Method for preparing high-compaction and high-rate lithium iron phosphate and application of high-compaction and high-rate lithium iron phosphate

Abstract

The invention relates to a method for preparing high-compaction and high-rate lithium iron phosphate and application of the high-compaction and high-rate lithium iron phosphate. The method comprises the following steps: adding a first lithium source, a first carbon source, a third carbon source and a solvent into doped iron phosphate with a high iron-phosphorus ratio, mixing and grinding to prepare precursor slurry A; adding a second lithium source, a second carbon source, a fourth carbon source and a solvent into doped iron phosphate with a low iron-phosphorus ratio, mixing and grinding to prepare precursor slurry B; mixing the precursor slurry A and the precursor slurry B, and performing spray drying to obtain dry powder; and sintering the dry powder in an inert gas environment to obtain the lithium iron phosphate. Compared with a conventional high-compaction lithium iron phosphate material, the lithium iron phosphate material prepared by the invention has more excellent electrical performance and lower polarization performance at high rate.

Description

Technical Field

[0001] The present application relates to lithium iron phosphate positive electrode materials, and in particular to a method for preparing high-density, high-rate lithium iron phosphate and its application. Background Art

[0002] Lithium iron phosphate (LiFePO 4 ) As one of the positive electrode materials for lithium-ion batteries, it has become one of the research hotspots of positive electrode materials for rechargeable lithium-ion batteries because of its relatively high theoretical capacity (170mAh / g), stable charge and discharge voltage platform, which makes organic electrolytes safer in battery applications, high safety, good electrode reaction reversibility, high thermal stability, and environmental friendliness. Especially in the past two years, with the rapid development of electric vehicles, customers have higher and higher requirements for fast charging. Some manufacturers have required charging to 80% SOC within 12 minutes or even 10 minutes, and energy density must be guaranteed. The development of high-rate and high-energy-density lithium-ion batteries is imminent, and the development of high-rate and high-density positive electrode materials is a key step in the development of high-rate and high-energy-density lithium-ion batteries.

[0003] Lithium iron phosphate is a battery material with an olivine structure. Although it has the above advantages, its intrinsic conductivity is low and its ion diffusion coefficient is small, which makes its low-temperature performance and high-rate performance poor, thus limiting the use of lithium batteries under high-rate conditions.

[0004] As the compaction of the pole piece becomes higher and higher, the compaction requirements for iron phosphate phosphorus powder are getting higher and higher. Developing powder compaction greater than 2.60, or even greater than 2.65, has become a very urgent task for major battery manufacturers. However, if the primary sintering must increase the sintering temperature to increase the compaction, the power performance of the material cannot be met and guaranteed, which is a problem. At present, some manufacturers use secondary sintering to improve powder compaction, which can also improve the power drop problem caused by high compaction, but the production capacity decreases and the cost increases.

[0005] There is a need to develop a method for synthesizing high-compaction, high-rate iron-phosphorus phosphate that can improve compaction and maintain power without increasing costs or reducing production capacity. Summary of the invention

[0006] On the one hand, the present application provides a method for preparing lithium iron phosphate, comprising:

[0007] Adding a first lithium source, a first carbon source, a third carbon source and a solvent to doped iron phosphate with a high iron-to-phosphorus ratio, mixing and grinding to obtain a precursor slurry A;

[0008] Adding a second lithium source, a second carbon source, a fourth carbon source and a solvent to the doped iron phosphate with a low iron-phosphorus ratio, mixing and grinding to obtain a precursor slurry B;

[0009] The precursor slurry A and the precursor slurry B are mixed and then spray-dried to obtain a dry powder;

[0010] Sintering the dry powder in an inert gas environment to obtain lithium iron phosphate;

[0011] Wherein, the doping elements in the high iron-phosphorus ratio doped iron phosphate are a first doping element and a second doping element, the first doping element is any one or more of Mn, Mg, Ni, and rare earth, and the second doping element is Ti or V;

[0012] The doping elements in the doped ferric phosphate with a low iron-phosphorus ratio are a second doping element and a third doping element, and the third doping element is F.

[0013] In one embodiment, the D50 of the precursor slurry A is 0.10-0.25 μm, for example, 0.10 μm, 0.12 μm, 0.14 μm, 0.15 μm, 0.20 μm, 0.24 μm, 0.25 μm; the D50 of the precursor slurry B is 0.35-0.50 μm, for example, 0.35 μm, 0.37 μm, 0.39 μm, 0.40 μm, 0.42 μm, 0.44 μm, 0.45 μm, 0.47 μm, 0.49 μm, 0.50 μm.

[0014] In one embodiment, the doping amount of Ti or V is 500-5000 ppm of the mass of the doped iron phosphate, for example, 500 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 3500 ppm, 4000 ppm, 4500 ppm, 5000 ppm.

[0015] In one embodiment, the doping amount of the first doping element is 500-3000ppm of the mass of the doped iron phosphate, for example, 500ppm, 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm; the doping amount of the third doping element is 1000-6000ppm of the mass of the doped iron phosphate, for example, 1000ppm, 1500ppm, 2000ppm, 2500ppm, 3000ppm, 3500ppm, 4000ppm, 4500ppm, 5000ppm, 5500ppm, 6000ppm.

[0016] In one embodiment, the mass ratio of the precursor slurry A to the precursor slurry B is 2:3-1:1, for example, 2:3, 4:5, 1:1.

[0017] In one embodiment, the high iron-phosphorus ratio is 0.975≤iron-phosphorus ratio≤1, for example, the high iron-phosphorus ratio is 0.975, 0.98, 0.985, 1; the low iron-phosphorus ratio is 0.96≤iron-phosphorus ratio<0.975, for example, the low iron-phosphorus ratio is 0.96, 0.965, 0.97.

[0018] In one embodiment, the first lithium source and the second lithium source are one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate, lithium acetate, lithium sulfate, lithium nitrate and lithium chloride;

[0019] The first carbon source and the second carbon source are at least one of glucose, sucrose, lactose, maltose, and citric acid;

[0020] The third carbon source and the fourth carbon source are at least one of PEG2000 and PVA2000;

[0021] The solvent is deionized water or anhydrous ethanol.

[0022] Another aspect of the present application provides lithium iron phosphate prepared by the above preparation method. The lithium iron phosphate can be used in lithium ion batteries.

[0023] On the other hand, the present application provides a positive electrode of a lithium ion battery, wherein the positive electrode of the lithium ion battery is made of a positive electrode material containing the lithium iron phosphate.

[0024] The lithium iron phosphate prepared in the present application is a high-density graded lithium iron phosphate, which has better electrical properties and lower polarization performance at high rates compared with conventional high-density lithium iron phosphate materials. DETAILED DESCRIPTION

[0025] The technical solution of the present application will be described in detail below.

[0026] The method for preparing lithium iron phosphate provided in the present application comprises:

[0027] Adding a first lithium source, a first carbon source, a third carbon source and a solvent to doped iron phosphate with a high iron-to-phosphorus ratio, mixing and grinding to obtain a precursor slurry A;

[0028] Adding a second lithium source, a second carbon source, a fourth carbon source and a solvent to the doped iron phosphate with a low iron-phosphorus ratio, mixing and grinding to obtain a precursor slurry B;

[0029] The precursor slurry A and the precursor slurry B are mixed and then spray-dried to obtain a dry powder;

[0030] Sintering the dry powder in an inert gas environment to obtain lithium iron phosphate;

[0031] Wherein, the doping elements in the high iron-phosphorus ratio doped iron phosphate are a first doping element and a second doping element, the first doping element is any one or more of Mn, Mg, Ni, and rare earth, and the second doping element is Ti or V;

[0032] The doping elements in the doped ferric phosphate with a low iron-phosphorus ratio are a second doping element and a third doping element, and the third doping element is F.

[0033] Phosphorus-oxygen bond is the basic covalent bond that constitutes the structure of lithium iron phosphate. In a synthetic environment where the phosphorus content is relatively high relative to the iron content, phosphorus promotes the growth, sintering and fusion of lithium iron phosphate primary grains, and lithium iron phosphate with larger primary particle size can be obtained; while in a synthetic environment where the phosphorus content is relatively low relative to the iron content, the increase in the iron content will inhibit the growth of lithium iron phosphate primary grains, and lithium iron phosphate with smaller primary particle size can be synthesized.

[0034] The present application uses iron phosphate with high iron-phosphorus ratio and iron phosphate with low iron-phosphorus ratio as precursors and grinds them into particles of different particle sizes. The low iron-phosphorus ratio precursor mixture is ground into large-sized particles, and the high phosphorus content promotes the growth and sintering fusion of primary particles, making it easier to sinter and form large-particle iron phosphate phosphorus; the high iron-phosphorus ratio precursor mixture is ground into small-sized particles, and the high iron content inhibits the growth of primary particles, synthesizing small-particle lithium iron phosphate. Therefore, the purpose of particle grading can be achieved by controlling the iron-phosphorus ratio of the precursor, thereby achieving a balance between high compaction and high rate.

[0035] In addition, doped iron phosphate with high iron-phosphorus ratio also contains any one or more of Ti, Mn, Mg, Ni, and rare earth that inhibit particle growth, and doped iron phosphate with low iron-phosphorus ratio also contains F that promotes particle growth. The application of doping elements will change the activation energy of the reaction. For example, Mn, Mg, Ni, etc., will inhibit the growth of primary particles, sintering fusion, etc. after doping. The application of F element will promote the growth and sintering fusion of primary particles.

[0036] In the process of synthesizing lithium iron phosphate, the comprehensive application of different iron-phosphorus ratios and doping elements can better synthesize particles of different sizes, thereby achieving grading between particles and obtaining high-density, high-rate iron phosphate phosphorus materials.

[0037] In order to enable those skilled in the art to more clearly understand the technical solution of the present application, the technical solution of the present application will be described in detail below in conjunction with specific embodiments.

[0038] Example 1

[0039] Doped iron phosphate with high iron-phosphorus ratio (iron-phosphorus ratio of 0.985:1) and doped iron phosphate with low iron-phosphorus ratio (iron-phosphorus ratio of 0.96:1) were selected. The doping elements in the doped iron phosphate with high iron-phosphorus ratio are Ti, Mn, Mg, and Ni, of which Ti accounts for 4000ppm of the mass of iron phosphate powder, Mn is 1500ppm, Mg is 1500ppm, and Ni is 1500ppm. The doping elements in the doped iron phosphate with low iron-phosphorus ratio are Ti and F, of which Ti accounts for 2000ppm of the mass of iron phosphate powder, and F is 5000ppm.

[0040] Adding Li to doped iron phosphate with high iron-phosphorus ratio 2 CO 3 , glucose (8% of the powder mass in slurry A), PEG2000 (1% of the powder mass in slurry A), deionized water as solvent, stirring to prepare slurry A with a solid content of 35%, and controlling the amount ratio of Li and Fe in the pre-slurry A to be 1.03:1. Coarse grinding in a liquid phase sand mill for 30 minutes, grinding speed 500rpm, then switching to fine grinding, grinding speed 700rpm, until the D50 particle size of slurry A is 0.10μm, to obtain precursor slurry A.

[0041] Adding Li to doped iron phosphate with low iron-phosphorus ratio 2 CO 3 , glucose (8% of the powder mass in slurry B), PEG2000 (1% of the powder mass in slurry B), deionized water as solvent, and stirred to prepare a precursor slurry B with a solid content of 35%, and the ratio of Li and Fe substances in the precursor slurry B was controlled to be 1.03:1. Coarse grinding was performed in a liquid phase sand mill for 30 minutes at a grinding speed of 500 rpm, and then fine grinding was performed at a grinding speed of 700 rpm until the D50 particle size of slurry B was 0.50 μm, thereby obtaining a precursor slurry B.

[0042] Precursor slurry A and precursor slurry B are mixed in a mass ratio of 2:3. The obtained mixed slurry is peristaltically pumped into a spray drying furnace with an inlet temperature of 200°C, an outlet temperature of 100°C, and a spray atmosphere of nitrogen. The dry powder obtained by thermal spraying is then collected by a cyclone and pulverized by air flow to obtain a lithium iron phosphate precursor.

[0043] The lithium iron phosphate precursor was transferred to a high-temperature kiln, and sintered at 650°C for 4 hours and then at 790°C for 16 hours under a nitrogen atmosphere. The sintered powder was then pulverized by air flow, demagnetized and sieved to obtain the lithium iron phosphate material. The heating rate in the temperature zone was 5°C / min.

[0044] Example 2

[0045] Doped iron phosphate with high iron-phosphorus ratio (iron-phosphorus ratio of 0.98:1) and doped iron phosphate with low iron-phosphorus ratio (iron-phosphorus ratio of 0.965:1) were selected. The doping elements in the doped iron phosphate with high iron-phosphorus ratio are Ti, Mn, and Mg, wherein the doping amount of Ti in the iron phosphate powder is 4000ppm, Mn is 1500ppm, and Mg is 1500ppm. The doping elements in the doped iron phosphate with low iron-phosphorus ratio are Ti and F, wherein the doping amount of Ti in the iron phosphate powder is 2000ppm, and F is 5000ppm.

[0046] Lithium acetate, sucrose (8% of the mass of the powder in slurry A), and PEG2000 (1% of the mass of the powder in slurry A) were added to the doped iron phosphate with a high iron-phosphorus ratio, and deionized water was used as a solvent to stir to obtain a slurry A with a solid content of 35%, and the ratio of the amount of Li and Fe substances in the pre-slurry A was controlled to be 1.03:1. The pre-slurry was coarsely ground in a liquid phase sand mill for 30 minutes at a grinding speed of 500 rpm, and then finely ground at a grinding speed of 700 rpm until the D50 particle size of the slurry A was 0.15 μm, thereby obtaining a precursor slurry A.

[0047] Lithium acetate, sucrose (8% of the powder mass in slurry B), and PEG2000 (1% of the powder mass in slurry B) were added to the doped iron phosphate with a low iron-phosphorus ratio, and the precursor slurry B with a solid content of 35% was prepared by stirring with deionized water as a solvent, and the ratio of the amount of Li and Fe substances in the precursor slurry B was controlled to be 1.03:1. The mixture was coarsely ground in a liquid phase sand mill for 30 minutes at a grinding speed of 500 rpm, and then finely ground at a grinding speed of 700 rpm until the D50 particle size of the slurry B was 0.45 μm, thereby obtaining the precursor slurry B.

[0048] Precursor slurry A and precursor slurry B are mixed in a mass ratio of 2:3. The obtained mixed slurry is peristaltically pumped into a spray drying furnace with an inlet temperature of 200°C, an outlet temperature of 100°C, and a spray atmosphere of nitrogen. The dry powder obtained by thermal spraying is then collected by a cyclone and pulverized by air flow to obtain a lithium iron phosphate precursor.

[0049] The lithium iron phosphate precursor was transferred to a high-temperature kiln, and sintered at 650°C for 4 hours and then at 790°C for 16 hours under a nitrogen atmosphere. The sintered powder was then pulverized by air flow, demagnetized and sieved to obtain the lithium iron phosphate material. The heating rate in the temperature zone was 5°C / min.

[0050] Example 3

[0051] Doped iron phosphate with high iron-phosphorus ratio (iron-phosphorus ratio of 0.975:1) and doped iron phosphate with low iron-phosphorus ratio (iron-phosphorus ratio of 0.970:1) were selected. The doping elements in the doped iron phosphate with high iron-phosphorus ratio are Ti, Mg, and Ni, where the doping amount of Ti in the iron phosphate powder is 4000ppm, Mg is 1500ppm, and Ni is 1500ppm. The doping elements in the doped iron phosphate with low iron-phosphorus ratio are Ti and F, where the doping amount of Ti in the iron phosphate powder is 2000ppm, and F is 5000ppm.

[0052] Lithium carbonate, glucose (8% of the powder mass in slurry A), and PEG2000 (1% of the powder mass in slurry A) were added to the doped iron phosphate with a high iron-phosphorus ratio, and deionized water was used as a solvent to stir to obtain a slurry A with a solid content of 35%, and the ratio of the amount of Li and Fe substances in the pre-slurry A was controlled to be 1.03:1. The pre-slurry was coarsely ground in a liquid phase sand mill for 30 minutes at a grinding speed of 500 rpm, and then finely ground at a grinding speed of 700 rpm until the D50 particle size of the slurry A was 0.20 μm, thereby obtaining a precursor slurry A.

[0053] Lithium carbonate, glucose (8% of the mass of the powder in slurry B), and PEG2000 (1% of the mass of the powder in slurry B) were added to the doped iron phosphate with a low iron-phosphorus ratio, and the precursor slurry B with a solid content of 35% was prepared by stirring with deionized water as a solvent, and the ratio of the amount of Li and Fe substances in the precursor slurry B was controlled to be 1.03:1. The mixture was coarsely ground in a liquid phase sand mill for 30 minutes at a grinding speed of 500 rpm, and then finely ground at a grinding speed of 700 rpm until the D50 particle size of the slurry B was 0.40 μm, thereby obtaining the precursor slurry B.

[0054] Precursor slurry A and precursor slurry B are mixed in a mass ratio of 2:3. The obtained mixed slurry is peristaltically pumped into a spray drying furnace with an inlet temperature of 200°C, an outlet temperature of 100°C, and a spray atmosphere of nitrogen. The dry powder obtained by thermal spraying is then collected by a cyclone and pulverized by air flow to obtain a lithium iron phosphate precursor.

[0055] The lithium iron phosphate precursor was transferred to a high-temperature kiln, and sintered at 650°C for 4 hours and then at 790°C for 16 hours under a nitrogen atmosphere. The sintered powder was then pulverized by air flow, demagnetized and sieved to obtain the lithium iron phosphate material. The heating rate in the temperature zone was 5°C / min.

[0056] Example 4

[0057] Doped iron phosphate with high iron-phosphorus ratio (iron-phosphorus ratio of 0.975:1) and doped iron phosphate with low iron-phosphorus ratio (iron-phosphorus ratio of 0.970:1) were selected. The doping elements in the doped iron phosphate with high iron-phosphorus ratio are Ti, Mn, and Ni, where the doping amount of Ti in the iron phosphate powder is 4000ppm, Mn is 1500ppm, and Ni is 1500ppm. The doping elements in the doped iron phosphate with low iron-phosphorus ratio are Ti and F, where the doping amount of Ti in the iron phosphate powder is 2000ppm, and F is 5000ppm.

[0058] Adding Li to doped iron phosphate with high iron-phosphorus ratio 2 CO 3 , maltose (8% of the powder mass in slurry A), PEG2000 (1% of the powder mass in slurry A), deionized water as solvent, stirring to prepare slurry A with a solid content of 35%, and controlling the ratio of Li and Fe substances in the pre-slurry A to be 1.03:1. Coarse grinding in a liquid phase sand mill for 30 minutes, grinding speed 500rpm, then switching to fine grinding, grinding speed 700rpm, until the D50 particle size of slurry A is 0.25μm, to obtain precursor slurry A.

[0059] Adding Li to doped iron phosphate with low iron-phosphorus ratio 2 CO 3 , maltose (8% of the powder mass in slurry B), PEG2000 (1% of the powder mass in slurry B), deionized water as solvent, and stirred to prepare a precursor slurry B with a solid content of 35%, and the ratio of Li and Fe substances in the precursor slurry B was controlled to be 1.03:1. Coarse grinding was performed in a liquid phase sand mill for 30 minutes at a grinding speed of 500 rpm, and then fine grinding was performed at a grinding speed of 700 rpm until the D50 particle size of slurry B was 0.35 μm, thereby obtaining a precursor slurry B.

[0060] Precursor slurry A and precursor slurry B are mixed in a mass ratio of 1:1. The obtained mixed slurry is peristaltically pumped into a spray drying furnace with an inlet temperature of 200°C, an outlet temperature of 100°C, and a spray atmosphere of nitrogen. The dry powder obtained by thermal spraying is then collected by a cyclone and pulverized by air flow to obtain a lithium iron phosphate precursor.

[0061] The lithium iron phosphate precursor was transferred to a high-temperature kiln, and sintered at 650°C for 4 hours and then at 790°C for 16 hours under a nitrogen atmosphere. The sintered powder was then pulverized by air flow, demagnetized and sieved to obtain the lithium iron phosphate material. The heating rate in the temperature zone was 5°C / min.

[0062] Comparative Example 1

[0063] Iron phosphate doped with Ti and having an iron-phosphorus ratio of 0.97 was selected, wherein the doping amount of Ti in the iron phosphate powder was 4000 ppm.

[0064] Adding Li to the selected iron phosphate 2 CO 3 , glucose (8% of the powder mass in slurry A), PEG2000 (1% of the powder mass in slurry A), deionized water as solvent, stirring to prepare slurry A with a solid content of 35%, and controlling the amount ratio of Li and Fe in slurry A to be 1.03:1. Coarse grinding in a liquid phase sand mill for 30 minutes, grinding speed 500rpm, then switching to fine grinding, grinding speed 700rpm, until the D50 particle size of slurry A is 0.30μm, to obtain precursor slurry A.

[0065] The precursor slurry A was peristaltically pumped into a spray drying furnace at a feed rate of 5 L / h, an inlet temperature of 240°C, and an outlet temperature of 120°C. The dry powder obtained by thermal spraying was then collected by a cyclone and pulverized by air flow to obtain a lithium iron phosphate precursor.

[0066] The lithium iron phosphate precursor was transferred into a high-temperature kiln, sintered at 500°C for 4 hours and then at 700°C for 14 hours under a nitrogen atmosphere, and the sintered powder was pulverized by air flow to obtain a capacity-type lithium iron phosphate material. The heating rate of the temperature zone was 5°C / min.

[0067] Comparative Example 2

[0068] Doped iron phosphate with high iron-phosphorus ratio (iron-phosphorus ratio of 0.98) and doped iron phosphate with low iron-phosphorus ratio (iron-phosphorus ratio of 0.965) were selected. The doping element in the doped iron phosphate with high iron-phosphorus ratio is Ti, wherein the doping amount of Ti in the iron phosphate powder is 4000ppm. The doping element in the doped iron phosphate with low iron-phosphorus ratio is Ti, wherein the doping amount of Ti in the iron phosphate powder is 2000ppm.

[0069] Precursor slurry A and precursor slurry B were mixed, the mass ratio of precursor slurry A to precursor slurry B was 3:2, and then Li 2 CO 3 , sucrose (8% of the powder mass in slurry A), PEG (1% of the powder mass in slurry A), deionized water as solvent, stirring to prepare slurry A with a solid content of 25%, and controlling the amount ratio of Li and Fe in the pre-slurry A to be 1.03:1. Coarse grinding in a liquid phase sand mill for 30 minutes, grinding speed 500rpm, then switching to fine grinding, grinding speed 700rpm, until the D50 particle size of slurry A is 0.30μm, to obtain a precursor slurry.

[0070] The obtained mixed slurry was peristaltically pumped into a spray drying furnace with a feed rate of 5 L / h, an inlet temperature of 220°C, an outlet temperature of 100°C, and a spray atmosphere of nitrogen. The dry powder obtained by thermal spraying was then collected by a cyclone and pulverized by air flow to obtain a lithium iron phosphate precursor.

[0071] The lithium iron phosphate precursor was transferred to a high-temperature kiln, and sintered in a 650℃ temperature zone for 4 hours under a helium atmosphere, and then sintered in a 850℃ temperature zone for 11 hours. The sintered powder was then airflow crushed and transferred to a high-temperature kiln for secondary sintering, and sintered in a 650℃ temperature zone for 5 hours. The lithium iron phosphate material was obtained after demagnetization and sieving. The temperature zone heating rate was 5℃ / min.

[0072] Physical performance comparison:

[0073] The powder specific surface area, powder compaction density PD, powder tap density, and powder resistance of the sintered materials obtained in the examples and comparative examples were tested. The test data are shown in Table 1.

[0074] Table 1

[0075]

[0076] As can be seen from Table 1, the compaction of the lithium iron phosphate materials prepared in Examples 1-4 is much higher than that of the lithium iron phosphate materials prepared in Comparative Examples 1 and 2, and has better processing performance. The present application obtains high-compacted, high-rate lithium iron phosphate powder by controlling the design of precursors, the design of doping elements, and the particle size of grinding particles.

[0077] Electrical performance comparison:

[0078] Button cell preparation:

[0079] Add conductive carbon black to the polyvinylidene fluoride (PVDF) aqueous solution, then add the lithium iron phosphate material prepared in the example or comparative example, stir evenly, and evenly coat the slurry on the aluminum foil on a coating machine to make a pole piece. Put the coated pole piece into a vacuum drying oven at a temperature of 120°C for vacuum drying for 6 hours, take out the pole piece and roll it on a roller press for standby use.

[0080] The button cell assembly was carried out in a glove box under an argon atmosphere, and the electrolyte was 1 mL i PF 6 +EC:DEC:DMC=1:1:1 (volume ratio), metal lithium sheet is the counter electrode. The capacity test was carried out on an American Arbin BT2000 battery tester, with a charge and discharge voltage range of 2 to 3.75V and a charge and discharge rate of 0.1C and 1C respectively.

[0081] Table 2

[0082]

[0083] It can be seen from Table 2 that the button batteries using lithium iron phosphate materials as positive electrode materials prepared in Examples 1-4 have slightly higher 0.1C charge and discharge gram capacity, better rate performance, a higher proportion of the 1C discharge 3.2V platform, smaller polarization, and better power performance than the button batteries using lithium iron phosphate materials as positive electrode materials prepared in Comparative Examples 1 and 2.

[0084] It should be noted that the technical solutions or technical features described in the above embodiments can be combined or supplemented with each other without causing conflicts. The scope of protection of this application is not limited to the above embodiments, and all modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of this application should be included in the scope of protection of this application.

Claims

1. A method for preparing lithium iron phosphate, characterized in that: The method comprises: Adding a first lithium source, a first carbon source, a third carbon source and a solvent to doped iron phosphate with a high iron-to-phosphorus ratio, mixing and grinding to obtain a precursor slurry A; Adding a second lithium source, a second carbon source, a fourth carbon source and a solvent to the doped iron phosphate with a low iron-phosphorus ratio, mixing and grinding to obtain a precursor slurry B; The precursor slurry A and the precursor slurry B are mixed and then spray-dried to obtain a dry powder; Sintering the dry powder in an inert gas environment to obtain lithium iron phosphate; Wherein, the doping elements in the high iron-phosphorus ratio doped iron phosphate are a first doping element and a second doping element, the first doping element is any one or more of Mn, Mg, Ni, and rare earth, and the second doping element is Ti or V; The doping elements in the doped ferric phosphate with a low iron-phosphorus ratio are a second doping element and a third doping element, and the third doping element is F.

2. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The precursor slurry A D 50 is 0.10-0.25 μm; D of the precursor slurry B 50 0.35-0.50μm.

3. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The doping amount of Ti or V is 500-5000ppm of the mass of the doped iron phosphate.

4. The method for preparing lithium iron phosphate according to claim 1, characterized in that: The doping amount of the first doping element is 500-3000 ppm of the mass of the doped iron phosphate; the doping amount of the third doping element is 1000-6000 ppm of the mass of the doped iron phosphate.

5. The method for preparing lithium iron phosphate according to any one of claims 1 to 4, characterized in that: The mass ratio of the precursor slurry A to the precursor slurry B is 2:3-1:

1.

6. The method for preparing lithium iron phosphate according to any one of claims 1 to 4, characterized in that: The high iron-phosphorus ratio is 0.975≤iron-phosphorus ratio≤1, and the low iron-phosphorus ratio is 0.96≤iron-phosphorus ratio<0.

975.

7. The method for preparing lithium iron phosphate according to any one of claims 1 to 4, characterized in that: The first lithium source and the second lithium source are one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium phosphate, lithium acetate, lithium sulfate, lithium nitrate and lithium chloride; The first carbon source and the second carbon source are at least one of glucose, sucrose, lactose, maltose, and citric acid; The third carbon source and the fourth carbon source are at least one of PEG2000, PEG4000, PEG6000 and PVA2000; The solvent is deionized water or anhydrous ethanol.

8. A lithium iron phosphate, characterized in that: The lithium iron phosphate is prepared by the preparation method described in any one of claims 1-7.

9. Application of lithium iron phosphate in lithium ion batteries, characterized in that: The lithium iron phosphate is the lithium iron phosphate according to claim 8.

10. A positive electrode of a lithium ion battery, characterized in that: The positive electrode of the lithium-ion battery is made of a positive electrode material comprising the lithium iron phosphate according to claim 8.

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