. They are considered the most likely energy storage devices to replace traditional fossil fuels and be widely used in the automotive and energy storage fields. However, there are still some problems that need to be solved. For example, the intrinsic conductivity of LFP is low and the lithium ion diffusion coefficient is small, which makes the low-temperature performance of LFP batteries poor and the high-rate performance is average. This greatly limits its application in automotive power batteries (BEV, HEV), starting power supplies, smart grids and other fields.

[0004] Iron phosphate (FePO4) is a precursor to lithium iron phosphate, and its preparation method is currently a hot topic of research. The purity, particle size, morphology, structure, and other performance indicators of iron phosphate play a crucial role in the electrochemical performance of the synthesized lithium iron phosphate material. The structure of iron phosphate materials prepared by traditional methods does not change during subsequent processing. During the final synthesis and manufacturing of lithium iron phosphate, iron phosphate will transform into lithium iron phosphate, and the grains will continue to grow, eventually becoming a large-grain lithium iron phosphate material with a dense structure. Due to the slow migration of lithium ions, this lithium iron phosphate material has poor high-rate performance and low-temperature performance. Its low electronic conductivity and slow one-dimensional ion diffusion hinder its high-rate charge and discharge.

[0005] Summary of the Invention

[0006] The present disclosure aims to solve at least one of the technical problems existing in the above-mentioned prior art. To this end, the present disclosure proposes a method for preparing a core-shell structured iron phosphate and its application, which can increase the one-dimensional diffusion rate of lithium ions and improve the rate performance of lithium iron phosphate.

[0007] According to one aspect of the present disclosure, a method for preparing core-shell ferric phosphate is proposed, comprising the following steps:

[0008] S1: mixing a surfactant, an organic iron source, an organic phosphorus source, a waste oil coagulant and liquid oil at 60-90°C, adding the resulting mixture to water, and stirring at 60-90°C to form an emulsion;

[0009] S2: adjusting the pH of the emulsion to 0.8-1.5, adding a trivalent iron salt solution and a phosphate solution to the emulsion, respectively, and adjusting the pH to 1.5-2.2 for reaction;

[0010] S3: After the reaction in step S2 is completed, the reaction solution is naturally cooled and filtered to obtain a solid wet material;

[0011] S4: The solid wet material is subjected to microwave heating in an oxygen atmosphere at a heating temperature of 500-800° C. to obtain the core-shell structured iron phosphate.

[0012] In some embodiments of the present disclosure, in step S1, the surfactant is at least one of dodecylbenzenesulfonic acid, polyoxyethylene sorbitan monooleate, fatty alcohol polyoxyethylene ether, polyoxyethylene sorbitan monolaurate, polyoxyethylene oleyl ether, polyoxyethylene castor oil, polyoxyethylene oleyl ether or polyoxyethylene lauryl ether.

[0013] In some embodiments of the present disclosure, in step S1, the organic iron source is at least one of ferrocene, iron oleate, iron naphthenate, or iron 2-ethylhexanoate.

[0014] In some embodiments of the present disclosure, in step S1, the organic phosphorus source is at least one of di(2-ethylhexyl)phosphate, trioctyl phosphate, triisopropyl phosphite, isooctyl phosphate, tri(4-nonphenyl)phosphite or mono-n-dodecyl phosphate.

[0015] In some embodiments of the present disclosure, in step S1, the waste oil coagulant is composed of a vinyl polymer having ≥30 mol% stearyl (meth)acrylate units and a number average molecular weight of 3,000-80,000. A waste oil coagulant composed of a vinyl polymer containing 30 mol% or more stearyl (meth)acrylate units has sufficient oil coagulant capacity even at 50°C, thus safely and easily solidifying the emulsion particles within the iron phosphate shell at low temperatures. The waste oil coagulant disclosed herein can be commercially available or homemade. The preparation method is described in Patent JPH05311191A and will not be further described herein.

[0016] In some embodiments of the present disclosure, in step S1, the liquid oil is at least one of edible oil, motor oil, or synthetic mineral oil.

[0017] In some embodiments of the present disclosure, in step S1, the organic phosphorus source and the organic iron source are respectively present in a molar ratio of phosphorus to iron of 1:(0.8-1.2); and the amount of the organic iron source and the liquid oil is 0.1-5 mol / L.

[0018] In some embodiments of the present disclosure, in step S1, the solid-liquid ratio of the waste oil coagulant to the liquid oil is (0.5-2) g:30 ml.

[0019] In some embodiments of the present disclosure, in step S1, the stirring speed is 800-3000 rpm; and the stirring time is 0.5-10 h.

[0020] In some embodiments of the present disclosure, in step S1, the weight ratio of the liquid oil to the surfactant is (1-9):1; and the total weight of the liquid oil and the surfactant in the emulsion accounts for 0.1-10% of the total weight of the emulsion.

[0021] In some embodiments of the present disclosure, in step S2, the reaction temperature is 70-90°C.

[0022] In some embodiments of the present disclosure, in step S2, the molar ratio of the phosphate in the phosphate solution to the ferric iron in the ferric iron solution is 1:(0.95-1.1).

[0023] In some embodiments of the present disclosure, in step S2, the phosphate solution is at least one of ammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, or phosphoric acid solution; and the concentration of phosphate in the phosphate solution is 0.1-5 mol / L.

[0024] In some embodiments of the present disclosure, in step S2, the ferric salt solution is at least one of ferric nitrate or ferric chloride solution; and the concentration of the ferric salt in the ferric salt solution is 0.1-5 mol / L.

[0025] In some embodiments of the present disclosure, in step S4, the microwave input power of the microwave heating is 20-30 KW.

[0026] In some embodiments of the present disclosure, in step S4, the microwave heating time is 15-30 minutes.

[0027] In some embodiments of the present disclosure, in step S4, the particle size D50 of the core-shell structured ferric phosphate is 1-7 μm. The particle size of the core-shell structured ferric phosphate is controlled by controlling the diameter of the emulsion particles.

[0028] The present disclosure also provides application of the preparation method in preparing lithium-ion batteries.

[0029] According to the embodiments of the present disclosure, there are at least the following beneficial effects:

[0030] 1. The present invention discloses dissolving organophosphorus and organoferric in liquid oil. The resulting mixture is mixed with water under the action of a surfactant and stirred at high speed to produce an emulsion. A trivalent iron salt solution and a phosphate solution are added to the emulsion for reaction. The resulting iron phosphate precipitate adheres to the emulsion particles, forming a stable iron phosphate shell on the outside and microspheres of the emulsion inside. After the reaction, the solution is cooled, and the emulsion microspheres inside solidify under the action of a waste oil coagulant, thereby preventing the internal organophosphorus and organoferric from flowing out during filtration. Finally, microwave heating is used to react the internal organophosphorus and organoferric to produce a loose iron phosphate core. The microwave heating method has the characteristics of rapid heating speed and high reaction rate. The emulsion microspheres inside the particles heat up and burn and decompose in a very short time. During heating, iron phosphate begins to form. The rapid decomposition of the emulsion helps to evenly distribute the iron phosphate in the cavity inside the particles, ultimately forming a loose iron phosphate core.

[0031] 2. The iron phosphate particles disclosed herein have a core-shell structure, with a loose iron phosphate core formed by organic phosphorus and organic iron inside and a dense iron phosphate shell formed by inorganic phosphorus and inorganic iron outside. Compared with completely dense iron phosphate, the iron phosphate particles disclosed herein have a loose inner core, which shortens the diffusion path of lithium ions and increases the one-dimensional diffusion rate of lithium ions, effectively overcoming the problem of capacity reduction caused by insufficient lithium ion diffusion in the core, thereby improving the rate performance of the battery.

[0032] 3. Compared with hollow lithium iron phosphate, the core-shell structured iron phosphate prepared in the present invention has a loose iron phosphate core in the iron phosphate shell, which increases the volume energy density of the lithium iron phosphate battery during the preparation process. DETAILED DESCRIPTION

[0033] The following will clearly and completely describe the concept and technical effects of the present disclosure in conjunction with embodiments to fully understand the purpose, features and effects of the present disclosure.

[0034] Example 1

[0035] This example prepares a core-shell structured iron phosphate, and the specific process is as follows:

[0036] (1) Polyoxyethylene oleyl alcohol ether, trioctyl phosphate, waste oil coagulant and ferrocene are mixed into soybean oil, wherein the waste oil coagulant is composed of a vinyl polymer having 40 mol% of stearyl (meth)acrylate units and a number average molecular weight of 22,000, the molar ratio of trioctyl phosphate to ferrocene is 1:1 based on phosphorus and iron, respectively, the mass ratio of polyoxyethylene oleyl alcohol ether to soybean oil is 1:4, the concentration of ferrocene in soybean oil is 2 mol / L, the solid-liquid ratio of waste oil coagulant to soybean oil is 1 g:30 ml, the temperature is maintained at 60° C., and the mixture is stirred and mixed uniformly; the above mixture is mixed with deionized water, the temperature is maintained at 60° C., and the mixture is stirred at a speed of 1600 rpm for 4 hours to form an emulsion, wherein the total mass of soybean oil and polyoxyethylene oleyl alcohol ether accounts for 5% of the total mass of the emulsion;

[0037] (2) adjusting the pH of the emulsion to 1.3 with phosphoric acid, then adding 1 mol / L ferric nitrate solution, and then adding 1 mol / L ammonium hydrogen phosphate solution to adjust the pH to 1.5-2.2, stirring and reacting at a reaction temperature of 70°C to form an iron phosphate shell around the emulsion particles;

[0038] (3) The reaction solution was naturally cooled and then filtered out. The obtained solid wet material was placed in an oxygen atmosphere for microwave heating. The microwave input power was 20 kW, the heating temperature was 600° C., and the heating time was 25 min to obtain a core-shell structured iron phosphate product with a particle size D50 of 3 μm.

[0039] Example 2

[0040] This example prepares a core-shell structured iron phosphate, and the specific process is as follows:

[0041] (1) Polyoxyethylene oleyl alcohol ether, trioctyl phosphate, waste oil coagulant and ferrocene are mixed into soybean oil, wherein the waste oil coagulant is composed of a vinyl polymer having 40 mol% of stearyl (meth)acrylate units and a number average molecular weight of 22,000, the molar ratio of trioctyl phosphate to ferrocene is 1:1 based on phosphorus and iron, respectively, the mass ratio of polyoxyethylene oleyl alcohol ether to soybean oil is 1:9, the concentration of ferrocene in soybean oil is 2 mol / L, the solid-liquid ratio of waste oil coagulant to soybean oil is 1 g:30 ml, the temperature is maintained at 60° C., and the mixture is stirred and mixed uniformly; the above mixture is mixed with deionized water, the temperature is maintained at 60° C., and the mixture is stirred at a speed of 1600 rpm for 5 hours to form an emulsion, wherein the total mass of soybean oil and polyoxyethylene oleyl alcohol ether accounts for 5% of the total mass of the emulsion;

[0042] (2) adjusting the pH of the emulsion to 1.3 with phosphoric acid, then adding 1 mol / L ferric nitrate solution, and then adding 1 mol / L ammonium hydrogen phosphate solution to adjust the pH to 1.5-2.2, stirring and reacting at a reaction temperature of 70°C to form an iron phosphate shell around the emulsion particles;

[0043] (3) The reaction solution was naturally cooled and then filtered out. The obtained solid wet material was placed in an oxygen atmosphere for microwave heating. The microwave input power was 20 kW, the heating temperature was 600° C., and the heating time was 25 min to obtain a core-shell structured iron phosphate product with a particle size D50 of 6 μm.

[0044] The amount of surfactant added in this embodiment is small, the diameter of the emulsion particles is large, and the diameter of the iron phosphate formed is large. The large diameter causes the gap and cavity volume to be large, and the internal iron phosphate is not evenly distributed, resulting in a reduced capacity.

[0045] Example 3

[0046] This example prepares a core-shell structured iron phosphate, and the specific process is as follows:

[0047] (1) Polyoxyethylene oleyl alcohol ether, trioctyl phosphate, waste oil coagulant and ferrocene are mixed into soybean oil, wherein the waste oil coagulant is composed of a vinyl polymer having 40 mol% of stearyl (meth)acrylate units and a number average molecular weight of 22,000, the molar ratio of trioctyl phosphate to ferrocene is 1:1 based on phosphorus and iron, respectively, the mass ratio of polyoxyethylene oleyl alcohol ether to soybean oil is 1:1, the concentration of ferrocene in soybean oil is 2 mol / L, the solid-liquid ratio of waste oil coagulant to soybean oil is 1 g:30 ml, the temperature is maintained at 60° C., and the mixture is stirred and mixed uniformly; the above mixture is mixed with deionized water, the temperature is maintained at 60° C., and the mixture is stirred at a speed of 1000 rpm for 2 h to form an emulsion, wherein the total mass of soybean oil and polyoxyethylene oleyl alcohol ether accounts for 5% of the total mass of the emulsion;

[0048] (2) adjusting the pH of the emulsion to 1.3 with phosphoric acid, then adding 1 mol / L ferric nitrate solution, and then adding 1 mol / L ammonium hydrogen phosphate solution to adjust the pH to 1.5-2.2, stirring and reacting at a reaction temperature of 70°C to form an iron phosphate shell around the emulsion particles;

[0049] (3) The reaction solution was naturally cooled and then filtered out. The obtained solid wet material was placed in an oxygen atmosphere for microwave heating. The microwave input power was 20 kW, the heating temperature was 600° C., and the heating time was 25 min to obtain a core-shell structured iron phosphate product with a particle size D50 of 2 μm.

[0050] The amount of surfactant in this embodiment is large, and the particle size D50 of the formed iron phosphate is small.

[0051] Example 4

[0052] This example prepares a core-shell structured iron phosphate, and the specific process is as follows:

[0053] (1) Polyoxyethylene oleyl alcohol ether, trioctyl phosphate, waste oil coagulant and ferrocene are mixed into soybean oil, wherein the waste oil coagulant is composed of a vinyl polymer having 40 mol% of stearyl (meth)acrylate units and a number average molecular weight of 22,000, the molar ratio of trioctyl phosphate to ferrocene is 1:1 based on phosphorus and iron, respectively, the mass ratio of polyoxyethylene oleyl alcohol ether to soybean oil is 1:4, the concentration of ferrocene in soybean oil is 2 mol / L, the solid-liquid ratio of waste oil coagulant to soybean oil is 1 g:30 ml, the temperature is maintained at 60° C., and the mixture is stirred and mixed uniformly; the above mixture is mixed with deionized water, the temperature is maintained at 60° C., and the mixture is stirred at a speed of 800 rpm for 5 hours to form an emulsion, wherein the total mass of soybean oil and polyoxyethylene oleyl alcohol ether accounts for 5% of the total mass of the emulsion;

[0054] (2) adjusting the pH of the emulsion to 1.3 with phosphoric acid, then adding 1 mol / L ferric nitrate solution, and then adding 1 mol / L ammonium hydrogen phosphate solution to adjust the pH to 1.5-2.2, stirring and reacting at a reaction temperature of 70°C to form an iron phosphate shell around the emulsion particles;

[0055] (3) The reaction solution was naturally cooled and then filtered out. The obtained solid wet material was placed in an oxygen atmosphere for microwave heating. The microwave input power was 20 kW, the heating temperature was 600° C., and the heating time was 20 min to obtain a core-shell structured iron phosphate product with a particle size D50 of 7 μm.

[0056] The cutting speed for forming the emulsion affects the particle size. In this embodiment, the cutting speed is low, and the formed emulsion is unstable, has a large particle size, and a small capacity.

[0057] Example 5

[0058] This example prepares a core-shell structured iron phosphate, and the specific process is as follows:

[0059] (1) Polyoxyethylene oleyl alcohol ether, trioctyl phosphate, waste oil coagulant and ferrocene are mixed into soybean oil, wherein the waste oil coagulant is composed of a vinyl polymer having 40 mol% of stearyl (meth)acrylate units and a number average molecular weight of 22,000, the molar ratio of trioctyl phosphate to ferrocene is 1:1 based on phosphorus and iron, respectively, the mass ratio of polyoxyethylene oleyl alcohol ether to soybean oil is 1:1, the concentration of ferrocene in soybean oil is 2 mol / L, the solid-liquid ratio of waste oil coagulant to soybean oil is 1 g:30 ml, the temperature is maintained at 60° C., and the mixture is stirred and mixed uniformly; the above mixture is mixed with deionized water, the temperature is maintained at 60° C., and the mixture is stirred at a speed of 2000 rpm for 3 hours to form an emulsion, wherein the total mass of soybean oil and polyoxyethylene oleyl alcohol ether accounts for 5% of the total mass of the emulsion;

[0060] (2) adjusting the pH of the emulsion to 1.3 with phosphoric acid, then adding 1 mol / L ferric nitrate solution, and then adding 1 mol / L ammonium hydrogen phosphate solution to adjust the pH to 1.5-2.2, stirring and reacting at a reaction temperature of 70°C to form an iron phosphate shell around the emulsion particles;

[0061] (3) The reaction solution was naturally cooled and then filtered out. The obtained solid wet material was placed in an oxygen atmosphere for microwave heating. The microwave input power was 20 kW, the heating temperature was 600° C., and the heating time was 25 min to obtain a core-shell structured iron phosphate product with a particle size D50 of 2 μm.

[0062] The cutting speed for forming the emulsion affects the size of the particle size. In this embodiment, the cutting speed is higher, the particle size is smaller, and the capacity is larger.

[0063] Comparative Example 1

[0064] This comparative example prepares a hollow ferric phosphate. The difference from Example 1 is that no organic phosphorus and organic iron are added. The specific process is as follows:

[0065] Polyoxyethylene oleyl alcohol ether is mixed into soybean oil in a mass ratio of 1:4, and the mixture is stirred to form a uniform mixture; the mixture is mixed with deionized water and stirred at a speed of 1600 rpm for 4 hours to form an emulsion, wherein the total mass of soybean oil and polyoxyethylene oleyl alcohol ether accounts for 5% of the total mass of the emulsion; the pH of the emulsion is adjusted to 1.3 using phosphoric acid, and then a 1 mol / L ferric nitrate solution is added, followed by a 1 mol / L ammonium hydrogen phosphate solution to adjust the pH to 1.5-2.2, and the reaction is stirred at a reaction temperature of 70°C to form an iron phosphate shell around the emulsion particles; the reaction solution is naturally cooled and then filtered and removed, and the resulting solid wet material is placed in an oxygen atmosphere and microwave heated at a microwave input power of 20 kW, a heating temperature of 600°C, and a heating time of 25 minutes to obtain hollow iron phosphate with a particle size D50 of 3 μm.

[0066] Test example

[0067] The iron phosphate and Li2CO3 of the above embodiment were loaded into a ball mill and wet-milled with anhydrous ethanol at a speed of 500 rpm for 6 hours; glucose, lithium carbonate and the prepared iron phosphate in a ratio of 0.05:1.05:1.0 were calcined at 700°C under a nitrogen atmosphere to obtain lithium iron phosphate material.

[0068] The prepared lithium iron phosphate cathode material was mixed with a cyclohexane solution of acetylene black and polyvinylidene fluoride (PVDF) at room temperature and pressure to form a slurry (the weight ratio of cathode material: acetylene black: PVDF was 75:15:10). This slurry was evenly coated onto an aluminum foil substrate to serve as the positive electrode of a simulated battery. The negative electrode of the simulated battery used a lithium sheet, the electrolyte consisted of 1 mol LiPF₆ dissolved in 1 L of a mixed solvent of EC and DMC (1:1 by volume), and the separator was a polyethylene ion exchange membrane. The cathode, negative electrode, electrolyte, and separator were assembled into a simulated battery in an argon-protected glove box.

[0069] Steps for simulating battery rate testing:

[0070] First, the battery was charged to 4.2V at 30mA / g, then discharged at a rate current to 2.0V. The released capacity was the discharge capacity at that rate. After discharge, the battery was discharged again at 30mA / g to 2.0V. The next rate test was then performed. The test results for this simulated battery are listed in Table 1.

[0071] Table 1

[0072] As shown in Examples 1-5, the amount of surfactant added affects the size of the emulsion particles. The larger the amount of surfactant added, the smaller the particle size of the emulsion particles formed; the smaller the amount of surfactant added, the larger the particle size of the emulsion particles formed. The cutting speed also has a certain influence on the particle size of the emulsion particles. The higher the cutting speed, the smaller the particle size of the emulsion particles; the lower the cutting speed, the larger the particle size of the emulsion particles formed.

[0073] The volume energy density was measured using a vernier caliper method to test the volume capacity difference between Example 1 and Comparative Example 1.

[0074] The volume energy density of Example 1 was measured to be 285 KWh / m 3 The volume energy density of comparative example 1 is 180KWh / m 3 This indicates that the core-shell lithium iron phosphate disclosed in the present invention has a higher volume energy density than the hollow lithium iron phosphate material.

[0075] While the embodiments of the present disclosure have been described in detail above, the present disclosure is not limited to the embodiments described above. Various modifications may be made within the scope of knowledge possessed by a person skilled in the art without departing from the spirit of the present disclosure. Furthermore, the embodiments of the present disclosure and the features within the embodiments may be combined with each other unless there is a conflict.

Claims

1. A preparation method of core-shell structured iron phosphate, characterized in that It includes the following steps: S1: Mix a surfactant, an organic iron source, an organic phosphorus source, a waste oil coagulant, and a liquid oil at 60 - 90 °C. Add the obtained mixture to water and stir at 60 - 90 °C to form an emulsion; S2: Adjust the pH of the emulsion to 0.8 - 1.

5. After adding a ferric salt solution and a phosphate solution to the emulsion respectively, adjust the pH to 1.5 - 2.2 for reaction; S3: After the reaction in step S2 ends, the reaction solution is naturally cooled and filtered to obtain a solid wet material; S4: The solid wet material is subjected to microwave heating in an oxygen atmosphere at a heating temperature of 500 - 800 °C to obtain the core - shell structured iron phosphate.

2. The preparation method according to claim 1, characterized in that In step S1, the surfactant is at least one of dodecylbenzenesulfonic acid, polyoxyethylene sorbitan monooleate, fatty alcohol polyoxyethylene ether, polyoxyethylene sorbitan monolaurate, polyoxyethylene oleyl ether, polyoxyethylene castor oil, polyoxyethylene oil - based ether, or polyoxyethylene lauryl ether.

3. The preparation method according to claim 1, characterized in that In step S1, the organic iron source is at least one of ferrocene, iron oleate, iron naphthenate, or iron 2 - ethylhexanoate.

4. The preparation method according to claim 1, characterized in that In step S1, the organic phosphorus source is at least one of bis(2 - ethylhexyl) phosphate, trioctyl phosphate, triisopropyl phosphite, isooctyl phosphate, tris(4 - nonylphenyl) phosphite, or monolauryl phosphate.

5. The preparation method according to claim 1, characterized in that In step S1, the waste oil coagulant is composed of a vinyl polymer having ≥30 mol% stearyl (meth) acrylate units, and the number - average molecular weight is 3000 - 80000.

6. The preparation method according to claim 1, characterized in that In step S1, the liquid oil is at least one of edible oil, machine oil, or synthetic mineral oil.

7. The preparation method according to claim 1, characterized in that In step S1, the organic phosphorus source and the organic iron source are in a molar ratio of phosphorus to iron of 1:(0.8 - 1.2); the dosage of the organic iron source and the liquid oil is 0.1 - 5 mol / L.

8. The preparation method according to claim 1, characterized in that In step S1, the solid - liquid ratio of the waste oil coagulant to the liquid oil is (0.5 - 2) g:30 ml.

9. The preparation method according to claim 1, characterized in that In step S1, the rotation speed of the stirring is 800 - 3000 rpm; the stirring time is 0.5 - 10 h.

10. The preparation method according to claim 1, characterized in that In step S1, the weight ratio of the liquid oil to the surfactant is (1 - 9):1; the total weight of the liquid oil and the surfactant in the emulsion accounts for 0.1 - 10% of the total weight of the emulsion.

11. The preparation method according to claim 1, characterized in that In step S2, the reaction temperature is 70 - 90 °C.

12. The preparation method according to claim 1, characterized in that In step S2, the molar ratio of the phosphate in the phosphate solution to the ferric salt in the ferric salt solution is 1:(0.95 - 1.1).

13. The preparation method according to claim 1, characterized in that In step S2, the phosphate solution is at least one of ammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, or phosphoric acid solution; the concentration of the phosphate in the phosphate solution is 0.1 - 5 mol / L.

14. The preparation method according to claim 1, characterized in that In step S2, the ferric salt solution is at least one of ferric nitrate or ferric chloride solution; the concentration of the ferric salt in the ferric salt solution is 0.1 - 5 mol / L.

15. The preparation method according to claim 1, characterized in that In step S4, the microwave input power for the microwave heating is 20 - 30 KW.

16. The preparation method according to claim 1, characterized in that In step S4, the microwave heating time is 15 - 30 min.

17. The preparation method according to claim 1, characterized in that In step S4, the D50 particle size of the core-shell structured iron phosphate is 1-7 μm.

18. Application of the preparation method according to any one of claims 1-17 in the preparation of lithium-ion batteries.

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