Doped lithium iron manganese phosphate material and preparation method thereof

Abstract

The invention belongs to the technical field of secondary battery positive electrode materials. The invention relates to a lithium manganese phosphate material, in particular to a doped lithium manganese phosphate material and a preparation method thereof. The product comprises a lithium manganese iron phosphate inner core and a lithium iron phosphate shell coating the surface of the lithium manganese iron phosphate inner core, wherein the lithium manganese iron phosphate inner core comprises pores and titanium dioxide deposited in the pores, the content of the titanium dioxide is 1.5-2.5% of the mass of the lithium manganese iron phosphate inner core, and the porosity of the lithium manganese iron phosphate is 20-25%; wherein the porosity is equal to (1-apparent density / true density) * 100%; the true density is 3.6 g.cm <-3 >; the apparent density is measured by a tap density instrument. The D50 of the lithium manganese iron phosphate core is 5-6 [mu] m; the particle size distribution Span value of the lithium manganese iron phosphate inner core is 0.8 to 1.0; the Span value is equal to (D90-D10) / D50.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of positive electrode materials of secondary batteries. More particularly, it relates to a doped lithium manganese iron phosphate material and a preparation method thereof. BACKGROUND

[0002] Taking lithium manganese iron phosphate as the main material, doping or compounding lithium iron phosphate is a strategy to optimize the performance of lithium manganese iron phosphate material, which aims to improve the cycle stability, inhibit the dissolution of manganese, improve the rate performance, and at the same time maintain the advantages of high voltage and high energy density.

[0003] Specifically, the stable structure of lithium iron phosphate can reduce the dissolution of manganese ions in lithium manganese iron phosphate, especially at high temperature or long cycle, in addition, the doping of ferrous ions can reduce the lattice distortion caused by manganese ions, and improve the material structure stability. For example, it can be considered to take lithium manganese iron phosphate as the core, and coat a lithium iron phosphate nanolayer with a thickness of, for example, 5-10 nm on the surface, so as to inhibit the corrosion of electrolyte to Mn.

[0004] However, during long-term use, the inventors found that there are differences in the lattice parameters between lithium manganese iron phosphate and lithium iron phosphate, and the lattice volume of lithium manganese iron phosphate is slightly larger. During long-term charging and discharging, the repeated deintercalation of lithium ions may cause interface stress accumulation, eventually leading to peeling of the coating layer or cracks; in addition, manganese ions are prone to Jahn-Teller distortion during the cycle, while lithium iron phosphate does not have this phenomenon, and the structural responses of the two at the interface are inconsistent, which may accelerate the phase separation, and finally affect the service life of the product. SUMMARY

[0005] The technical problem to be solved by the present application is that, when the surface of the existing lithium manganese iron phosphate is coated with lithium iron phosphate, due to the differences in the lattice parameters and other reasons, the surface lithium iron phosphate coating layer and the lithium manganese iron phosphate core may separate during use, thereby reducing the cycle life of the product. The present application provides a doped lithium manganese iron phosphate material and a preparation method thereof.

[0006] The purpose of the present application is to provide a doped lithium manganese iron phosphate material.

[0007] Another purpose of the present application is to provide a preparation method of a doped lithium manganese iron phosphate material.

[0008] The above purposes of the present application are achieved by the following technical solutions: A doped lithium manganese iron phosphate material, comprising a lithium manganese iron phosphate core and a lithium iron phosphate shell coated on the surface of the lithium manganese iron phosphate core; The manganese iron lithium phosphate core comprises pores and titanium dioxide deposited in the pores, the content of the titanium dioxide is 1.5-2.5% of the mass of the manganese iron lithium phosphate core, and the porosity of the manganese iron lithium phosphate is 20-25%. The porosity = (1-apparent density / true density) * 100%. The true density is 3.6 g·cm -3 ; The apparent density is measured by a tap density instrument.

[0009] By adopting a suitable porosity, it can be ensured that the titanium dioxide can fully penetrate and diffuse into the manganese iron lithium phosphate during the deposition process, but if the porosity is too high, the physical structure stability of the manganese iron lithium phosphate core itself will be reduced; on the basis of the corresponding porosity, by adjusting the deposition amount of the titanium dioxide relative to the mass of the core, enough titanium dioxide can be deposited in the pores, but too much titanium dioxide will not be deposited on the surface of the material, thereby significantly reducing the proportion of active material and affecting the energy density of the product. Further, the D50 of the manganese iron lithium phosphate core is 5-6 μm; and the particle size distribution span value of the manganese iron lithium phosphate core is 0.8-1.0. The span value = (D90-D10) / D50.

[0010] By controlling the particle size within a suitable range, the uniformity during the coating or deposition of lithium iron phosphate can be ensured, but the inventors have found that the particle size distribution is also one of the influencing factors, a lower span value corresponds to a narrower particle size distribution, at this time, for the same reaction environment, the deposition process is similar, and the deposition process can be more uniform, and a too wide particle size distribution will lead to uneven thickness of the coating layer, so that part of the particles are easy to crack due to the too thin coating layer, and part of the particles are affected by lithium ion transmission or capacity due to the too thick coating layer; but if the span value is too low, the grading effect between particles will be poor when the material is used as a positive active material, the porosity formed by the particle accumulation is high, the particle impedance of the whole electrode sheet is high, and the energy density cannot be developed. Further, the D50 of the doped lithium iron phosphate material is 1.03-1.05 times the D50 of the manganese iron lithium phosphate core; and the particle size distribution span value of the doped lithium iron phosphate material is less than the particle size distribution span value of the manganese iron lithium phosphate core.

[0011] By selecting the raw materials in the preparation process and controlling the preparation process, the particle size of the coated lithium iron phosphate is increased, which is also a way to control the thickness of the surface coating layer. Moreover, by controlling the coating process and even screening and grading the coated particles, the particle size distribution of the finished product particles after coating is more uniform than before coating, making the stress between different particles relatively uniform.

[0012] Further, the sphericity of the lithium manganese iron phosphate core is 90-92%, and the sphericity of the doped lithium iron phosphate material is greater than that of the lithium manganese iron phosphate core.

[0013] Further, the lithium iron phosphate shell includes amorphous carbon, and the amorphous carbon is doped with N elements.

[0014] By introducing amorphous carbon and N element doping through the following preparation method, the adhesion of the interface adsorption between the lithium iron phosphate coating layer and the core is improved, and the surface ion diffusion resistance is further reduced.

[0015] Further, the titanium dioxide is anatase phase titanium dioxide.

[0016] A preparation method of a doped lithium manganese iron phosphate material, the specific preparation steps include: A dilute nitric acid solution is used as the corrosion medium to corrode the lithium manganese iron phosphate material to form a porous lithium manganese iron phosphate material with a porosity of 20-25%; Titanium source precursors are deposited in the pores of the porous lithium manganese iron phosphate material, and then calcined to form titanium dioxide in the pores to obtain a lithium manganese iron phosphate core; Specifically, tetrabutyl titanate can be used as the titanium source. By controlling the moisture content of the lithium manganese iron phosphate material, a small amount of water is left inside. For details, refer to the related preparation method of the embodiment. During heating, tetrabutyl titanate diffuses and penetrates into the pores, while water begins to diffuse outward. When the two meet, tetrabutyl titanate hydrolyzes in the pores to form a precursor. The precursor is further calcined to form titanium dioxide, thereby achieving deposition in the pores. A lithium iron phosphate coating is coated on the surface of the lithium manganese iron phosphate core to obtain a doped lithium manganese iron phosphate material.

[0017] Further, the specific preparation steps further include: Titanium source precursors are deposited in the pores of the porous lithium manganese iron phosphate material, and then calcined at a temperature of 400-450°C to form titanium dioxide in the pores to obtain a lithium manganese iron phosphate core; Mixing the lithium manganese iron phosphate core and water, then adding polyvinylpyrrolidone, ultrasonic dispersion, to obtain a lithium manganese iron phosphate core dispersion liquid; To the lithium manganese iron phosphate core dispersion liquid, add lithium hydroxide, ferrous sulfate and ammonium dihydrogen phosphate, mix well, then heat and stir at a temperature of 60-80℃ and a pH of 8-9 for 2-4h, then centrifugal separation, drying, then sintering in a mixed atmosphere of argon and hydrogen at a temperature of 400-450℃ for 2-3h, cooling, and discharging, to obtain the doped lithium manganese iron phosphate material. The mixed atmosphere of argon and hydrogen is mixed by argon and hydrogen at a volume ratio of 95:5.

[0018] Further, the specific preparation steps further comprise: Mixing the lithium manganese iron phosphate core and water at a mass ratio of 1:10 to obtain a mixed solution, then adding 4-6% of the mass of the mixed solution of polyvinylpyrrolidone and 6-8% of the mass of the lithium manganese iron phosphate core of dopamine, ultrasonic dispersion, to obtain a lithium manganese iron phosphate core dispersion liquid.

[0019] Beneficial technical effects: (1) The technical scheme of the present application takes lithium manganese iron phosphate as the core, and coats a lithium iron phosphate coating on the surface, and on this basis, deposits titanium dioxide in the pores of the lithium manganese iron phosphate core to solve the problem of phase separation between the core and the surface coating; Specifically, the titanium dioxide deposited in the pores can form a strong interaction with the manganese ions in the lithium manganese iron phosphate material, thereby anchoring the manganese element and reducing manganese dissolution, which can stabilize the structure of the lithium manganese iron phosphate material; In addition, the lithium ion diffusion coefficient of titanium dioxide is higher than that of pure lithium manganese iron phosphate, and the titanium dioxide in the pores can provide an additional lithium ion transmission path; More importantly, the hydroxyl groups on the surface of titanium dioxide can form a strong adsorption force with the precursor of lithium iron phosphate, thereby forming a lithium manganese iron phosphate-titanium dioxide-lithium iron phosphate modulus overgradient, reducing the risk of interface cracking during the cycle process; (2) By further adjusting the temperature during calcination, the crystal form of the generated titanium dioxide is anatase phase, which has a lower strain characteristic, and based on this, it can effectively absorb the lattice stress change of lithium manganese iron phosphate during charging and discharging. DETAILED DESCRIPTION

[0020] The present application will be further described in conjunction with specific examples, but the examples do not limit the present application in any form. Unless otherwise specified, the reagents, methods and equipment used in the present application are conventional reagents, methods and equipment in the technical field.

[0021] Unless otherwise specified, the reagents and materials used in the following examples are commercially available. Example 1

[0022] Lithium manganese iron phosphate particles and 0.2 mol / L nitric acid solution were mixed at a mass ratio of 1:10. The mixture was then subjected to ultrasonic corrosion at 55℃ and 80 kHz for 65 min. After filtration, the filter cake was washed three times with deionized water. The washed filter cake was then dried in an oven at 55℃ until the moisture content was 6% to form a porous lithium manganese iron phosphate material with a porosity of 20%. When adjusting the above process parameters, they should be adjusted reasonably according to the porosity. The porosity is characterized by the following formula: porosity = (1 - apparent density / true density) × 100%. The true density is 3.6 g·cm³. -3 ; The apparent density was obtained by tap density meter; Tetrabutyl titanate and anhydrous ethanol were mixed uniformly at a mass ratio of 1:10 to obtain a tetrabutyl titanate solution. Porous lithium manganese iron phosphate material and the tetrabutyl titanate solution were mixed at a mass ratio of 1:12 and subjected to ultrasonic reaction at 55°C and 70kHz for 80 minutes to deposit a titanium precursor in the pores of the porous lithium manganese iron phosphate material. After the reaction was completed, the mixture was filtered, the filter cake was collected, and washed three times with anhydrous ethanol. The washed filter cake was dried to constant weight at 90°C, then heated to 400°C and calcined for 100 minutes under a nitrogen atmosphere. After cooling to room temperature, the material was discharged and sieved to obtain lithium manganese iron phosphate core particles with a D50 of 5 μm, a particle size distribution Span value of 0.8, and a sphericity of 90%. The Span value is calculated as (D90 - D10) / D50. In addition, during the above preparation process, the content of titanium dioxide was controlled to 1.5% of the mass of lithium manganese iron phosphate core particles by controlling the amount of raw materials used. Lithium manganese iron phosphate core particles and water were mixed at a mass ratio of 1:10 to obtain a mixture. Then, 4% by mass of polyvinylpyrrolidone and 6% by mass of dopamine were added to the mixture and ultrasonically dispersed at an ultrasonic frequency of 80kHz for 20 minutes to obtain a lithium manganese iron phosphate core dispersion. Lithium hydroxide, ferrous sulfate and ammonium dihydrogen phosphate were added to the lithium manganese iron phosphate core dispersion and mixed evenly. The mixture was heated and stirred for 2 hours at 60°C and pH 8. After centrifugation and drying, the mixture was sintered for 2 hours at 400°C in a mixed atmosphere of argon and hydrogen. After cooling, the material was discharged to obtain the doped lithium manganese iron phosphate material. By controlling the coating process conditions above, and cooperating with screening, to obtain the D50 greater than the particle of the lithium manganese iron phosphate core, specifically, the D50 of the doped lithium iron phosphate material is 1.03 times of the D50 of the lithium manganese iron phosphate core, and the particle size distribution Span value of the doped lithium iron phosphate material is less than the particle size distribution Span value of the lithium manganese iron phosphate core; by uniform coating, so that the sphericity of the doped lithium iron phosphate material is greater than the sphericity of the lithium iron phosphate core; The molar ratio of lithium hydroxide and ferrous sulfate is 1:1, the amount of ammonium dihydrogen phosphate is 10% of the mass of lithium hydroxide, a small amount of ammonium dihydrogen phosphate is used in the reaction process, and the phosphorus source diffused out of the lithium manganese iron phosphate is insufficient; the amount of lithium hydroxide is 1.0% of the mass of the lithium manganese iron phosphate core; The mixed gas atmosphere of argon and hydrogen is mixed by argon and hydrogen in a volume ratio of 95:5. Example 2

[0023] The lithium manganese iron phosphate particles and the nitric acid solution with a concentration of 0.25 mol / L are mixed in a mass ratio of 1:10, then after ultrasonic corrosion at a temperature of 60℃ and an ultrasonic frequency of 90kHz for 70min, filtration is performed, and the filter cake is washed with deionized water for 3 times, then the washed filter cake is dried in an oven at a temperature of 58℃ until the water content is 7%, to form a porous lithium manganese iron phosphate material with a porosity of 22%; When the process parameters are regulated, the porosity is reasonably regulated according to the size, and the characterization method of the porosity is referred to as porosity=(1-apparent density / true density)×100%; The true density is 3.6g·cm -3 ; The apparent density is measured by a tap density instrument; The titanium tetrabutoxide and anhydrous ethanol are uniformly mixed in a mass ratio of 1:10 to obtain a titanium tetrabutoxide solution; the porous lithium manganese iron phosphate material and the titanium tetrabutoxide solution are mixed in a mass ratio of 1:12, then after ultrasonic reaction at a temperature of 58℃ and an ultrasonic frequency of 72kHz for 90min, a titanium source precursor is deposited in the pores of the porous lithium manganese iron phosphate material; after the reaction is completed, filtration is performed, the filter cake is collected, and the filter cake is washed with anhydrous ethanol for 3 times, then the washed filter cake is dried at a temperature of 90℃ until the constant weight, then heated to 420℃ under a nitrogen atmosphere, and after being kept at a temperature of 420℃ for 110min, cooled to room temperature, discharged, and screened to obtain lithium manganese iron phosphate core particles with a D50 of 5μm, a particle size distribution Span value of 0.9, and a sphericity of 91%; the Span value=(D90-D10) / D50; In addition, during the above preparation process, the content of titanium dioxide is controlled to be 2% of the mass of lithium manganese iron phosphate core particles by controlling the amount of raw materials used; Lithium manganese iron phosphate core particles and water were mixed at a mass ratio of 1:10 to obtain a mixture. Then, 5% polyvinylpyrrolidone and 7% dopamine were added to the mixture and ultrasonically dispersed at an ultrasonic frequency of 80kHz for 20 minutes to obtain a lithium manganese iron phosphate core dispersion. Lithium hydroxide, ferrous sulfate and ammonium dihydrogen phosphate were added to the lithium manganese iron phosphate core dispersion and mixed evenly. The mixture was then heated and stirred for 3 hours at 70°C and pH 8.5. After centrifugation and drying, the mixture was sintered at 420°C for 2.5 hours in a mixed atmosphere of argon and hydrogen. After cooling, the material was discharged to obtain the doped lithium manganese iron phosphate material. By controlling the above coating process conditions and combining them with sieving, particles with a D50 greater than that of the lithium manganese iron phosphate core are obtained. Specifically, the D50 of the doped lithium iron phosphate material is 1.04 times that of the lithium manganese iron phosphate core, and the particle size distribution Span value of the doped lithium iron phosphate material is smaller than that of the lithium manganese iron phosphate core. Through uniform coating, the sphericity of the doped lithium iron phosphate material is made greater than that of the lithium iron phosphate core. The molar ratio of lithium hydroxide to ferrous sulfate is 1:1, and the amount of ammonium dihydrogen phosphate is 10% of the mass of lithium hydroxide. The small amount of ammonium dihydrogen phosphate used in the reaction process results in insufficient phosphorus source diffused from lithium manganese iron phosphate. The amount of lithium hydroxide used is 1.1% of the mass of the lithium manganese iron phosphate core. The mixed atmosphere of argon and hydrogen is formed by mixing argon and hydrogen in a volume ratio of 95:5. Example 3

[0024] Lithium manganese iron phosphate particles and 0.3 mol / L nitric acid solution were mixed at a mass ratio of 1:10. The mixture was then subjected to ultrasonic corrosion at 65℃ and 100 kHz for 75 min. After filtration, the filter cake was washed three times with deionized water. The washed filter cake was then dried in an oven at 60℃ until the moisture content was 8%, thus forming a porous lithium manganese iron phosphate material with a porosity of 25%. When adjusting the above process parameters, they should be adjusted reasonably according to the porosity. The porosity is characterized by the following formula: porosity = (1 - apparent density / true density) × 100%. The true density is 3.6 g·cm³. -3 ; The apparent density was obtained by tap density meter; The tetrabutyl titanate and anhydrous ethanol are mixed uniformly according to a mass ratio of 1:10 to obtain a tetrabutyl titanate solution; the porous manganese iron lithium phosphate material and the tetrabutyl titanate solution are mixed according to a mass ratio of 1:12, then under the condition of a temperature of 60°C and an ultrasonic frequency of 75 kHz, the mixture is ultrasonically reacted for 100 min to deposit a titanium source precursor in the pores of the porous manganese iron lithium phosphate material; after the reaction is completed, the mixture is filtered, the filter cake is collected, and the filter cake is washed with anhydrous ethanol for 3 times; after the washed filter cake is dried to a constant weight under the condition of a temperature of 90°C, the filter cake is heated to 450°C in a nitrogen atmosphere, and then is incubated and calcined for 120 min, and then is cooled to room temperature, discharged, and sieved to obtain manganese iron lithium phosphate core particles with a D50 of 6 μm, a particle size distribution Span value of 1.0, and a sphericity of 92%; the Span value=(D90-D10) / D50; In addition, in the above preparation process, the content of titanium dioxide is controlled to be 2.5% of the mass of the manganese iron lithium phosphate core particles by controlling the amount of raw materials; The manganese iron lithium phosphate core particles and water are mixed according to a mass ratio of 1:10 to obtain a mixed solution, and then 6% of polyvinylpyrrolidone based on the mass of the mixed solution and 8% of dopamine based on the mass of the manganese iron lithium phosphate core particles are added, and then the mixture is ultrasonically dispersed for 20 min under the condition of an ultrasonic frequency of 80 kHz to obtain a manganese iron lithium phosphate core dispersion liquid; Lithium hydroxide, ferrous sulfate, and ammonium dihydrogen phosphate are added to the manganese iron lithium phosphate core dispersion liquid, the mixture is mixed uniformly, and then is heated and stirred for 4 h under the condition of a temperature of 80°C and a pH of 9, and then is centrifugally separated, dried, sintered in a mixed atmosphere of argon and hydrogen at a temperature of 450°C for 3 h, cooled, and discharged to obtain the doped manganese iron lithium phosphate material; By controlling the above coating process conditions and sieving, manganese iron lithium phosphate core particles with a D50 greater than that of the manganese iron lithium phosphate core are obtained, specifically, the D50 of the doped lithium iron phosphate material is 1.05 times the D50 of the manganese iron lithium phosphate core, and the particle size distribution Span value of the doped lithium iron phosphate material is less than that of the manganese iron lithium phosphate core; by uniform coating, the sphericity of the doped lithium iron phosphate material is greater than that of the manganese iron lithium phosphate core; The molar ratio of lithium hydroxide to ferrous sulfate is 1:1, and the amount of ammonium dihydrogen phosphate is 10% of the mass of lithium hydroxide; a small amount of ammonium dihydrogen phosphate is used in the reaction process because the manganese iron lithium phosphate diffuses out of the phosphorus source; The mixed atmosphere of argon and hydrogen is obtained by mixing argon and hydrogen according to a volume ratio of 95:5. Example 4

[0025] The difference between this example and Example 1 is that dopamine is not added, and the rest of the conditions remain unchanged. Example 5

[0026] The difference between this example and Example 1 is that: The tetrabutyl titanate and anhydrous ethanol were mixed uniformly according to a mass ratio of 1:10 to obtain a tetrabutyl titanate solution; the porous manganese iron phosphate lithium material and the tetrabutyl titanate solution were mixed according to a mass ratio of 1:12, and then under the condition of a temperature of 55°C and an ultrasonic frequency of 70 kHz, the mixture was incubated and ultrasonically reacted for 80 min, so as to deposit a titanium source precursor in the pores of the porous manganese iron phosphate lithium material; after the reaction was completed, the filter cake was collected by filtration, and the filter cake was washed with anhydrous ethanol for 3 times; the washed filter cake was dried to a constant weight under the condition of a temperature of 90°C, and then heated to 550°C in a nitrogen atmosphere, incubated and calcined for 100 min, cooled to room temperature, discharged, and sieved to obtain manganese iron phosphate lithium core particles with a D50 of 5 μm, a particle size distribution Span value of 0.8, and a sphericity of 90%; the Span value=(D90-D10) / D50; The rest of the conditions remain unchanged; due to the increase of the calcination temperature, part of the titanium dioxide is converted into rutile phase.

[0027] Comparative Example 1 The difference between this comparative example and Example 1 is that aluminum isopropoxide is used instead of tetrabutyl titanate, and the rest of the conditions remain unchanged.

[0028] Comparative Example 2 The difference between this comparative example and Example 1 is that tetrabutyl titanate is not used, and the lithium iron phosphate is directly coated, and the rest of the conditions remain unchanged.

[0029] The products obtained in the above examples and comparative examples were tested and evaluated for performance, and the specific testing and evaluation methods and results are as follows: An aluminum foil with a thickness of 13 um was used as the positive current collector, and a positive active material: conductive agent Super P: binder PVDF=90:5:5 formula was used to form a positive electrode with a thickness of 80 um and a surface density of 18 mg / cm 2 A CR2032 button cell was assembled using a lithium metal sheet as the negative electrode, EC / DMC as the electrolyte with a volume ratio of 1:1, lithium salt lithium hexafluorophosphate with a concentration of 1 mol / L, and Celgard2325 as the separator; EIS testing was performed using a Bio-Logic SP-300 type electrochemical workstation at a temperature of 25°C, a frequency range of 0.1 Hz-100 Hz, an amplitude of 10 mV, and a test potential of 50% SOC; Among them, after every 50 cycles, EIS test is carried out, and 2h is rested before test to stabilize OCV; Specifically, during the cycle process, the following charge-discharge strategy is used for reference: the voltage interval is 2.5-4.0V, the cycle rate is 1C, first charged to the upper limit voltage at 1C rate, then charged at constant voltage to 0.05C cut-off, after 5min of static, discharged to the lower limit voltage at 1C rate, and 5min of static is kept; Through EIS test, Rct value is obtained to evaluate interface reaction resistance, i.e. charge transfer impedance, and detailed test results are shown in Table 1; Table 1: Product performance evaluation results

[0030] From the test results in Table 1, it can be seen that the product obtained by the application can relatively maintain the interface stable and the relevant interface impedance growth is low during the cycle process.

[0031] The above embodiments are the preferred embodiments of the application, but the embodiments of the application are not limited to the above embodiments, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the application shall be equivalent replacement methods, and all shall be included in the protection scope of the application.

Claims

1. A doped lithium iron phosphate material, characterized in that, It includes a lithium iron phosphate core and a lithium iron phosphate shell covering the surface of the lithium iron phosphate core; The lithium manganese iron phosphate core includes pores and titanium dioxide deposited in the pores. The content of titanium dioxide is 1.5-2.5% of the mass of the lithium manganese iron phosphate core, and the porosity of the lithium manganese iron phosphate is 20-25%. Wherein, porosity = (1 - apparent density / true density) × 100%; The true density is 3.6 g·cm³. -3 ; The apparent density was obtained by measuring the tap density using a tap density meter.

2. The doped lithium manganese iron phosphate material according to claim 1, characterized in that, The D50 of the lithium manganese iron phosphate core is 5-6 μm; and the Span value of the particle size distribution of the lithium manganese iron phosphate core is 0.8-1.

0. The Span value is calculated as (D90 - D10) / D50.

3. The lithium iron phosphate doped material according to claim 2, characterized in that, The D50 of the doped lithium iron phosphate material is 1.03-1.05 times that of the lithium manganese iron phosphate core; and the particle size distribution span value of the doped lithium iron phosphate material is smaller than that of the lithium manganese iron phosphate core.

4. The lithium iron phosphate doped material according to claim 2, characterized in that, The sphericity of the lithium iron phosphate core is 90-92%, and the sphericity of the doped lithium iron phosphate material is greater than that of the lithium iron phosphate core.

5. The doped lithium manganese iron phosphate material according to claim 1, characterized in that, The lithium iron phosphate shell includes amorphous carbon, and the amorphous carbon is doped with nitrogen (N) element.

6. The doped lithium manganese iron phosphate material according to claim 1, characterized in that, The titanium dioxide is anatase phase titanium dioxide.

7. A method for preparing a doped lithium manganese iron phosphate material as described in any one of claims 1-6, characterized in that, The specific preparation steps include: Dilute nitric acid solution was used as the corrosive medium to corrode lithium manganese iron phosphate material to form porous lithium manganese iron phosphate material with a porosity of 20-25%. In the pores of the porous lithium manganese iron phosphate material, a titanium source precursor is deposited and then calcined to form titanium dioxide in the pores to obtain the lithium manganese iron phosphate core. By coating the surface of the lithium manganese iron phosphate core with lithium iron phosphate, a doped lithium manganese iron phosphate material is obtained.

8. The method for preparing a doped lithium manganese iron phosphate material according to claim 7, characterized in that, The specific preparation steps also include: In the pores of the porous lithium manganese iron phosphate material, a titanium source precursor is deposited and then calcined at a temperature of 400-450℃ to form titanium dioxide in the pores, so as to obtain the lithium manganese iron phosphate core. The lithium manganese iron phosphate core was mixed with water, then polyvinylpyrrolidone was added, and the mixture was ultrasonically dispersed to obtain a lithium manganese iron phosphate core dispersion. Lithium hydroxide, ferrous sulfate and ammonium dihydrogen phosphate are added to the lithium manganese iron phosphate core dispersion. After mixing evenly, the mixture is heated and stirred for 2-4 hours at a temperature of 60-80℃ and a pH of 8-9. After centrifugation and drying, the mixture is sintered for 2-3 hours at a temperature of 400-450℃ in a mixed atmosphere of argon and hydrogen. After cooling, the material is discharged to obtain the doped lithium manganese iron phosphate material. The mixed atmosphere of argon and hydrogen is formed by mixing argon and hydrogen in a volume ratio of 95:

5.

9. The method for preparing a doped lithium manganese iron phosphate material according to claim 8, characterized in that, The specific preparation steps also include: Lithium manganese iron phosphate cores and water are mixed at a mass ratio of 1:10 to obtain a mixture. Then, 4-6% by mass of polyvinylpyrrolidone and 6-8% by mass of dopamine are added to the mixture and ultrasonically dispersed to obtain a lithium manganese iron phosphate core dispersion.

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