Preparation method and application of core-shell structure iron phosphate

Abstract

The present disclosure belongs to the technical field of lithium ion batteries, and particularly relates to a preparation method of core-shell structure iron phosphate and application thereof. The method comprises mixing a surfactant, an organic iron source, an organic phosphorus source, a waste oil coagulant and a liquid oil at 60-90 DEG C, adding the obtained mixed solution into water to form a emulsion, adding a ferric salt solution and a phosphate solution into the emulsion respectively for reaction, naturally cooling the reaction solution, filtering, and subjecting the obtained solid wet material to microwave heating in an oxygen atmosphere to obtain the core-shell structure iron phosphate.

Description

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 structure iron phosphate and application thereof. BACKGROUND

[0002] In recent years, with the rapid development of the new energy industry and the increasing improvement of people's living quality, new energy vehicles have entered the public view, and people's demand for new energy vehicles is increasing. Lithium iron phosphate (LiFePO4) as a positive electrode material of lithium ion battery is considered to be one of the ideal positive electrode materials of new energy vehicles, because it has low cost, environmental friendliness, high safety and good electrochemical performance.

[0003] Lithium iron phosphate is the most popular positive electrode material of lithium ion battery in recent years, which has an extremely stable olivine structure. Compared with other lithium ion batteries with positive electrode materials, the battery prepared by using lithium iron phosphate has excellent safety and longer cycle life, and is considered to be the most likely to replace traditional fossil energy and be widely used in vehicle and energy storage fields. However, there are still some problems to be solved, such as the low intrinsic conductivity of lithium iron phosphate material and the small lithium ion diffusion coefficient, which makes the low-temperature performance of lithium iron phosphate battery poor and the large-rate performance general. This greatly limits its application in the fields of vehicle power battery (BEV, HEV), starting power supply, smart grid and the like.

[0004] Iron phosphate (FePO4) as a precursor of lithium iron phosphate, its preparation method is currently a hot research topic. The performance indicators such as purity, particle size, morphology and structure of iron phosphate play a crucial role in the electrochemical performance of the synthesized lithium iron phosphate material. The structure of the iron phosphate material prepared by the traditional method will not change in the subsequent processing process. In the final synthesis and manufacturing process of lithium iron phosphate, iron phosphate will be converted into lithium iron phosphate, and the crystal grain will continue to grow, and finally become a large-grained lithium iron phosphate material with a dense structure. Due to the slow migration speed of lithium ions, the high-rate performance and low-temperature performance of such lithium iron phosphate material are poor, and its low electronic conductivity and slow one-dimensional ion diffusion hinder its high-rate charge and discharge. SUMMARY

[0005] The present disclosure aims to at least solve one of the technical problems existing in the prior art. To this end, the present disclosure proposes a preparation method of core-shell structure iron phosphate and application thereof, which can improve the one-dimensional diffusion rate of Li ions and improve the rate performance of lithium iron phosphate.

[0006] According to one aspect of the present disclosure, a preparation method of core-shell structure iron phosphate is provided, comprising the following steps:

[0007] S1: mixing a surfactant, an organic iron source, an organic phosphorus source, a waste oil coagulant, and a liquid oil at 60-90°C, adding the obtained mixture to water, stirring at 60-90°C to form an emulsion;

[0008] S2: adjusting the pH of the emulsion to 0.8-1.5, adding a ferric salt solution and a phosphate solution to the emulsion respectively, and adjusting the pH to 1.5-2.2 to react;

[0009] S3: after the reaction in step S2 is completed, the reaction solution is naturally cooled and filtered to obtain a solid wet material;

[0010] 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 structure iron phosphate.

[0011] 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.

[0012] 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.

[0013] 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, tris(4-nonylphenyl) phosphite, or mono-n-dodecyl phosphate.

[0014] 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 3000-80000. The waste oil coagulant composed of a vinyl polymer with 30 mol% or more stearyl(meth)acrylate units has sufficient oil coagulation ability even at 50°C, so it can safely and easily solidify the emulsion particles inside the iron phosphate shell at low temperature. The waste oil coagulant of the present disclosure can be a commercially available waste oil coagulant or a self-made one, and the preparation method is described in patent JPH05311191A, which is not repeated here.

[0015] In some embodiments of the present disclosure, in step S1, the liquid oil is at least one of edible oil, engine oil, or synthetic mineral oil.

[0016] In some embodiments of the present disclosure, in step S1, the molar ratio of the organic phosphorus source to the organic iron source is 1:(0.8-1.2) in terms of phosphorus and iron; and the amount of the organic iron source and the liquid oil is 0.1-5 mol / L.

[0017] 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.

[0018] 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.

[0019] 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.

[0020] In some embodiments of the present disclosure, in step S2, the reaction temperature is 70-90℃.

[0021] In some embodiments of the present disclosure, in step S2, the molar ratio of the phosphate salt in the phosphate salt solution to the trivalent iron salt in the trivalent iron salt solution is 1:(0.95-1.1).

[0022] In some embodiments of the present disclosure, in step S2, the phosphate salt solution is at least one of ammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate or phosphoric acid solution; and the concentration of the phosphate salt in the phosphate salt solution is 0.1-5 mol / L.

[0023] In some embodiments of the present disclosure, in step S2, the trivalent iron salt solution is at least one of ferric nitrate or ferric chloride solution; and the concentration of the iron salt in the trivalent iron salt solution is 0.1-5 mol / L.

[0024] In some embodiments of the present disclosure, in step S4, the microwave input power for the microwave heating is 20-30 KW.

[0025] In some embodiments of the present disclosure, in step S4, the microwave heating time is 15-30 min.

[0026] In some embodiments of the present disclosure, in step S4, the particle size D50 of the core-shell structure iron phosphate is 1-7 μm. The particle size of the core-shell structure iron phosphate is controlled by controlling the diameter of the emulsion particles.

[0027] The present disclosure also provides the use of the preparation method in the preparation of lithium ion batteries.

[0028] According to the embodiments of this disclosure, at least the following beneficial effects are achieved:

[0029] 1. This disclosure involves dissolving organophosphates and organoferric compounds in liquid oil. The mixture is then mixed with water under the action of a surfactant and stirred at high speed to obtain an emulsion. A ferric salt solution and a phosphate solution are added to the emulsion to react. The resulting ferric phosphate precipitate adheres to the emulsion particles, forming a stable ferric phosphate shell on the outside and microspheres of the emulsion inside. After the reaction, the solution is cooled, and the internal emulsion microspheres solidify under the action of a waste oil coagulant, preventing the internal organoferric and organophosphate compounds from flowing out during filtration. Finally, microwave heating causes the internal organophosphates and organoferric compounds to react, resulting in a loosely structured ferric phosphate core. Microwave heating is characterized by rapid heating and a high reaction rate. The emulsion microspheres inside the particles heat up and decompose rapidly, and ferric phosphate begins to form during heating. The rapid decomposition of the emulsion helps the ferric phosphate to distribute evenly within the cavities of the particles, ultimately forming a loosely structured ferric phosphate core.

[0030] 2. The iron phosphate particles disclosed herein have a core-shell structure. The interior is a loose iron phosphate core formed by organic phosphorus and organic iron, and the exterior is a dense iron phosphate shell formed by inorganic phosphorus and inorganic iron. 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, improves the one-dimensional diffusion rate of lithium ions, and effectively overcomes the problem of capacity reduction caused by insufficient diffusion of lithium ions in the core, thereby improving the rate performance of the battery.

[0031] 3. Compared with hollow lithium iron phosphate, the core-shell structured lithium iron phosphate prepared in this disclosure has a loose iron iron phosphate core inside the iron phosphate shell, which increases the volumetric energy density during the preparation of lithium iron phosphate batteries. Detailed Implementation

[0032] The following will describe the concept and technical effects of this disclosure clearly and completely with reference to the embodiments, so as to fully understand the purpose, features and effects of this disclosure.

[0033] Example 1

[0034] This embodiment prepares a core-shell structured iron phosphate, and the specific process is as follows:

[0035] (1) Polyoxyethylene oleyl alcohol ether, trioctyl phosphate, waste oil coagulant and ferrocene were mixed into soybean oil. The waste oil coagulant was composed of a vinyl polymer with 40 mol% stearyl (meth) acrylate units and a number average molecular weight of 22,000. The molar ratio of trioctyl phosphate to ferrocene was 1:1 based on phosphorus and iron, respectively. The mass ratio of polyoxyethylene oleyl alcohol ether to soybean oil was 1:4. The concentration of ferrocene in soybean oil was 2 mol / L. The solid-liquid ratio of waste oil coagulant to soybean oil was 1 g:30 ml. The temperature was maintained at 60°C and the mixture was stirred until homogeneous. The mixture was then mixed with deionized water and stirred at 1600 rpm for 4 hours to form an emulsion. The total mass of soybean oil and polyoxyethylene oleyl alcohol ether accounted for 5% of the total mass of the emulsion.

[0036] (2) The pH of the emulsion was adjusted to 1.3 using phosphoric acid, followed by the addition of 1 mol / L ferric nitrate solution and then 1 mol / L ammonium hydrogen phosphate solution to adjust the pH to 1.5-2.2. The reaction was stirred and the reaction temperature was 70℃, and an iron phosphate shell was formed around the emulsion particles.

[0037] (3) After the reaction solution is cooled naturally, it is filtered out and the resulting solid wet material is placed in an oxygen atmosphere for microwave heating. The microwave input power is 20KW, the heating temperature is 600℃, and the heating time is 25min, to obtain a core-shell structured iron phosphate product with a particle size D50 of 3μm.

[0038] Example 2

[0039] This embodiment prepares a core-shell structured iron phosphate, and the specific process is as follows:

[0040] (1) Polyoxyethylene oleyl alcohol ether, trioctyl phosphate, waste oil coagulant and ferrocene were mixed into soybean oil. The waste oil coagulant was composed of a vinyl polymer with 40 mol% stearyl (meth) acrylate units and a number average molecular weight of 22,000. The molar ratio of trioctyl phosphate to ferrocene was 1:1 (phosphorus and iron, respectively). The mass ratio of polyoxyethylene oleyl alcohol ether to soybean oil was 1:9. The concentration of ferrocene in soybean oil was 2 mol / L. The solid-liquid ratio of waste oil coagulant to soybean oil was 1 g:30 ml. The temperature was maintained at 60°C and the mixture was stirred until homogeneous. The mixture was then mixed with deionized water and stirred at 1600 rpm for 5 h to form an emulsion. The total mass of soybean oil and polyoxyethylene oleyl alcohol ether accounted for 5% of the total mass of the emulsion.

[0041] (2) The pH of the emulsion was adjusted to 1.3 using phosphoric acid, followed by the addition of 1 mol / L ferric nitrate solution and then 1 mol / L ammonium hydrogen phosphate solution to adjust the pH to 1.5-2.2. The reaction was stirred and the reaction temperature was 70℃, and an iron phosphate shell was formed around the emulsion particles.

[0042] (3) After the reaction solution is cooled naturally, it is filtered out and the resulting solid wet material is placed in an oxygen atmosphere for microwave heating. The microwave input power is 20KW, the heating temperature is 600℃, and the heating time is 25min, to obtain a core-shell structured iron phosphate product with a particle size D50 of 6μm.

[0043] In this embodiment, the amount of surfactant added is relatively small, resulting in larger emulsion particle diameters and larger diameters of the iron phosphate particles. Larger diameters lead to larger voids and cavities, and less uniform distribution of iron phosphate internally, thus reducing capacity. (Example 3)

[0044] This embodiment prepares a core-shell structured iron phosphate, and the specific process is as follows:

[0045] (1) Polyoxyethylene oleyl alcohol ether, trioctyl phosphate, waste oil coagulant and ferrocene were mixed into soybean oil. The waste oil coagulant was composed of a vinyl polymer with 40 mol% stearyl (meth) acrylate units and a number average molecular weight of 22,000. The molar ratio of trioctyl phosphate to ferrocene was 1:1 (phosphorus and iron respectively). The mass ratio of polyoxyethylene oleyl alcohol ether to soybean oil was 1:1. The concentration of ferrocene in soybean oil was 2 mol / L. The solid-liquid ratio of waste oil coagulant to soybean oil was 1 g:30 ml. The temperature was maintained at 60°C and the mixture was stirred until homogeneous. The mixture was then mixed with deionized water and stirred at 1000 rpm for 2 hours to form an emulsion. The total mass of soybean oil and polyoxyethylene oleyl alcohol ether accounted for 5% of the total mass of the emulsion.

[0046] (2) The pH of the emulsion was adjusted to 1.3 using phosphoric acid, followed by the addition of 1 mol / L ferric nitrate solution and then 1 mol / L ammonium hydrogen phosphate solution to adjust the pH to 1.5-2.2. The reaction was stirred and the reaction temperature was 70℃, and an iron phosphate shell was formed around the emulsion particles.

[0047] (3) After the reaction solution is cooled naturally, it is filtered out and the resulting solid wet material is placed in an oxygen atmosphere for microwave heating. The microwave input power is 20KW, the heating temperature is 600℃, and the heating time is 25min, to obtain a core-shell structured iron phosphate product with a particle size D50 of 2μm.

[0048] In this embodiment, the amount of surfactant is relatively large, resulting in a smaller particle size D50 of the iron phosphate.

[0049] Example 4

[0050] This embodiment prepares a core-shell structured iron phosphate, and the specific process is as follows:

[0051] (1) Polyoxyethylene oleyl alcohol ether, trioctyl phosphate, waste oil coagulant and ferrocene were mixed into soybean oil. The waste oil coagulant was composed of a vinyl polymer with 40 mol% stearyl (meth) acrylate units and a number average molecular weight of 22,000. The molar ratio of trioctyl phosphate to ferrocene was 1:1 based on phosphorus and iron, respectively. The mass ratio of polyoxyethylene oleyl alcohol ether to soybean oil was 1:4. The concentration of ferrocene in soybean oil was 2 mol / L. The solid-liquid ratio of waste oil coagulant to soybean oil was 1 g:30 ml. The temperature was maintained at 60°C and the mixture was stirred until homogeneous. The mixture was then mixed with deionized water and stirred at 800 rpm for 5 h to form an emulsion. The total mass of soybean oil and polyoxyethylene oleyl alcohol ether accounted for 5% of the total mass of the emulsion.

[0052] (2) The pH of the emulsion was adjusted to 1.3 using phosphoric acid, followed by the addition of 1 mol / L ferric nitrate solution and then 1 mol / L ammonium hydrogen phosphate solution to adjust the pH to 1.5-2.2. The reaction was stirred and the reaction temperature was 70℃, and an iron phosphate shell was formed around the emulsion particles.

[0053] (3) After the reaction solution is cooled naturally, it is filtered out and the resulting solid wet material is placed in an oxygen atmosphere for microwave heating. The microwave input power is 20KW, the heating temperature is 600℃, and the heating time is 20min, to obtain a core-shell structured iron phosphate product with a particle size D50 of 7μm.

[0054] The cutting speed that forms the emulsion affects the particle size. In this embodiment, the cutting speed is relatively low, resulting in an unstable emulsion with a larger particle size and smaller volume.

[0055] Example 5

[0056] This embodiment prepares a core-shell structured iron phosphate, and the specific process is as follows:

[0057] (1) Polyoxyethylene oleyl alcohol ether, trioctyl phosphate, waste oil coagulant and ferrocene were mixed into soybean oil. The waste oil coagulant was composed of a vinyl polymer with 40 mol% stearyl (meth) acrylate units and a number average molecular weight of 22,000. The molar ratio of trioctyl phosphate to ferrocene was 1:1 (phosphorus and iron respectively). The mass ratio of polyoxyethylene oleyl alcohol ether to soybean oil was 1:1. The concentration of ferrocene in soybean oil was 2 mol / L. The solid-liquid ratio of waste oil coagulant to soybean oil was 1 g:30 ml. The temperature was maintained at 60°C and the mixture was stirred until homogeneous. The mixture was then mixed with deionized water and stirred at 2000 rpm for 3 hours to form an emulsion. The total mass of soybean oil and polyoxyethylene oleyl alcohol ether accounted for 5% of the total mass of the emulsion.

[0058] (2) The pH of the emulsion was adjusted to 1.3 using phosphoric acid, followed by the addition of 1 mol / L ferric nitrate solution and then 1 mol / L ammonium hydrogen phosphate solution to adjust the pH to 1.5-2.2. The reaction was stirred and the reaction temperature was 70℃, and an iron phosphate shell was formed around the emulsion particles.

[0059] (3) After the reaction solution is cooled naturally, it is filtered out and the resulting solid wet material is placed in an oxygen atmosphere for microwave heating. The microwave input power is 20KW, the heating temperature is 600℃, and the heating time is 25min, to obtain a core-shell structured iron phosphate product with a particle size D50 of 2μm.

[0060] The cutting speed that forms the emulsion affects the particle size. In this embodiment, the cutting speed is relatively high, the particle size is relatively small, and the volume is relatively large.

[0061] Comparative Example 1

[0062] This comparative example prepared a hollow iron phosphate, which differs from Example 1 in that it did not contain organic phosphorus and organic iron. The specific process is as follows:

[0063] Polyoxyethylene oleyl alcohol ether was mixed into soybean oil at a mass ratio of 1:4, and the mixture was stirred until homogeneous. The mixture was then combined with deionized water and stirred at 1600 rpm for 4 hours to form an emulsion, in which the total mass of soybean oil and polyoxyethylene oleyl alcohol ether accounted for 5% of the total mass of the emulsion. The pH of the emulsion was adjusted to 1.3 using phosphoric acid, followed by the addition of 1 mol / L ferric nitrate solution and then 1 mol / L ammonium hydrogen phosphate solution to adjust the pH to 1.5-2.2. The mixture was stirred and reacted at a temperature of 70°C, forming an iron phosphate shell around the emulsion particles. After the reaction solution was naturally cooled, it was filtered out, and the resulting solid wet material was placed in an oxygen atmosphere for microwave heating at a microwave input power of 20 kW, a heating temperature of 600°C, and a heating time of 25 min to obtain hollow iron phosphate with a particle size D50 of 3 μm.

[0064] Test case

[0065] The iron phosphate and Li2CO3 of the above embodiment were loaded into a ball mill and ball-milled with anhydrous ethanol at a speed of 500 rpm for 6 hours. The prepared iron phosphate was calcined at 700°C under a nitrogen atmosphere with glucose, lithium carbonate and the prepared iron phosphate in a ratio of 0.05:1.05:1.0.

[0066] 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 then uniformly coated onto an aluminum foil substrate to serve as the cathode of the simulated battery. The anode 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 (volume ratio 1:1), and the separator was a PE ion exchange membrane. The cathode, anode, electrolyte, and separator were assembled into a simulated battery within an argon-protected glove box.

[0067] Simulated battery rate testing steps:

[0068] First, the battery is charged to 4.2V at 30mA / g, then discharged to 2.0V at a higher rate. The capacity discharged is the discharge capacity at that rate. After discharging, the battery is discharged again at 30mA / g to 2.0V. Then, the test is performed at the next higher rate. The test results of this simulated battery are listed in Table 1.

[0069] Table 1

[0070]

[0071] As shown in Examples 1-5, the amount of surfactant added affects the particle size of the emulsion particles formed. The greater 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. Simultaneously, the cutting speed also has a certain influence on the particle size of the emulsion particles. The greater the cutting speed, the smaller the particle size of the emulsion particles; the smaller the cutting speed, the larger the particle size of the emulsion particles formed.

[0072] The volumetric energy density was measured using a vernier caliper to test the volumetric capacity difference between Example 1 and Comparative Example 1.

[0073] The volumetric energy density of Example 1 was measured to be 285 kWh / m³. 3 The volumetric energy density of Comparative Example 1 is 180 kWh / m³. 3 This indicates that the core-shell lithium iron phosphate material disclosed herein has a higher volumetric energy density compared to hollow lithium iron phosphate materials.

[0074] The embodiments of this disclosure have been described in detail above. However, this disclosure is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of this disclosure. Furthermore, the embodiments and features described herein can be combined with each other unless otherwise specified.

Claims

1. A method for preparing core-shell structured iron phosphate, characterized in that, Includes the following steps: S1: Mix surfactant, organic iron source, organic phosphorus source, waste oil coagulant and liquid oil at 60-90℃, add the resulting mixture to water, and stir at 60-90℃ to form an emulsion; S2: Adjust the pH of the emulsion to 0.8-1.5, add ferric salt solution and phosphate solution to the emulsion respectively, and then adjust the pH to 1.5-2.2 to carry out the reaction; S3: After the reaction described in step S2 is completed, the reaction solution is naturally cooled and filtered to obtain a solid wet material; S4: The solid wet material is microwave heated in an oxygen atmosphere at a temperature of 500-800℃ to obtain the core-shell structured iron phosphate. In step S1, the surfactant is at least one of dodecylbenzenesulfonic acid, polyoxyethylene dehydrated sorbitan monooleate, fatty alcohol polyoxyethylene ether, polyoxyethylene dehydrated sorbitan monolaurate, polyoxyethylene oleyl alcohol ether, polyoxyethylene castor oil, polyoxyethylene oleyl ether, or polyoxyethylene lauryl ether; the organic iron source is at least one of ferrocene, ferric oleate, ferric naphthenate, or ferric 2-ethylhexanoate; and the organic phosphorus source is at least one of di(2-ethylhexyl) phosphate, trioctyl phosphate, triisopropyl phosphite, isooctyl phosphate, tris(4-nonylphenyl) phosphite, or mono-n-dodecyl phosphate. In step S2, the reaction temperature is 70-90℃; the molar ratio of phosphate in the phosphate solution to ferric salt in the ferric salt solution is 1:(0.95-1.1); the phosphate solution is at least one of ammonium hydrogen phosphate, ammonium dihydrogen phosphate, ammonium phosphate, or phosphoric acid solution; the concentration of phosphate in the phosphate solution is 0.1-5 mol / L; the ferric salt solution is at least one of ferric nitrate or ferric chloride solution; the concentration of ferric salt in the ferric salt solution is 0.1-5 mol / L.

2. 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 a number average molecular weight of 3000-80000.

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

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

5. 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.

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

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

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

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

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

11. The application of the preparation method according to any one of claims 1-10 in the preparation of lithium-ion batteries.

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