CN109950514B

CN109950514B - Preparation method of lithium iron phosphate coated with lithium ferrite

Info

Publication number: CN109950514B
Application number: CN201910328849.4A
Authority: CN
Inventors: 王柯娜
Original assignee: Individual
Current assignee: Sichuan Lomon Phosphorous Chemistry Co ltd
Priority date: 2019-04-23
Filing date: 2019-04-23
Publication date: 2020-09-25
Anticipated expiration: 2039-04-23
Also published as: CN109950514A

Abstract

The invention discloses a preparation method of lithium ferrite coated lithium iron phosphate. Adding a ferrous sulfate solution, a lithium hydroxide solution, an ammonium monohydrogen phosphate solution and a titanyl sulfate solution into a high-pressure reaction kettle together, and carrying out hydrothermal reaction to obtain a slurry; adding a dispersing agent into the slurry, adding a ferric chloride solution, an ammonium bicarbonate solution and a lithium hydroxide solution into the base solution in a parallel flow manner, introducing carbon dioxide after the addition is finished, filtering, heating and washing filter residues with pure water, and drying, screening and removing iron to obtain a precursor; and calcining the obtained precursor in an inert atmosphere, and performing jet milling, screening and iron removal on the calcined material to obtain lithium iron phosphate coated by lithium ferrite. The method is simple and low in cost, amorphous titanium-doped lithium iron phosphate is prepared by a hydrothermal method, then lithium iron phosphate coprecipitation is coated by precipitation, lithium ferrite coated lithium iron phosphate is obtained by calcination, and the obtained lithium iron phosphate has high capacity and high compaction density.

Description

Preparation method of lithium iron phosphate coated with lithium ferrite

Technical Field

The invention relates to a preparation method of lithium ferrite coated lithium iron phosphate, belonging to the technical field of lithium battery anode materials.

Background

Lithium iron phosphate is a novel electrode material of lithium ion batteries. Its advantages are high discharge capacity, low cost, no poison and no environmental pollution. The industrialized production is being realized successively in all countries of the world. But its tap density is low, affecting the capacitance. The main production method is a high-temperature solid-phase synthesis method, and the product index is relatively stable. The performance of the lithium ion battery mainly depends on the anode and cathode materials, the lithium iron phosphate is used as the anode material of the lithium ion battery only in recent years, and the domestic development of the high-capacity lithium iron phosphate battery is 7 months in 2005. The safety performance and the cycle life of the battery are not comparable with those of other materials, and the safety performance and the cycle life of the battery are the most important technical indexes of the power battery. The 1C charging and discharging cycle life reaches 2000 times. The overcharged voltage of the single battery is 30V, and the single battery does not burn and does not explode when being punctured. The lithium iron phosphate anode material can be used in series connection of large-capacity lithium ion batteries more easily. So as to meet the requirement of frequent charging and discharging of the electric vehicle. The lithium ion battery anode material has the advantages of no toxicity, no pollution, good safety performance, wide raw material source, low price, long service life and the like, and is an ideal anode material of a new generation of lithium ion batteries.

However, the lithium iron phosphate has the greatest disadvantage of poor conductivity, so that the conventional treatment method is to coat carbon, but the coating of carbon causes low compacted density of the lithium iron phosphate, so that a lithium iron phosphate material with higher compacted density and high capacity is urgently needed.

Disclosure of Invention

In view of the above, the invention provides a preparation method of lithium iron phosphate coated with lithium ferrite, which is simple and low in cost, and the amorphous titanium-doped lithium iron phosphate is prepared by a hydrothermal method, then precipitated, coated with lithium iron, coprecipitated, and calcined to obtain lithium iron phosphate coated with lithium ferrite, and the obtained lithium iron phosphate has high capacity and high compaction density.

The invention solves the technical problems by the following technical means:

a preparation method of lithium iron phosphate coated with lithium ferrite comprises the following steps:

(1) adding a ferrous sulfate solution, a lithium hydroxide solution, an ammonium monohydrogen phosphate solution and a titanyl sulfate solution into a high-pressure reaction kettle, carrying out hydrothermal reaction at the temperature of 250-300 ℃ and the pressure of 0.4-0.5MPa for 5-6h under the condition of stirring, then carrying out pressure relief and cooling, and pouring out the slurry;

(2) adding a dispersing agent into the slurry obtained in the step (1), stirring for 15-30min to serve as a base solution, then preparing a ferric chloride solution, an ammonium bicarbonate solution and a lithium hydroxide solution, adding the ferric chloride solution, the ammonium bicarbonate solution and the lithium hydroxide solution into the base solution in a parallel flow manner under the stirring condition, maintaining the pH value of the feeding process to be 7-7.5, the temperature to be 40-55 ℃, the feeding time to be 2-3h, introducing carbon dioxide after the feeding is finished, continuing to react for 1-2h to enable the lithium content in the supernatant to be lower than 0.2g/L, then filtering, heating the filter residue to be washed by pure water, and obtaining a precursor through drying, screening and deironing;

(3) and (3) calcining the precursor obtained in the step (2) in an inert atmosphere, and performing jet milling, screening and iron removal on the calcined material to obtain lithium iron phosphate coated with lithium ferrite.

The concentrations of the ferrous sulfate solution, the lithium hydroxide solution, the ammonium monohydrogen phosphate solution and the titanyl sulfate solution in the step (1) are respectively 1.5-2mol/L, 1-1.5mol/L, 2-2.5mol/L and 0.5-1mol/L, the ferrous sulfate and the lithium hydroxide are battery grade, the ammonium monohydrogen phosphate is food grade, the titanyl sulfate is reagent pure, and the molar ratio of the ferrous sulfate, the lithium hydroxide, the ammonium monohydrogen phosphate to the titanyl sulfate is 1:1.02-1.03:1:0.001-0.002, adding the ferrous sulfate solution, the lithium hydroxide solution, the ammonium monohydrogen phosphate solution and the titanyl sulfate solution into the high-pressure kettle at a constant speed for 15-30min, stirring at the speed of 300-450r/min, and introducing nitrogen to maintain the pressure during the hydrothermal reaction.

In the step (2), the dispersing agent is polyethylene glycol, the concentration of the dispersing agent is 0.1-0.2%, the concentrations of the ferric chloride solution, the ammonium bicarbonate solution and the lithium hydroxide solution are 1.5-2mol/L, 2-2.5mol/L and 1.5-2mol/L respectively, the pH value of the ferric chloride solution is 1-1.5, the molar ratio of the added ferric chloride to the lithium hydroxide is 1:5.1-5.2, and the stirring speed in the reaction process is 200-300 r/min.

And (3) in the filter residue washing process in the step (2), the washing is stopped until the conductivity of washing water is lower than 50 mu S/cm, the drying process adopts vacuum microwave drying, the drying temperature is lower than 80 ℃, the drying is carried out until the moisture of the material is lower than 0.5%, an ultrasonic vibration sieve is adopted for sieving, the aperture of the sieve is 100-mesh and 200-mesh, an electromagnetic iron remover is adopted for circularly removing iron, and the iron is removed until the magnetic substance of the material is lower than 2 ppm.

In the step (3), at least one of nitrogen, carbon dioxide or argon is used as inert atmosphere, the calcination temperature is 750-800 ℃, the calcination time is 5-8h, the calcined material is cooled to a temperature less than 80 ℃ and then discharged, the high-pressure nitrogen is used as a gas source for jet milling, a 2-level electromagnetic iron remover is used for removing iron, and the material after iron removal is subjected to vacuum sealing and packaging.

In the air flow crushing process in the step (3), the D50 discharged after crushing is maintained to be 1-1.5 mu m, the D10 is more than 0.4 mu m, and the D100 is less than 10 mu m.

The molar ratio of the ferrous sulfate added in the step (1) to the ferric chloride added in the step (2) is 48-49: 1.

This patent adopts lithium ferrite to replace carbon and carries out the cladding of lithium iron phosphate, can be in order to play the effect of electric conductivity, avoided losing the compaction density of lithium iron phosphate again, because in the process of carrying out carbon cladding lithium iron phosphate, because the cladding is realized to the method that most adopt organic carbon source thermal decomposition, the carbon source that obtains is amorphous, the appearance is the floccus, thereby influenced the compaction density of lithium iron phosphate, simultaneously in high temperature hydrothermal method process, titanium has evenly been doped, further strengthened the electric conductivity of lithium iron phosphate.

According to actual measurement, the powder resistance of the lithium iron phosphate prepared by the method is in the same order of magnitude as that of the carbon-coated lithium iron phosphate, and because the process avoids the coating by carbon, the lithium iron phosphate is coated by lithium iron oxide, the compaction density is greatly improved, and the powder compaction density of the conventional carbon-coated lithium iron phosphate can not exceed 2.55g/mL and can generally reach 2.4-2.5g/mL under the condition of basically not losing the capacity (namely the 0.1C discharge capacity is more than 150mAh/g), but the powder compaction density can reach more than 2.7g/mL and far exceeds that of the carbon-coated lithium iron phosphate by adopting the process under the same electrical property condition.

The process line adopts a high-temperature hydrothermal method to prepare amorphous lithium iron phosphate, and simultaneously titanium is introduced in the step, so that the titanium is uniformly doped in the lithium iron phosphate;

FeSO₄+LiOH+(NH₄)₂HPO₄-----LiFePO₄+(NH₄)₂SO₄+H₂O

then taking lithium iron phosphate as a crystal nucleus, adding a ferric chloride solution, an ammonium bicarbonate solution and a lithium hydroxide solution, reacting at a certain pH and temperature, CO-precipitating and coating lithium iron on the lithium iron phosphate, and introducing CO into a small amount of lithium₂Lithium carbonate is formed to coat the particles to improve the recovery rate of lithium, and 2Fe (OH) is formed on the surface of the lithium iron phosphate₃·5Li₂CO₃A coating layer;

then, calcining at high temperature in inert atmosphere to form crystalline lithium iron phosphate, and 2Fe (OH)₃·5Li₂CO₃Obtaining Li through pyrolysis₅FeO₄Thereby forming the lithium iron ferrite-coated lithium iron phosphate powder.

The invention has the beneficial effects that: the method is simple and low in cost, amorphous titanium-doped lithium iron phosphate is prepared by a hydrothermal method, then lithium iron phosphate is deposited, coated and coprecipitated, and then lithium ferrite-coated lithium iron phosphate is obtained by calcining, and the obtained lithium iron phosphate has high capacity and high compaction density.

Detailed Description

The present invention will be described in detail with reference to specific examples, in which the preparation method of lithium iron phosphate coated with lithium ferrite of this example comprises the following steps:

Example 1

adding a ferrous sulfate solution, a lithium hydroxide solution, an ammonium monohydrogen phosphate solution and a titanyl sulfate solution into a high-pressure reaction kettle, carrying out hydrothermal reaction under the stirring condition, wherein the hydrothermal reaction temperature is 285 ℃, the pressure is 0.45MPa, the reaction is carried out for 5.5h, then carrying out pressure relief and cooling, and pouring out the slurry; the concentrations of the ferrous sulfate solution, the lithium hydroxide solution, the ammonium monohydrogen phosphate solution and the titanyl sulfate solution are respectively 1.8mol/L, 1.5mol/L, 2.5mol/L and 0.8mol/L, the ferrous sulfate and the lithium hydroxide are battery grade, the ammonium monohydrogen phosphate is food grade, the titanyl sulfate is reagent pure, and the molar ratio of the ferrous sulfate, the lithium hydroxide, the ammonium monohydrogen phosphate to the titanyl sulfate is 1: 1.025:1: 0.0015, adding the ferrous sulfate solution, the lithium hydroxide solution, the ammonium monohydrogen phosphate solution and the titanyl sulfate solution into the high-pressure kettle at a constant speed for 25min, stirring at 380r/min, and introducing nitrogen to maintain the pressure during hydrothermal reaction.

The obtained slurry was sampled and tested for particle size, and the results were as follows:

Dmin	D10	D50	D90	Dmax
					0.19μm	0.27μm	0.87μm	1.6μm	2.5μm

and (3) washing the slurry, drying in vacuum, and detecting elements, wherein the results are as follows:

adding a dispersing agent into the obtained slurry, stirring for 25min to serve as a base solution, then preparing an iron chloride solution, an ammonium bicarbonate solution and a lithium hydroxide solution, adding the iron chloride solution, the ammonium bicarbonate solution and the lithium hydroxide solution into the base solution in a cocurrent manner under the stirring condition, maintaining the pH value of 7.2 in the feeding process, keeping the temperature at 49 ℃, keeping the feeding time at 3h, introducing carbon dioxide after the feeding is finished, continuing to react for 1.5h to enable the lithium content in the supernatant to be lower than 0.2g/L, then filtering, heating the filter residue with pure water for washing, and drying, screening and removing iron to obtain a precursor; the dispersing agent is polyethylene glycol, the concentration of the dispersing agent is 0.15%, the concentrations of the ferric chloride solution, the ammonium bicarbonate solution and the lithium hydroxide solution are respectively 2mol/L, 2mol/L and 1.5mol/L, the pH value of the ferric chloride solution is 1.3, the molar ratio of the added ferric chloride to the lithium hydroxide is 1:5.15, the stirring speed in the reaction process is 280r/min, the filter residue washing process is stopped after the conductivity of washing water is lower than 50 mu S/cm, the drying process adopts vacuum microwave drying at the drying temperature of less than 80 ℃, the drying is carried out until the moisture of the material is lower than 0.5%, an ultrasonic vibration sieve is adopted for sieving, the aperture of a sieve mesh for removing iron is 150 meshes, an electromagnetic iron remover is adopted for carrying out circulating iron removal, and the magnetic substance for removing iron is lower than 2 ppm.

The molar ratio of the added ferrous sulfate to the added ferric chloride is 48.5:1

The detection data of the finally obtained dried material precursor are as follows:

item

Li

Fe

P

Ti

Ca

Ni

Numerical value

4.68％

35.21％

19.18％

0.281％

22.4ppm

13.7ppm

Cr

Cu

Zn

Co

Mg

Dmin

D10

5.7ppm

1.1ppm

14.1ppm

8.3ppm

25.9ppm

0.21μm

0.31μm

D50

D90

Dmax

Moisture content

Magnetic impurities

BET

Tap density

0.88μm

1.75μm

2.71μm

0.41％

1.65ppm

11.3m²/g

1.12g/mL

Calcining the obtained precursor in an inert atmosphere, carrying out jet milling, screening and iron removal on the calcined material, wherein the inert atmosphere adopts nitrogen, the calcining temperature is 790 ℃, the calcining time is 7h, the calcined material is cooled to the temperature of less than 80 ℃, then discharging is carried out, the jet milling adopts high-pressure nitrogen as a gas source, the iron removal adopts a 2-level electromagnetic iron remover to remove iron, the material after iron removal is subjected to vacuum sealing packaging, and the jet milling process maintains the D50 of the discharged material after grinding to be 1.3 mu m, the D10 to be 0.45 mu m and the D100 to be 8.5 mu m.

The detection data of the finally obtained lithium iron phosphate are as follows:

example 2

(1) adding a ferrous sulfate solution, a lithium hydroxide solution, an ammonium monohydrogen phosphate solution and a titanyl sulfate solution into a high-pressure reaction kettle, carrying out hydrothermal reaction for 6 hours under the stirring condition, wherein the hydrothermal reaction temperature is 280 ℃ and the pressure is 0.5MPa, then carrying out pressure relief and cooling, and pouring out the slurry;

(2) adding a dispersing agent into the slurry obtained in the step (1), stirring for 20min to obtain a base solution, then preparing a ferric chloride solution, an ammonium bicarbonate solution and a lithium hydroxide solution, adding the ferric chloride solution, the ammonium bicarbonate solution and the lithium hydroxide solution into the base solution in a parallel flow manner under the stirring condition, maintaining the pH value of the feeding process to be 7.5, the temperature to be 50 ℃, the feeding time to be 3h, introducing carbon dioxide after the feeding is finished, continuing to react for 1.5h to ensure that the lithium content in the supernatant is lower than 0.2g/L, then filtering, heating the filter residue to be washed by pure water, and drying, screening and removing iron to obtain a precursor;

The concentrations of the ferrous sulfate solution, the lithium hydroxide solution, the ammonium monohydrogen phosphate solution and the titanyl sulfate solution in the step (1) are respectively 1.8mol/L, 1.3mol/L, 2.3mol/L and 0.8mol/L, the ferrous sulfate and the lithium hydroxide are battery grade, the ammonium monohydrogen phosphate is food grade, the titanyl sulfate is reagent pure, and the molar ratio of the ferrous sulfate, the lithium hydroxide, the ammonium monohydrogen phosphate to the titanyl sulfate is 1: 1.022:1: 0.0018, adding the ferrous sulfate solution, the lithium hydroxide solution, the ammonium monohydrogen phosphate solution and the titanyl sulfate solution into the high-pressure kettle at a constant speed for 18min, stirring at a speed of 400r/min, and introducing nitrogen to maintain the pressure during hydrothermal reaction.

In the step (2), the dispersing agent is polyethylene glycol, the concentration of the dispersing agent is 0.18%, the concentrations of the ferric chloride solution, the ammonium bicarbonate solution and the lithium hydroxide solution are 2mol/L, 2.5mol/L and 2mol/L respectively, the pH value of the ferric chloride solution is 1.2, the molar ratio of the added ferric chloride to the lithium hydroxide is 1:5.15, and the stirring speed in the reaction process is 300 r/min.

And (3) in the step (2), the washing process of the filter residue is stopped until the conductivity of the washing water is lower than 50 mu S/cm, the drying process adopts vacuum microwave drying, the drying temperature is lower than 80 ℃, the drying is carried out until the moisture of the material is lower than 0.5%, an ultrasonic vibration sieve is adopted for sieving, the aperture of the sieve mesh is 150 meshes, the iron removal is carried out circularly by adopting an electromagnetic iron remover, and the iron removal is carried out until the magnetic substance of the material is lower than 2 ppm.

And (3) adopting carbon dioxide as inert atmosphere in the step (3), wherein the calcination temperature is 790 ℃, the calcination time is 7 hours, discharging the calcined material after cooling to the temperature of less than 80 ℃, adopting high-pressure nitrogen as a gas source for jet milling, adopting a 2-level electromagnetic iron remover for iron removal, and carrying out vacuum sealing and packaging on the material after iron removal.

In the air flow crushing process in the step (3), D50, D10 and D100 of the crushed discharged materials are maintained to be 1.15 mu m, 0.43 mu m and 9.1 mu m respectively.

The molar ratio of the ferrous sulfate added in the step (1) to the ferric chloride added in the step (2) is 49: 1.

item	Li	Fe	P	Ti	D10
						Numerical value	4.72％	35.21％	19.22％	0.321％	0.43μm
D50	D100	Tap density	BET	Compacted density of powder	Internal resistance of powder
						1.15μm	9.1μm	1.33g/mL	7.89m²/g	2.72g/mL	16.1Ω㎝

Example 3

(1) adding a ferrous sulfate solution, a lithium hydroxide solution, an ammonium monohydrogen phosphate solution and a titanyl sulfate solution into a high-pressure reaction kettle, carrying out hydrothermal reaction for 5.5 hours at the temperature of 295 ℃ and the pressure of 0.5MPa under the condition of stirring, then carrying out pressure relief and cooling, and pouring out the slurry;

(2) adding a dispersing agent into the slurry obtained in the step (1), stirring for 25min to obtain a base solution, then preparing a ferric chloride solution, an ammonium bicarbonate solution and a lithium hydroxide solution, adding the ferric chloride solution, the ammonium bicarbonate solution and the lithium hydroxide solution into the base solution in a parallel flow manner under the stirring condition, maintaining the pH value of the feeding process to be 7.3, the temperature to be 50 ℃, the feeding time to be 3h, introducing carbon dioxide after the feeding is finished, continuing to react for 2h to ensure that the lithium content in the supernatant is lower than 0.2g/L, then filtering, heating the filter residue with pure water, washing, drying, screening and removing iron to obtain a precursor;

The concentrations of the ferrous sulfate solution, the lithium hydroxide solution, the ammonium monohydrogen phosphate solution and the titanyl sulfate solution in the step (1) are respectively 1.8mol/L, 1.3mol/L, 2.3mol/L and 0.8mol/L, the ferrous sulfate and the lithium hydroxide are battery grade, the ammonium monohydrogen phosphate is food grade, the titanyl sulfate is reagent pure, and the molar ratio of the ferrous sulfate, the lithium hydroxide, the ammonium monohydrogen phosphate to the titanyl sulfate is 1: 1.02:1: 0.0012, adding the ferrous sulfate solution, the lithium hydroxide solution, the ammonium monohydrogen phosphate solution and the titanyl sulfate solution into the high-pressure kettle at a constant speed for 25min, stirring at a speed of 420r/min, and introducing nitrogen to maintain the pressure during hydrothermal reaction.

In the step (2), the dispersing agent is polyethylene glycol, the concentration of the dispersing agent is 0.1%, the concentrations of the ferric chloride solution, the ammonium bicarbonate solution and the lithium hydroxide solution are 1.8mol/L, 2mol/L and 1.5mol/L respectively, the pH value of the ferric chloride solution is 1.2, the molar ratio of the added ferric chloride to the lithium hydroxide is 1:5.12, and the stirring speed in the reaction process is 230 r/min.

And (3) in the step (2), the washing process of the filter residue is stopped until the conductivity of the washing water is lower than 50 mu S/cm, the drying process adopts vacuum microwave drying, the drying temperature is lower than 80 ℃, the drying is carried out until the moisture of the material is lower than 0.5%, an ultrasonic vibration sieve is adopted for sieving, the aperture of the sieve is 200 meshes, the iron removal is carried out in a circulating manner by adopting an electromagnetic iron remover, and the iron removal is carried out until the magnetic substance of the material is lower than 2 ppm.

And (3) in the step (3), nitrogen is used as inert atmosphere, the calcination temperature is 780 ℃, the calcination time is 8 hours, the calcined material is discharged after being cooled to the temperature of less than 80 ℃, high-pressure nitrogen is used as a gas source for jet milling, a 2-level electromagnetic iron remover is used for removing iron, and the material after iron removal is packaged in a vacuum sealing mode.

In the air flow crushing process in the step (3), D50, D10 and D100 of the crushed discharged materials are maintained to be 1.25 mu m, 0.48 mu m and 9.46 mu m respectively.

item	Li	Fe	P	Ti	D10
						Numerical value	4.69％	35.25％	19.29％	0.261％	0.48μm
D50	D100	Tap density	BET	Compacted density of powder	Internal resistance of powder
						1.25μm	9.46μm	1.31g/mL	7.37m²/g	2.78g/mL	24.8Ω㎝

The commercially available lithium iron phosphate with a carbon content of about 1.5% and doped titanium of 2500ppm was similarly tested, and the results were as follows:

finally, the above embodiments are only for illustrating the technical solutions of the present invention and not for limiting, although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications or equivalent substitutions may be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all of them should be covered in the claims of the present invention.

Claims

1. A preparation method of lithium iron phosphate coated with lithium ferrite is characterized by comprising the following steps:

(2) adding a dispersing agent into the slurry obtained in the step (1), wherein the dispersing agent is polyethylene glycol, stirring for 15-30min to obtain a base solution, then preparing an iron chloride solution, an ammonium bicarbonate solution and a lithium hydroxide solution, adding the iron chloride solution, the ammonium bicarbonate solution and the lithium hydroxide solution into the base solution in a parallel flow manner under the stirring condition, maintaining the pH value of the feeding process to be 7-7.5, the temperature to be 40-55 ℃, the feeding time to be 2-3h, introducing carbon dioxide after the feeding is finished, continuing to react for 1-2h to ensure that the lithium content in the supernatant is lower than 0.2g/L, then filtering, heating the filter residue to be washed by pure water, and obtaining a precursor through drying, screening and deironing;

(3) calcining the precursor obtained in the step (2) in an inert atmosphere, and performing jet milling, screening and iron removal on the calcined material to obtain lithium iron phosphate coated with lithium ferrite, wherein the lithium iron phosphate comprises Li₅FeO₄。

2. The method for preparing lithium iron phosphate coated with lithium ferrite according to claim 1, wherein the method comprises the following steps: the concentrations of the ferrous sulfate solution, the lithium hydroxide solution, the ammonium monohydrogen phosphate solution and the titanyl sulfate solution in the step (1) are respectively 1.5-2mol/L, 1-1.5mol/L, 2-2.5mol/L and 0.5-1mol/L, the ferrous sulfate and the lithium hydroxide are battery grade, the ammonium monohydrogen phosphate is food grade, the titanyl sulfate is reagent pure, the molar ratio of the ferrous sulfate, the lithium hydroxide, the ammonium monohydrogen phosphate and the titanyl sulfate is 1:1.02-1.03:1:0.001-0.002, the ferrous sulfate solution, the lithium hydroxide solution, the ammonium monohydrogen phosphate solution and the titanyl sulfate solution are all uniformly added into an autoclave, the adding time is 15-30min, the stirring speed is 300-450r/min, the reaction is hydrothermal, and nitrogen is introduced to maintain the pressure.

3. The method for preparing lithium iron phosphate coated with lithium ferrite according to claim 1, wherein the method comprises the following steps: the concentration of the dispersing agent in the step (2) is 0.1-0.2%, the concentrations of the ferric chloride solution, the ammonium bicarbonate solution and the lithium hydroxide solution are 1.5-2mol/L, 2-2.5mol/L and 1.5-2mol/L respectively, the pH value of the ferric chloride solution is 1-1.5, the molar ratio of the added ferric chloride to the lithium hydroxide is 1:5.1-5.2, and the stirring speed in the reaction process is 200-300 r/min.

4. The method for preparing lithium iron phosphate coated with lithium ferrite according to claim 1, wherein the method comprises the following steps: and (3) in the filter residue washing process in the step (2), the washing is stopped until the conductivity of washing water is lower than 50 mu S/cm, the drying process adopts vacuum microwave drying, the drying temperature is lower than 80 ℃, the drying is carried out until the moisture of the material is lower than 0.5%, an ultrasonic vibration sieve is adopted for sieving, the aperture of the sieve is 100-mesh and 200-mesh, an electromagnetic iron remover is adopted for circularly removing iron, and the iron is removed until the magnetic substance of the material is lower than 2 ppm.

5. The method for preparing lithium iron phosphate coated with lithium ferrite according to claim 1, wherein the method comprises the following steps: in the step (3), at least one of nitrogen, carbon dioxide or argon is used as inert atmosphere, the calcination temperature is 750-800 ℃, the calcination time is 5-8h, the calcined material is cooled to a temperature less than 80 ℃ and then discharged, the high-pressure nitrogen is used as a gas source for jet milling, a 2-level electromagnetic iron remover is used for removing iron, and the material after iron removal is subjected to vacuum sealing and packaging.

6. The method for preparing lithium iron phosphate coated with lithium ferrite according to claim 1, wherein the method comprises the following steps: in the air flow crushing process in the step (3), the D50 discharged after crushing is maintained to be 1-1.5 mu m, the D10 is more than 0.4 mu m, and the D100 is less than 10 mu m.

7. The method for preparing lithium iron phosphate coated with lithium ferrite according to claim 1, wherein the method comprises the following steps: the molar ratio of the ferrous sulfate added in the step (1) to the ferric chloride added in the step (2) is 48-49: 1.