WO2025091236A1 - Modified iron phosphate, and preparation method therefor and use thereof - Google Patents

Abstract

Modified iron phosphate, and a preparation method therefor and the use thereof. The preparation method comprises the following steps: (1) mixing starch and water to obtain a starch suspension, injecting the starch suspension, an iron source solution and a phosphate solution into a reaction device in a parallel flow manner, controlling the pH value in the system to perform a first-step reaction, and performing solid-liquid separation, so as to obtain a precipitate; (2) mixing the precipitate with an amylase solution, and heating same to perform a second-step reaction, so as to obtain a precursor; and (3) sintering the precursor, so as to obtain modified iron phosphate. By introducing starch during the co-precipitation process of iron phosphate, the starch is continuously and slowly hydrolyzed under acidic conditions and is then thoroughly hydrolyzed by means of amylase, and finally, iron phosphate of a porous hollow structure is spontaneously synthesized after the complete hydrolysis thereof, thereby shortening the diffusion path of lithium ions and improving the electrical properties.

Description

A modified ferric phosphate and its preparation method and application Technical Field

The present invention belongs to the technical field of battery materials and relates to a modified iron phosphate and a preparation method and application thereof.

Background Art

Lithium-ion batteries (LIBs) are an important rechargeable battery technology currently, and are widely used in mobile electronic devices, electric vehicles, and energy storage systems. Compared with other rechargeable battery technologies, lithium-ion batteries have many advantages, such as high energy density, which can store more energy; long cycle life, which can be charged and discharged for many times; fast charging speed, which can usually be completed within a few hours. As a leader in performance among lithium-ion batteries and with low raw material costs, lithium iron phosphate (LiFePO ₄ ) has attracted widespread attention and has become a commercial darling in recent years. However, LiFePO ₄ also has its own defects. Its excellent cycle performance is due to its stable olivine crystal structure, but it is also because of this structure that its lithium ions can only diffuse in one-dimensional channels, resulting in slow lithium ion and electron transmission rates. These problems have greatly limited the electrical performance of lithium iron phosphate batteries.

The most widely used method for synthesizing lithium iron phosphate is to use iron phosphate as raw material and react with lithium source for sintering. This method has the advantages of high purity, stable performance, simple operation, etc., and can be applied to large-scale industrial production. The microstructure and chemical composition of iron phosphate will have a huge impact on the performance of LiFePO ₄ /C, so the preparation of excellent iron phosphate is very important for lithium-ion batteries. Most methods to improve the lithium ion transmission rate use porous structure morphology to shorten the diffusion path of lithium ions.

CN111362243A introduces a silicon dioxide core and uses it as a carrier and template to prepare iron phosphate, and then transfers it to a hydrofluoric acid solution to etch away the template to obtain a hollow iron phosphate and then synthesize a lithium iron phosphate material. This method can improve the electrochemical performance of the battery, but this method introduces additional templates and template removal agents to increase the preparation cost, and hydrofluoric acid is a strong acid with strong corrosiveness and limited operation.

CN109052358A discloses a method for preparing mesoporous-macroporous ferric phosphate, comprising the following steps: S1, dissolving P123 as a template in an acidic solution, and stirring until the solution is clear; S2, preparing a ferrous salt solution, mixing the ferrous salt solution with phosphoric acid in proportion, and obtaining an iron salt base solution; weighing phosphate according to the molar ratio of total iron to total phosphorus, dissolving the weighed phosphate and adding excess hydrogen peroxide to obtain a phosphate solution; S3, adding the phosphate solution to the iron salt base solution, and slowly adding the P123 solution treated in step S1; heating up after the addition is completed, reducing the stirring speed after the heating is completed, and keeping the temperature under uniform stirring for reaction; S4, filtering and washing the product obtained after the reaction in step S3, and then calcining to remove the template to obtain anhydrous ferric phosphate.

The above scheme introduces templates and removes templates, which increases the process time and cost, and the structural stability of the iron phosphate obtained is poor. Therefore, seeking a simple method for preparing porous iron phosphate is of great significance for the industrialization and commercialization of lithium-ion batteries.

Summary of the invention

The following is a summary of the subject matter described in detail herein. This summary is not intended to limit the scope of the claims.

The purpose of the present disclosure is to provide a modified ferric phosphate and a preparation method and application thereof. The present disclosure introduces starch during the co-precipitation of ferric phosphate, and the starch is slowly hydrolyzed under acidic conditions. Then, amylase is used to completely hydrolyze the starch, and finally, the hydrolysis is completely and spontaneously used to synthesize ferric phosphate with a porous hollow structure, thereby shortening the diffusion path of lithium ions and improving electrical performance.

To achieve this purpose, the present invention adopts the following technical solutions:

In a first aspect, the present disclosure provides a method for preparing modified ferric phosphate, the preparation method comprising the following steps:

(1) Mix starch and water to obtain a starch suspension, inject the starch suspension, an iron source solution and a phosphate solution into a reaction device in parallel, control the pH in the system to perform a one-step reaction, separate the solid and the liquid to obtain a precipitate Sediment;

(2) mixing the precipitate with an amylase solution and heating the mixture to perform a two-step reaction to obtain a precursor;

(3) Sintering the precursor to obtain the modified iron phosphate.

The present invention adopts starch as the core structure of the hollow structure of iron phosphate, and then utilizes the hydrolysis property of starch itself to slowly hydrolyze the insoluble matter into glucose and dissolve in the aqueous solution. In the process of co-precipitation of iron phosphate, starch coexists with iron phosphate from an insoluble state to a gradual hydrolysis process, and the two are in a state of coating each other. As the reaction proceeds, the starch gradually hydrolyzes the iron phosphate into a hollow porous structure. When the starch is in a gelatinized state, the viscosity increases, which acts as a barrier to suppress the agglomeration of particles during the pre-synthesis of the material.

In one embodiment, the starch in step (1) includes any one of corn starch, potato starch or tapioca starch, or a combination of at least two of them.

In one embodiment, the starch has a particle size of 50 to 250 nm, for example, 50 nm, 80 nm, 100 nm, 150 nm or 250 nm.

In one embodiment, the mass concentration of the starch suspension is 2-15%, for example, 2%, 5%, 8%, 10% or 15%.

The present invention can control the hydrolysis rate of starch and the pore size of generated iron phosphate by controlling the particle size and concentration of starch in the starch suspension.

In one embodiment, the starch suspension is subjected to ultrasonic treatment.

In one embodiment, the ultrasonic treatment time is 0.5 to 2 h, for example, 0.5 h, 0.8 h, 1 h, 1.5 h or 2 h.

The present invention pre-ultrasonicates the starch suspension to effectively improve the dispersibility of the starch suspension.

In one embodiment, the solute of the iron source solution in step (1) includes ferric nitrate and/or ferric chloride.

In one embodiment, the concentration of the iron source solution is 0.5 to 2.5 mol/L, for example: 0.5 mol/L, 1mol/L, 1.5mol/L, 2mol/L or 2.5mol/L, etc.

In one embodiment, the solute of the phosphate solution includes any one of diammonium phosphate, ammonium hydrogen phosphate, phosphoric acid, sodium dihydrogen phosphate or sodium hydrogen phosphate, or a combination of at least two thereof.

In one embodiment, the concentration of the phosphate solution is 0.5 to 2.5 mol/L, for example, 0.5 mol/L, 1 mol/L, 1.5 mol/L, 2 mol/L or 2.5 mol/L.

In one embodiment, the molar ratio of the iron element in the iron source solution to the phosphate in the phosphate solution is (0.97-1.02):1, for example: 0.97:1, 0.99:1, 1:1, 1.01:1 or 1.02:1, etc.

The present invention can prepare iron phosphate with a suitable phosphorus-iron ratio and avoid the generation of impurities by controlling the molar ratio of the iron element in the iron source solution and the phosphate radical in the phosphate solution within the above range.

In one embodiment, the reaction device in step (1) includes a clearing device, a material receiving device and a reactor.

In one embodiment, the feed rate and the clearing device are controlled to ensure that the concentration of the starch suspension in the reactor remains constant, and the slurry precipitated at the bottom of the reactor is returned to the reactor through the feed inlet to continue growing, thereby achieving internal circulation of the material and improving the material recovery rate.

The present disclosure uses the above-mentioned device to improve production efficiency, realize internal circulation of materials and improve material recovery rate.

In one embodiment, the method for controlling the pH in the system comprises adding an alkaline solution.

The present invention can generate a precipitate by adding an alkaline solution to adjust the pH, and then carry out subsequent processes through post-treatment such as filtration and washing.

In one embodiment, the alkaline solution comprises aqueous ammonia.

In one embodiment, the pH is 1.4 to 2.2, for example, 1.4, 1.6, 1.8, 2, 2.1 or 2.2, etc.

In one embodiment, during the one-step reaction, the mass concentration of starch in the system is 2～10%, for example: 2%, 4%, 6%, 8% or 10%, etc.

In the one-step reaction process disclosed in the present invention, the concentration of starch in the reaction system will affect its performance. When the concentration of starch is controlled at 2-10%, the performance of the modified iron phosphate obtained is better. If the concentration of starch is too low and the starch content in the solution is too little, the hydrolysis pore-forming effect cannot be achieved, and the tight stacking performance of the obtained iron phosphate is poor. If the concentration of starch is too high, the modified iron phosphate has a porous structure of uneven size, the volume increases, and the active substance of the material decreases, which affects the battery capacity and performance of the later synthesized lithium iron phosphate.

In one embodiment, the aging temperature of the one-step reaction is 50-90°C, for example, 50°C, 60°C, 70°C, 80°C or 90°C.

In one embodiment, the aging time of the one-step reaction is 3 to 12 hours, for example, 3 hours, 5 hours, 8 hours, 10 hours or 12 hours.

The present invention can improve the crystal structure stability of the material through aging.

In one embodiment, the mass concentration of the amylase solution in step (2) is 0.5-2.5%, for example: 0.5%, 1%, 1.5%, 2% or 2.5%, etc.

In one embodiment, the mass ratio of the precipitate to the amylase solution is 1:(1-3), for example: 1:1, 1:1.5, 1:2, 1:2.5 or 1:3, etc.

The present invention controls the mass concentration of the amylase solution and the mass ratio of the precipitate to the amylase solution within the above ranges, thereby controlling the hydrolysis rate of starch and producing iron phosphate having a loose, hollow and porous structure and good stability.

In one embodiment, the temperature of the two-step reaction is 35-75°C, for example, 35°C, 40°C, 50°C, 60°C or 75°C.

In one embodiment, ultrasonic treatment is performed during the two-step reaction.

In one embodiment, the two-step reaction time is 15 to 90 min, for example, 15 min, 20 min, 40 min, 50 min or 90 min.

In one embodiment, the sintering temperature in step (3) is 500-550°C, for example, 500°C, 510°C, 520°C, 540°C or 550°C.

The present invention discloses sintering at a relatively low temperature to remove crystal water (the conventional method of sintering to form pores using carbon materials as templates, when the sintering temperature is too high, some pores inside the material will collapse, thereby causing the material structure stability to deteriorate), which can reduce the impact of high temperature on the material structure and avoid problems such as pore collapse caused by high-temperature sintering.

In one embodiment, the sintering treatment time is 4 to 8 hours, for example: 4 hours, 5 hours, 6 hours, 7 hours or 8 hours.

In a second aspect, the present disclosure provides a modified iron phosphate, wherein the modified iron phosphate is prepared by the method described in the first aspect.

The iron phosphate prepared by the method disclosed in the present invention has a loose, hollow and porous structure, an increased specific surface area and a more sufficient contact area, a reduced ion diffusion path and an increased diffusion path, and the lithium iron phosphate prepared thereby also has a loose, porous, hollow structure. The presence of this structure can greatly shorten the diffusion path of lithium ions, increase the lithium ion transmission speed and thereby enhance the electrochemical performance of the battery.

In a third aspect, the present disclosure provides a lithium iron phosphate, which is prepared by mixing the modified iron phosphate as described in the second aspect with a lithium source and a carbon source and subjecting the mixture to a calcination treatment.

In one embodiment, the lithium source includes any one of lithium carbonate, lithium hydroxide or lithium acetate, or a combination of at least two thereof.

In one embodiment, the carbon source includes any one of glucose, sucrose, phenolic resin, starch, dextrin, citric acid, oxalic acid, cellulose or vitamins, or a combination of at least two thereof.

In one embodiment, the mass of the carbon source is 3% to 15% of the total mass of the iron phosphate and the lithium source, for example, 3%, 5%, 8%, 10% or 15%.

In one embodiment, the mixing comprises mixing the modified iron phosphate with the lithium source and grinding it and then adding carbon source.

In one embodiment, the calcination process includes one-step calcination and two-step calcination.

The present invention discloses a lithium iron phosphate positive electrode material having both capacity performance and cycle performance, through a two-step calcination process without affecting the material structure.

In one embodiment, the temperature of the one-step calcination is 300-500°C, for example, 300°C, 350°C, 450°C, 480°C or 500°C.

In one embodiment, the one-step calcination time is 0.5 to 3 hours, for example, 0.5 hours, 0.85 hours, 1 hour, 2 hours or 3 hours.

In one embodiment, the temperature of the two-step calcination is 500-850°C, for example, 500°C, 600°C, 700°C, 800°C or 850°C.

In one embodiment, the two-step calcination time is 6 to 18 hours, for example: 6 hours, 8 hours, 10 hours, 15 hours or 18 hours.

In a fourth aspect, the present disclosure provides a lithium-ion battery, wherein the lithium-ion battery comprises the lithium iron phosphate as described in the third aspect.

Compared with the prior art, the present invention has the following beneficial effects:

(1) The method disclosed in the present invention is simple and easy to implement, and avoids the introduction of templates and template removal agents to increase unnecessary component introduction and preparation steps. In addition, by controlling the degree of starch hydrolysis and the number of subsequent washings, part of the hydrolysis product can be retained on the iron phosphate, and after high-temperature calcination, carbon is formed to in situ coat the iron phosphate to improve conductivity.

(2) The iron phosphate prepared by the method disclosed in the present invention has a loose, hollow and porous structure, with an increased specific surface area and a more sufficient contact area, and the ion diffusion path is shortened and the diffusion path is increased. The lithium iron phosphate prepared by the method also has a loose, porous, hollow structure. The existence of this structure can greatly shorten the diffusion path of lithium ions, increase the lithium ion transmission speed, and thereby improve the electrochemical performance of the battery.

(3) The battery made of the modified iron phosphate disclosed in the present invention has a 0.1C discharge capacity of more than 156.9 mAh/g, a 0.5C discharge capacity of more than 150.4 mAh/g, a 1C discharge capacity of more than 145.9 mAh/g, and a 2C discharge capacity of more than 141.2 mAh/g.

Other aspects will be apparent upon reading and understanding the drawings and detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are used to provide further understanding of the technical solution of this article and constitute a part of the specification. Together with the embodiments of the present application, they are used to explain the technical solution of this article and do not constitute a limitation on the technical solution of this article.

FIG. 1 is a 5000-fold SEM image of the modified iron phosphate described in Example 1 of the present disclosure.

DETAILED DESCRIPTION

The technical solution of the present disclosure is further described below through specific implementation methods. Those skilled in the art should understand that the embodiments are only to help understand the present disclosure and should not be regarded as specific limitations of the present disclosure.

Example 1

This embodiment provides a modified iron phosphate, and the preparation method of the modified iron phosphate is as follows:

(1) Under ultrasonic conditions, potato starch with a particle size of 150 nm is uniformly dispersed in deionized water to prepare a starch suspension with a concentration of 8%, iron chloride and ammonium dihydrogen phosphate are weighed to prepare 1 mol/L iron chloride and 1 mol/L ammonium dihydrogen phosphate solutions, respectively, and the iron chloride solution and the ammonium dihydrogen phosphate solution are slowly injected into a stirring tank at a molar ratio of 1:1, and stirred at a stirring speed of 1200 rpm until the mixture is uniformly mixed, and the starch suspension is injected at a controlled flow rate so that the starch concentration in the reaction tank is always maintained at 3%, and 25% ammonia water is slowly added dropwise to adjust the solution to pH = 1.6, and the mixture is heated to 55° C. with stirring for 8 hours to obtain a precipitate;

(2) placing the precipitate in a 1% amylase solution, controlling the pH to 5, heating to 60° C. and maintaining the temperature for 30 min, and using ultrasound to promote the final hydrolysis of starch into glucose to obtain a precursor;

(3) The precursor was washed four times with deionized water, dried in a vacuum drying oven at 85° C. for 8 h, placed in a muffle furnace, heated to 520° C. at a heating rate of 5° C./min, and kept warm for 6 h to obtain the modified iron phosphate. The SEM image of the modified iron phosphate is shown in FIG1 .

Example 2

This embodiment provides a modified iron phosphate, and the preparation method of the modified iron phosphate is as follows:

(1) Under ultrasonic conditions, corn starch with a particle size of 50 nm is uniformly dispersed in deionized water to prepare a 5% starch suspension, iron nitrate and ammonium hydrogen phosphate are weighed to prepare 1 mol/L iron nitrate and 1 mol/L ammonium hydrogen phosphate solutions, respectively, and the iron nitrate solution and the ammonium hydrogen phosphate solution are slowly injected into a stirring tank at a molar ratio of 1:1, and stirred at a stirring speed of 1200 rpm until the mixture is uniformly mixed, and at the same time, the starch suspension is injected to control the flow rate so that the starch concentration in the reaction tank is always maintained at 5%, and at the same time, 25% ammonia water is slowly added dropwise to adjust the solution to pH = 2.0, and the mixture is heated to 90° C. with stirring for 3 h to obtain a precipitate;

(2) placing the precipitate in a 2% amylase solution, controlling the pH to 5, heating to 40° C. and maintaining the temperature for 80 min, and using ultrasound to promote the final hydrolysis of starch into glucose to obtain a precursor;

(3) The precursor was washed four times with deionized water, dried in a vacuum drying oven at 85° C. for 8 h, placed in a muffle furnace, heated to 500° C. at a heating rate of 5° C./min, and kept warm for 8 h to obtain the modified iron phosphate.

Example 3

This embodiment provides a modified iron phosphate, and the preparation method of the modified iron phosphate is as follows:

(1) Under ultrasonic conditions, cassava starch is uniformly dispersed in deionized water to prepare a starch suspension with a concentration of 15%, ferric chloride and diammonium phosphate are weighed to prepare 1 mol/L ferric chloride and 1 mol/L diammonium phosphate solutions, respectively, and the ferric chloride solution and the diammonium phosphate solution are slowly injected into a stirring tank at a molar ratio of 1:1, and stirred at a stirring speed of 1200 rpm until the mixture is uniformly mixed, and the starch suspension is injected to control the concentration of the starch suspension. The flow rate was set so that the starch concentration in the reactor was always maintained at 8%, and 25% ammonia water was slowly added dropwise to adjust the solution to pH = 1.4, and the mixture was stirred and heated to 50°C for 12 hours to obtain a precipitate;

(2) placing the precipitate in a 2.5% amylase solution, controlling the pH to 5, heating to 75° C. and maintaining the temperature for 25 min, and using ultrasound to promote the final hydrolysis of starch into glucose to obtain a precursor;

(3) The precursor is washed four times with deionized water, dried in a vacuum drying oven at 85° C. for 8 h, placed in a muffle furnace, heated to 550° C. at a heating rate of 5° C./min, and kept warm for 4 h to obtain the modified iron phosphate.

Example 4

The only difference between this embodiment and embodiment 1 is that the concentration of starch in the reactor is 1%, and the other conditions and parameters are exactly the same as those in embodiment 1.

Example 5

The only difference between this embodiment and embodiment 1 is that the concentration of starch in the reactor is 15%, and the other conditions and parameters are exactly the same as those in embodiment 1.

Comparative Example 1

The only difference between this comparative example and Example 1 is that no amylase is added, and other conditions and parameters are exactly the same as those in Example 1.

Comparative Example 2

This comparative example provides a kind of iron phosphate, and the preparation method of the iron phosphate is as follows:

(1) Weighing ferric chloride and ammonium dihydrogen phosphate to prepare 1 mol/L ferric chloride and 1 mol/L ammonium dihydrogen phosphate solutions, respectively, mixing the ferric chloride solution and the ammonium dihydrogen phosphate solution at a molar ratio of 1:1, and stirring at a stirring speed of 1200 rpm until the mixture is uniformly mixed;

(2) slowly adding 25% ammonia water to adjust the solution to pH = 1.8, stirring and heating to 60° C. for 8 hours to obtain a precipitate, and filtering the precipitate;

(3) The filtered precipitate was washed three times with deionized water, and dried in a vacuum drying oven at 85° C. for 8 h to obtain a precursor. The precursor prepared above was then placed in a muffle furnace, heated to 550° C. at a heating rate of 5° C./min, and kept warm for 8 h to obtain the iron phosphate.

Performance Test:

The modified iron phosphate prepared in the embodiment and the comparative example is uniformly dispersed in water according to the stoichiometric ratio of lithium, iron and carbon elements of 1:1.03:0.07, ball milled at a speed of 4000rpm for 5h until uniformly mixed, and then spray dried to obtain a precursor powder. The precursor powder is kept at 400℃ for 1.5h at a heating rate of 5℃/min under a nitrogen atmosphere, and then heated to 750℃ for high temperature calcination for 10h to obtain lithium iron phosphate, and the lithium iron phosphate is uniformly mixed with the conductive agent acetylene black and the adhesive polyvinylidene fluoride in a ratio of 92:4:4 in N-methylpyrrolidone, and then coated on aluminum foil and placed in a vacuum drying oven for drying. After drying, the battery is assembled in an argon glove box, pressed into a positive electrode sheet by a tablet press, the negative electrode is a metal lithium sheet, the electrolyte is 1M _LiPF6 -EC:DMC (volume ratio 1:1), and a polypropylene porous membrane is used as a diaphragm. The charge and discharge voltage was controlled between 2.5-4.5V to test its electrochemical performance. The test results are shown in Table 1:

Table 1

As can be seen from Table 1, from Examples 1-3, the battery prepared by the modified iron phosphate disclosed in the present invention has a 0.1C discharge capacity of more than 156.9 mAh/g, a 0.5C discharge capacity of more than 150.4 mAh/g, a 1C discharge capacity of more than 145.9 mAh/g, and a 2C discharge capacity of more than 141.2 mAh/g.

By comparing Example 1 with Examples 4-5, it can be seen that in the preparation process of the modified iron phosphate described in the present invention, the concentration of starch in the reaction system will affect its performance. When the concentration of starch is controlled at 2-10%, the performance of the modified iron phosphate obtained is better. If the concentration of starch is too low and the starch content in the solution is too little, the hydrolysis pore-forming effect cannot be achieved, and the tight stacking performance of the obtained iron phosphate is poor. If the concentration of starch is too high, the modified iron phosphate has a porous structure of uneven size, the volume becomes larger, and the active substance of the material becomes less, which affects the battery capacity and performance of the later synthesized lithium iron phosphate.

By comparing Example 1 and Comparative Example 1, it can be seen that when amylase is not added, starch is not completely hydrolyzed and will remain in the iron phosphate and will be removed by high-temperature dehydration later, but it will affect the crystallinity of the iron phosphate. Compared with iron phosphate with residual starch, iron phosphate without starch has better crystallinity after high-temperature dehydration.

By comparing Example 1 and Comparative Example 2, it can be seen that the present invention introduces starch during the co-precipitation of iron phosphate, the starch is slowly hydrolyzed under acidic conditions, and then the starch is completely hydrolyzed using amylase, and finally the hydrolysis is completely and spontaneously synthesized into iron phosphate with a porous hollow structure, thereby shortening the lithium ion diffusion path and improving the electrical performance.

Claims (17)

A method for preparing modified ferric phosphate comprises the following steps: (1) mixing starch and water to obtain a starch suspension, injecting the starch suspension, an iron source solution and a phosphate solution into a reaction device in parallel, controlling the pH in the system to perform a one-step reaction, and performing solid-liquid separation to obtain a precipitate; (2) mixing the precipitate with an amylase solution and heating the mixture to perform a two-step reaction to obtain a precursor; (3) Sintering the precursor to obtain the modified iron phosphate. The preparation method according to claim 1, wherein the particle size of the starch in step (1) is 50 to 250 nm. The preparation method according to claim 1 or 2, wherein the mass concentration of the starch suspension is 2 to 15%. The preparation method according to any one of claims 1 to 3, wherein in step (1), the starch suspension is subjected to ultrasonic treatment; And/or, the ultrasonic treatment time is 0.5 to 2 hours. The preparation method according to any one of claims 1 to 4, wherein the molar ratio of the iron element in the iron source solution in step (1) to the phosphate in the phosphate solution is (0.97 to 1.02):1. The preparation method according to any one of claims 1 to 5, wherein the reaction device in step (1) comprises a clearing device, a material receiving device and a reaction kettle; And/or, the feed rate and the clearing device are controlled to ensure that the concentration of the starch suspension in the reactor remains constant, and the slurry precipitated at the bottom of the reactor is returned to the reactor through the feed inlet to continue growing, thereby achieving internal circulation of the material and improving the material recovery rate. The preparation method according to any one of claims 1 to 6, wherein the method for controlling the pH in the system in step (1) comprises adding an alkaline solution; And/or, the pH is 1.4 to 2.2. The preparation method according to any one of claims 1 to 7, wherein during the one-step reaction in step (1), the mass concentration of starch in the system is 2 to 10%. The preparation method according to any one of claims 1 to 8, wherein the aging temperature of the one-step reaction in step (12) is 50 to 90° C.; And/or, the aging time of the one-step reaction is 3 to 12 hours. The preparation method according to any one of claims 1 to 9, wherein the mass concentration of the amylase solution in step (2) is 0.5 to 2.5%. The preparation method according to any one of claims 1 to 9, wherein the mass ratio of the precipitate to the amylase solution in step (2) is 1:(1 to 3). The preparation method according to any one of claims 1 to 11, wherein the temperature of the second step reaction in step (2) is 35 to 75° C.; And/or, ultrasonic treatment is performed during the two-step reaction; And/or, the time of the two-step reaction is 15 to 90 minutes. The preparation method according to any one of claims 1 to 12, wherein the temperature of the sintering treatment in step (3) is 500 to 550° C.; And/or, the sintering treatment time is 4 to 8 hours. A modified iron phosphate prepared by the method according to any one of claims 1 to 13. A lithium iron phosphate, wherein the lithium iron phosphate is prepared by mixing the modified iron phosphate as claimed in claim 14 with a lithium source and a carbon source and subjecting the mixture to a calcination treatment. The lithium iron phosphate according to claim 15, wherein the calcination treatment comprises a one-step calcination and a two-step calcination; And/or, the temperature of the one-step calcination is 300-500°C; And/or, the one-step calcination time is 0.5 to 3 hours; And/or, the temperature of the second-step calcination is 500-850°C; And/or, the time of the second-step calcination is 6 to 18 hours. A lithium ion battery comprising the lithium iron phosphate as claimed in claim 15 or 16.