Lithium iron phosphate, preparation method and application thereof

Abstract

The application belongs to the technical field of battery materials, and particularly relates to a lithium iron phosphate and a preparation method and application thereof. The preparation method of the lithium iron phosphate comprises the following steps: (1) mixing iron phosphate A, a lithium source, a carbon source and water, and then grinding to obtain slurry C, wherein the iron-phosphorus molar ratio of the iron phosphate A is 0.950-0.980, and the specific surface area is 7-15 m 2 ; (2) mixing iron phosphate B, a lithium source, a carbon source and water, and then grinding to obtain slurry D, wherein the iron-phosphorus molar ratio of the iron phosphate B is 0.970-0.998, and the specific surface area is 1-7 m 2 ; (3) mixing the slurry C and the slurry D to obtain slurry E, spray drying the slurry E, roasting, and then crushing to obtain the lithium iron phosphate.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of battery materials, and particularly relates to a lithium iron phosphate and a preparation method and application thereof. BACKGROUND

[0002] With the increasing global energy demand and the pursuit of renewable energy, the development of electric vehicles, portable electronic devices and other fields has become a trend in today's society. Lithium ion batteries, as a kind of high energy density, long cycle life and environmentally friendly energy storage device, are widely used in these fields. Lithium iron phosphate, as an important positive electrode material of lithium ion battery, has attracted widespread attention due to its good safety, low price and long cycle life. The electrochemical performance of lithium iron phosphate determines the performance of the battery, including energy density, cycle life, safety and the like. Therefore, improving the performance of lithium iron phosphate has been one of the research hotspots in the field of lithium ion batteries. However, the traditional preparation method of lithium iron phosphate has some problems, such as low energy density, fast capacity decay and the like, which limits its application in lithium ion batteries.

[0003] The traditional preparation method of lithium iron phosphate is usually to prepare a lithium iron phosphate precursor by a solid phase method or a wet synthesis method, and then to add a lithium source to obtain a lithium iron phosphate material by calcination under a nitrogen atmosphere at a high temperature. However, this method has problems such as uneven particle size of lithium iron phosphate, low crystallinity, weak binding force between particles and the like, which leads to unstable battery performance, low capacity and short cycle life, and cannot meet the requirements of battery application. SUMMARY

[0004] The present application aims to at least solve one of the problems in the related art. To this end, the present application provides a lithium iron phosphate and a preparation method and application thereof, which effectively improves the compaction density and capacity of lithium iron phosphate, and meets the requirements of battery application.

[0005] The above technical purpose of the present application is achieved by the following technical scheme:

[0006] A preparation method of lithium iron phosphate, comprising the following steps: (1) mixing lithium iron phosphate A with a lithium source, a carbon source and water to obtain slurry C after grinding, the iron-phosphorus molar ratio of the lithium iron phosphate A is 0.950-0.980, and the specific surface area is 7-15 m 2 / g; (2) mixing lithium iron phosphate B with a lithium source, a carbon source and water to obtain slurry D after grinding, the iron-phosphorus molar ratio of the lithium iron phosphate B is 0.970-0.998, and the specific surface area is 1-7 m 2 / g; (3) mixing the slurry C and the slurry D to obtain slurry E, and then spray drying and calcining the slurry E to obtain lithium iron phosphate after crushing.

[0007] In an embodiment, the particle size D50 of the iron phosphate A is 1-6 μm, and the particle size D50 of the iron phosphate B is 6-15 μm.

[0008] In an embodiment, the particle size D50 of the iron phosphate A is 2-6 μm, and the particle size D50 of the iron phosphate B is 6-12 μm.

[0009] In an embodiment, the iron to phosphorus molar ratio of the iron phosphate A is 0.960-0.975, and the specific surface area is 7-11 m 2 / g; and the iron to phosphorus molar ratio of the iron phosphate B is 0.980-0.995, and the specific surface area is 3-7 m 2 / g.

[0010] In an embodiment, the morphology of the primary particles of the iron phosphate A is lamellar.

[0011] In an embodiment, the morphology of the primary particles of the iron phosphate B is massive.

[0012] In an embodiment, the molar ratio of Fe, P and Li in the slurry C is 1:(1.025-1.040):(1.010-1.070).

[0013] In an embodiment, the molar ratio of Fe, P and Li in the slurry C is 1:(1.027-1.038):(1.015-1.065).

[0014] In an embodiment, the molar ratio of Fe, P and Li in the slurry D is 1:(1.005-1.020):(1.010-1.070).

[0015] In an embodiment, the molar ratio of Fe, P and Li in the slurry D is 1:(1.008-1.015):(1.015-1.065).

[0016] In an embodiment, in the slurry C, the mass of the carbon source is 3%-15% of the total mass of the iron phosphate and the lithium source, and the solid content of the slurry C is 30%-45%.

[0017] In an embodiment, in the slurry C, the mass of the carbon source is 4%-13% of the total mass of the iron phosphate and the lithium source, and the solid content of the slurry C is 33%-42%.

[0018] In an embodiment, in the slurry D, the mass of the carbon source is 3%-15% of the total mass of the iron phosphate and the lithium source, and the solid content of the slurry D is 30%-45%.

[0019] In an embodiment, in the slurry D, the mass of the carbon source is 4%-13% of the total mass of the iron phosphate and the lithium source, and the solid content of the slurry D is 33%-42%.

[0020] In an embodiment, the slurry C and the slurry D are mixed in a mass ratio of (0.5-0.8):(0.2-0.5) to obtain the slurry E.

[0021] In an embodiment, the particle size D50 of the slurry C is 0.38-0.60 μm.

[0022] In an embodiment, the particle size D50 of the slurry C is 0.40-0.55 μm.

[0023] In an embodiment, the particle size D50 of the slurry D is 0.80-1.40 μm.

[0024] In an embodiment, the particle size D50 of the slurry D is 0.85-1.35 μm.

[0025] In an embodiment, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate and lithium acetate.

[0026] In an embodiment, the carbon source is at least one of sucrose, glucose, maltose, starch, polyvinyl alcohol, polyethylene glycol and Tween.

[0027] In an embodiment, the calcination comprises two-stage calcination or three-stage calcination, wherein the two-stage calcination refers to first calcination at a temperature raising rate of 5-15 ℃ / min to 300-500 ℃ for 2-6 h, and then calcination at a temperature raising rate of 10-30 ℃ / min to 700-850 ℃ for 8-16 h; and the three-stage calcination refers to first calcination at a temperature raising rate of 5-15 ℃ / min to 300-500 ℃ for 2-6 h, then calcination at a temperature raising rate of 5-15 ℃ / min to 500-700 ℃ for 4-10 h, and then calcination at a temperature raising rate of 10-30 ℃ / min to 700-850 ℃ for 8-16 h.

[0028] A lithium iron phosphate prepared by the preparation method as described above.

[0029] In an embodiment, the D50 of the lithium iron phosphate is 0.8-1.5 μm, the BET is 11-14 m 2 / g, the tap density is 2.50-2.60 g / cc, and the discharge capacity at 3.75 V / 0.1 C is 155-161 mAh / g.

[0030] A lithium ion battery comprising the lithium iron phosphate as described above.

[0031] The present application has the following advantages:

[0032] (1) The preparation method of the lithium iron phosphate of the present application can realize the matching and optimization of the particle size distribution of the lithium iron phosphate positive material by using two specific iron-phosphorus molar ratio, particle size and specific surface area of iron phosphate to carry out grading and roasting, thereby effectively improving the compaction density and capacity of the lithium iron phosphate positive material.

[0033] (2) The preparation method of the lithium iron phosphate of the present application can realize the matching and optimization of the particle size distribution of the lithium iron phosphate positive material by using two specific iron-phosphorus molar ratio, particle size and specific surface area of iron phosphate to carry out grading and roasting, thereby effectively improving the compaction density and capacity of the lithium iron phosphate positive material.

[0034] (3) The preparation method of the lithium iron phosphate of the present application can realize the matching and optimization of the particle size distribution of the lithium iron phosphate positive material by using two specific iron-phosphorus molar ratio, particle size and specific surface area of iron phosphate to carry out grading and roasting, thereby effectively improving the compaction density and capacity of the lithium iron phosphate positive material. BRIEF DESCRIPTION OF DRAWINGS

[0035] Figure 1 SEM image of the lithium iron phosphate prepared in Example 1 of the present application;

[0036] Figure 2 XRD image of the lithium iron phosphate prepared in Example 1 of the present application. DETAILED DESCRIPTION

[0037] The present application will be further described below in conjunction with specific examples.

[0038] Example 1:

[0039] A preparation method of lithium iron phosphate, comprising the following steps:

[0040] (1) 75 kg of iron phosphate A, 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol and 181.5 kg of pure water were mixed and then coarsely ground and finely ground to slurry C with a particle size D50 of 0.5 μm and a solid content of 36%, wherein the iron phosphate A has an iron-phosphorus molar ratio of 0.970, a particle size D50 of 4.5 μm and a specific surface area of 8.6 m 2 / g, and the iron phosphate A has a flaky morphology;

[0041] (2) 25 kg of iron phosphate B, 6.25 kg of lithium carbonate, 1.645 kg of glucose, 1.125 kg of polyethylene glycol and 60.5 kg of pure water were mixed and then coarsely ground and finely ground to slurry D with a particle size D50 of 1.2 μm and a solid content of 36%, wherein the iron phosphate B has an iron-phosphorus molar ratio of 0.991, a particle size D50 of 8.1 μm and a specific surface area of 5.4 m 2 / g, and the iron phosphate B has a blocky morphology;

[0042] (3) The slurry C and the slurry D were mixed to obtain slurry E with a mass ratio of 0.75:0.25, and the slurry E was spray dried to obtain a powder, and the powder was calcined in a nitrogen atmosphere in two stages, specifically, first, the temperature was raised to 350 ℃ at a rate of 10 ℃ / min and calcined for 4 h, and then the temperature was raised to 760 ℃ at a rate of 20 ℃ / min and calcined for 12 h. After calcination, the material was crushed, classified and sieved to obtain lithium iron phosphate. The SEM image of the lithium iron phosphate is shown in Figure 1 , and the XRD image of the lithium iron phosphate is shown in Figure 2 . It can be seen from Figure 1 that the synthesized lithium iron phosphate is spherical particles, both large particles and small particles, the small particles fill the gap between the large particles, and the secondary particles are soft agglomerates. It can be seen from Figure 2 that the lithium iron phosphate is a 29-0715 crystal form, and no new crystal form and impurity peak is generated.

[0043] Example 2:

[0044] A preparation method of lithium iron phosphate, comprising the following steps:

[0045] (1) 50 kg of iron phosphate A, 12.5 kg of lithium carbonate, 3.29 kg of glucose, 2.25 kg of polyethylene glycol and 121 kg of pure water were mixed and then coarsely ground and finely ground to slurry C with a particle size D50 of 0.5 μm and a solid content of 36%, wherein the iron phosphate A has an iron-phosphorus molar ratio of 0.970, a particle size D50 of 4.5 μm and a specific surface area of 8.6 m 2 / g, and the iron phosphate A has a flaky morphology;

[0046] (2) 25 kg of iron phosphate B, 6.25 kg of lithium carbonate, 1.645 kg of glucose, 1.125 kg of polyethylene glycol and 60.5 kg of pure water are mixed and then coarsely ground and finely ground to slurry D with a particle size D50 of 1.2 μm and a solid content of 36%, wherein the iron phosphate B has an iron-phosphorus molar ratio of 0.991, a particle size D50 of 8.1 μm, a specific surface area of 5.4 m2 / g, and a morphology of blocky morphology; 2 / g, the iron phosphate B has a morphology of blocky morphology;

[0047] (3) The slurry C and the slurry D are mixed at a mass ratio of 0.50:0.50 to obtain slurry E, the slurry E is spray dried to obtain a powder, and the powder is subjected to two-stage calcination in a nitrogen atmosphere, specifically, first calcined at a rate of 10 ℃ / min to 350 ℃ for 4 h, and then calcined at a rate of 20 ℃ / min to 760 ℃ for 12 h. After calcination, the material is crushed, classified and sieved to obtain lithium iron phosphate.

[0048] Example 3:

[0049] A method for preparing lithium iron phosphate, comprising the following steps:

[0050] (1) 75 kg of iron phosphate A, 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol and 181.5 kg of pure water are mixed and then coarsely ground and finely ground to slurry C with a particle size D50 of 0.5 μm and a solid content of 36%, wherein the iron phosphate A has an iron-phosphorus molar ratio of 0.970, a particle size D50 of 4.5 μm, a specific surface area of 8.6 m2 / g, and a morphology of flaky morphology; 2 / g, the iron phosphate A has a morphology of flaky morphology;

[0051] (2) 25 kg of iron phosphate B, 6.25 kg of lithium carbonate, 1.645 kg of glucose, 1.125 kg of polyethylene glycol and 60.5 kg of pure water are mixed and then coarsely ground and finely ground to slurry D with a particle size D50 of 1.2 μm and a solid content of 36%, wherein the iron phosphate B has an iron-phosphorus molar ratio of 0.991, a particle size D50 of 8.1 μm, a specific surface area of 5.4 m2 / g, and a morphology of blocky morphology; 2 / g, the iron phosphate B has a morphology of blocky morphology;

[0052] (3) The slurry C and the slurry D are mixed at a mass ratio of 0.75:0.25 to obtain slurry E, the slurry E is spray dried to obtain a powder, and the powder is subjected to three-stage calcination in a nitrogen atmosphere, specifically, first calcined at a rate of 10 ℃ / min to 350 ℃ for 4 h, then calcined at a rate of 10 ℃ / min to 600 ℃ for 6 h, and finally calcined at a rate of 10 ℃ / min to 760 ℃ for 12 h. After calcination, the material is crushed, classified and sieved to obtain lithium iron phosphate.

[0053] Example 4:

[0054] A method for preparing lithium iron phosphate, comprising the following steps:

[0055] (1) 75 kg of iron phosphate A, 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol and 181.5 kg of pure water are mixed and then sequentially subjected to rough grinding and fine grinding to obtain slurry C with a particle size D50 of 0.5 μm and a solid content of 36%, wherein the iron-phosphorus molar ratio of the iron phosphate A is 0.960, the particle size D50 is 3.5 μm, and the specific surface area is 9.7 m 2 / g, and the morphology of the iron phosphate A is flaky;

[0056] (2) 25 kg of iron phosphate B, 6.25 kg of lithium carbonate, 1.645 kg of glucose, 1.125 kg of polyethylene glycol and 60.5 kg of pure water are mixed and then sequentially subjected to rough grinding and fine grinding to obtain slurry D with a particle size D50 of 1.2 μm and a solid content of 36%, wherein the iron-phosphorus molar ratio of the iron phosphate B is 0.995, the particle size D50 is 8.5 μm, and the specific surface area is 3.4 m 2 / g, and the morphology of the iron phosphate B is blocky;

[0057] (3) The slurry C and the slurry D are mixed at a mass ratio of 0.75:0.25 to obtain slurry E, the slurry E is spray dried to obtain a powder, and the powder is subjected to two-stage calcination in a nitrogen atmosphere, specifically, first calcined at a rate of 10 ℃ / min to 350 ℃ for 4 h, and then calcined at a rate of 20 ℃ / min to 760 ℃ for 12 h, and then the calcined material is crushed, classified and sieved to obtain lithium iron phosphate.

[0058] Example 5: (The difference from Example 1 is that in step (3), the slurry C and the slurry D are mixed at a ratio of 0.25:0.75 to obtain the slurry E, and then sprayed)

[0059] A method for preparing lithium iron phosphate, comprising the following steps:

[0060] (1) 75 kg of iron phosphate A, 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol and 181.5 kg of pure water are mixed and then sequentially subjected to rough grinding and fine grinding to obtain slurry C with a particle size D50 of 0.5 μm and a solid content of 36%, wherein the iron-phosphorus molar ratio of the iron phosphate A is 0.970, the particle size D50 is 4.5 μm, and the specific surface area is 8.6 m 2 / g, and the morphology of the iron phosphate A is flaky;

[0061] (2) 25 kg of iron phosphate B, 6.25 kg of lithium carbonate, 1.645 kg of glucose, 1.125 kg of polyethylene glycol, and 60.5 kg of pure water are mixed, and then coarse grinding and fine grinding are sequentially performed to obtain slurry D with a particle size D50 of 1.2 μm and a solid content of 36%, wherein the iron phosphate B has an iron-phosphorus molar ratio of 0.991, a particle size D50 of 8.1 μm, a specific surface area of 5.4 m2 / g, and a morphology of blocky morphology; 2 / g, the iron phosphate B has a morphology of blocky morphology;

[0062] (3) The slurry C and the slurry D are mixed at a mass ratio of 0.25:0.75 to obtain slurry E, the slurry E is spray dried to obtain a powder, and the powder is calcined in a nitrogen atmosphere, specifically, first calcined at a rate of 10 ℃ / min to 350 ℃ for 4 h, and then calcined at a rate of 20 ℃ / min to 760 ℃ for 12 h, and then the calcined material is crushed, classified, and sieved to obtain lithium iron phosphate.

[0063] Example 6: (The difference from Example 1 is that in step (3), the temperature is raised to 760 ℃ at a rate of 15 ℃ / min and sintered for 12 h)

[0064] A method for preparing lithium iron phosphate, comprising the following steps:

[0065] (1) 75 kg of iron phosphate A, 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol, and 181.5 kg of pure water are mixed, and then coarse grinding and fine grinding are sequentially performed to obtain slurry C with a particle size D50 of 0.5 μm and a solid content of 36%, wherein the iron phosphate A has an iron-phosphorus molar ratio of 0.970, a particle size D50 of 4.5 μm, a specific surface area of 8.6 m2 / g, and a morphology of flaky morphology; 2 / g, the iron phosphate A has a morphology of flaky morphology;

[0066] (2) 25 kg of iron phosphate B, 6.25 kg of lithium carbonate, 1.645 kg of glucose, 1.125 kg of polyethylene glycol, and 60.5 kg of pure water are mixed, and then coarse grinding and fine grinding are sequentially performed to obtain slurry D with a particle size D50 of 1.2 μm and a solid content of 36%, wherein the iron phosphate B has an iron-phosphorus molar ratio of 0.991, a particle size D50 of 8.1 μm, a specific surface area of 5.4 m2 / g, and a morphology of blocky morphology; 2 / g, the iron phosphate B has a morphology of blocky morphology;

[0067] (3) The slurry C and the slurry D are mixed at a mass ratio of 0.75:0.25 to obtain slurry E, the slurry E is spray dried to obtain a powder, and the powder is calcined in a nitrogen atmosphere, specifically, first calcined at a rate of 15 ℃ / min to 760 ℃ for 12 h, and then the calcined material is crushed, classified, and sieved to obtain lithium iron phosphate.

[0068] Example 7: (The difference from Example 1 is that the two-stage sintering temperature in step (3) is 900℃, and the sintering time is 12h)

[0069] A preparation method of lithium iron phosphate, comprising the following steps:

[0070] (1) 75 kg of iron phosphate A, 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol, and 181.5 kg of pure water are mixed, and then coarse grinding and fine grinding are sequentially performed to obtain a slurry C with a particle size D50 of 0.5 μm and a solid content of 36%, wherein the iron-phosphorus molar ratio of the iron phosphate A is 0.970, the particle size D50 is 4.5 μm, and the specific surface area is 8.6 m 2 / g, and the morphology of the iron phosphate A is flaky;

[0071] (2) 25 kg of iron phosphate B, 6.25 kg of lithium carbonate, 1.645 kg of glucose, 1.125 kg of polyethylene glycol, and 60.5 kg of pure water are mixed, and then coarse grinding and fine grinding are sequentially performed to obtain a slurry D with a particle size D50 of 1.2 μm and a solid content of 36%, wherein the iron-phosphorus molar ratio of the iron phosphate B is 0.991, the particle size D50 is 8.1 μm, and the specific surface area is 5.4 m 2 / g, and the morphology of the iron phosphate B is blocky;

[0072] (3) The slurry C and the slurry D are mixed in a mass ratio of 0.75:0.25 to obtain a slurry E, the slurry E is spray dried to obtain a powder, and the powder is subjected to two-stage calcination in a nitrogen atmosphere, specifically, first calcined at 350℃ for 4h at a rate of 10℃ / min, and then calcined at 900℃ for 12h at a rate of 20℃ / min, and then the calcined material is crushed, classified, and sieved to obtain lithium iron phosphate.

[0073] Comparative Example 1: (The difference from Example 1 is that only iron phosphate A is used as a raw material)

[0074] A preparation method of lithium iron phosphate, comprising the following steps:

[0075] (1) 75 kg of iron phosphate A, 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol, and 181.5 kg of pure water are mixed, and then coarse grinding and fine grinding are sequentially performed to obtain a slurry C with a particle size D50 of 0.5 μm and a solid content of 36%, wherein the iron-phosphorus molar ratio of the iron phosphate A is 0.970, the particle size D50 is 4.5 μm, and the specific surface area is 8.6 m 2 / g, and the morphology of the iron phosphate A is flaky;

[0076] (2) 25 kg of iron phosphate A, 6.25 kg of lithium carbonate, 1.645 kg of glucose, 1.125 kg of polyethylene glycol, and 60.5 kg of pure water are mixed, and then coarse grinding and fine grinding are sequentially performed to obtain slurry D with a particle size D50 of 1.2 μm and a solid content of 36%;

[0077] (3) The slurry C and the slurry D are mixed at a mass ratio of 0.75:0.25 to obtain slurry E, the slurry E is spray dried to obtain a powder, and the powder is subjected to two-stage calcination in a nitrogen atmosphere, specifically, first calcined at a rate of 10 ℃ / min to 350 ℃ for 4 h, and then calcined at a rate of 20 ℃ / min to 760 ℃ for 12 h, and then the calcined material is crushed, classified, and sieved to obtain lithium iron phosphate.

[0078] Comparative Example 2: (The difference from Example 1 is that only iron phosphate B is used as a raw material)

[0079] A method for preparing lithium iron phosphate, comprising the following steps:

[0080] (1) 75 kg of iron phosphate B, 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol, and 181.5 kg of pure water are mixed, and then coarse grinding and fine grinding are sequentially performed to obtain slurry C with a particle size D50 of 0.5 μm and a solid content of 36%;

[0081] (2) 25 kg of iron phosphate B, 6.25 kg of lithium carbonate, 1.645 kg of glucose, 1.125 kg of polyethylene glycol, and 60.5 kg of pure water are mixed, and then coarse grinding and fine grinding are sequentially performed to obtain slurry D with a particle size D50 of 1.2 μm and a solid content of 36%, wherein the iron-phosphorus molar ratio of the iron phosphate B is 0.991, the particle size D50 of the iron phosphate B is 8.1 μm, the specific surface area of the iron phosphate B is 5.4 m 2 / g, and the morphology of the iron phosphate B is block morphology;

[0082] (3) The slurry C and the slurry D are mixed at a mass ratio of 0.75:0.25 to obtain slurry E, the slurry E is spray dried to obtain a powder, and the powder is subjected to two-stage calcination in a nitrogen atmosphere, specifically, first calcined at a rate of 10 ℃ / min to 350 ℃ for 4 h, and then calcined at a rate of 20 ℃ / min to 760 ℃ for 12 h, and then the calcined material is crushed, classified, and sieved to obtain lithium iron phosphate.

[0083] Comparative Example 3: (The difference from Example 1 is that the iron-phosphorus molar ratio of the iron phosphate A used is 1.001, the specific surface area of the iron phosphate A is 1.8 m 2 / g, and the morphology of the iron phosphate A is block morphology)

[0084] A method for preparing lithium iron phosphate, comprising the following steps:

[0085] (1) 75 kg of iron phosphate A, 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol and 181.5 kg of pure water are mixed and then coarsely ground and finely ground to slurry C with a particle size D50 of 0.5 μm and a solid content of 36%, wherein the iron phosphate A has an iron-phosphorus molar ratio of 1.001, a particle size D50 of 4.5 μm and a specific surface area of 1.8 m 2 / g, and the iron phosphate A has a blocky morphology;

[0086] (2) 25 kg of iron phosphate B, 6.25 kg of lithium carbonate, 1.645 kg of glucose, 1.125 kg of polyethylene glycol and 60.5 kg of pure water are mixed and then coarsely ground and finely ground to slurry D with a particle size D50 of 1.2 μm and a solid content of 36%, wherein the iron phosphate B has an iron-phosphorus molar ratio of 0.991, a particle size D50 of 8.1 μm and a specific surface area of 5.4 m 2 / g, and the iron phosphate B has a blocky morphology;

[0087] (3) The slurry C and the slurry D are mixed in a mass ratio of 0.75:0.25 to obtain slurry E, the slurry E is spray dried to obtain a powder, and the powder is subjected to two-stage calcination in a nitrogen atmosphere, specifically, first calcined at a rate of 10 ℃ / min to 350 ℃ for 4 h, and then calcined at a rate of 20 ℃ / min to 760 ℃ for 12 h. After calcination, the material is crushed, classified and sieved to obtain lithium iron phosphate.

[0088] Comparative Example 4: (The difference from Example 1 is that the iron phosphate A used has an iron-phosphorus molar ratio of 1.001, a specific surface area of 1.8 m 2 / g, and the iron phosphate A has a blocky morphology; the iron phosphate B has an iron-phosphorus molar ratio of 0.970, a specific surface area of 8.6 m 2 / g, and the iron phosphate B has a flaky morphology)

[0089] A preparation method of lithium iron phosphate, comprising the following steps:

[0090] (1) 75 kg of iron phosphate A, 18.75 kg of lithium carbonate, 4.935 kg of glucose, 3.375 kg of polyethylene glycol and 181.5 kg of pure water are mixed and then coarsely ground and finely ground to slurry C with a particle size D50 of 0.5 μm and a solid content of 36%, wherein the iron phosphate A has an iron-phosphorus molar ratio of 1.001, a particle size D50 of 4.5 μm and a specific surface area of 1.8 m 2 / g, and the iron phosphate A has a blocky morphology;

[0091] (2) 25 kg of iron phosphate B, 6.25 kg of lithium carbonate, 1.645 kg of glucose, 1.125 kg of polyethylene glycol, and 60.5 kg of pure water are mixed, and then coarse grinding and fine grinding are sequentially performed to obtain slurry D with a particle size D50 of 1.2 μm and a solid content of 36%, wherein the iron phosphate B has an iron-phosphorus molar ratio of 0.970, a particle size D50 of 8.1 μm, and a specific surface area of 8.6 m2 / g, and the iron phosphate B has a flaky morphology; 2 / g, and the iron phosphate B has a flaky morphology;

[0092] (3) The slurry C and the slurry D are mixed at a mass ratio of 0.75:0.25 to obtain slurry E, the slurry E is spray dried to obtain a powder, and the powder is subjected to two-stage calcination in a nitrogen atmosphere, specifically, first calcined at a rate of 10 ℃ / min to 350 ℃ for 4 h, and then calcined at a rate of 20 ℃ / min to 760 ℃ for 12 h, and then the calcined material is crushed, classified, and sieved to obtain lithium iron phosphate.

[0093] Test Example:

[0094] The tap density of the lithium iron phosphate of Examples 1-7 and Comparative Examples 1-4 is measured, and the lithium iron phosphate of Examples 1-7 and Comparative Examples 1-4 is used as a positive electrode material to assemble a button cell, and the specific steps include: the lithium iron phosphate, the conductive agent acetylene black, and the adhesive agent polyvinylidene fluoride are uniformly mixed in N-methylpyrrolidone at a mass ratio of 92:4:4 to prepare a slurry, which is then coated on an aluminum foil and dried in a vacuum drying box, and then pressed into a positive electrode sheet by a tablet press, and a metal lithium sheet is used as a negative electrode sheet, 1 mol / L of LiPF6-EC:DMC (volume ratio of 1:1) is used as an electrolyte, a polypropylene porous membrane is used as a separator, and the assembly of the battery is performed in an argon glove box.

[0095] The above battery is subjected to electrochemical performance testing, and the specific data is obtained by testing with an electrochemical workstation and the like. The first discharge capacity and the first efficiency are tested at room temperature 25 ℃, a charge-discharge voltage of 2.0-3.65 V, and a first charge-discharge rate of 0.1 C; the cycle performance of 200 cycles and the powder tap density are tested at room temperature 25 ℃, a charge-discharge voltage of 2.0-3.65 V, and a charge-discharge rate of 1 C, and the test results are shown in Table 1.

[0096] Table 1: Performance test results of lithium iron phosphate

[0097]

[0098] From Table 1, it can be seen that the compaction and capacity of Comparative Example 1 are slightly lower than those of Example 1, but the difference is not large. The main reason is that the activity of iron phosphate A is relatively high, and the particles are easy to grow during the sintering process, so the capacity is slightly deteriorated, but it does not cause a significant decrease in compaction. Comparative Example 2 only uses iron phosphate B as raw material. Since the activity of the material is poor, the particles are not easy to grow, the lithium ion migration ability is not significantly deteriorated, and the content of iron phosphate B is relatively high, the lithium storage capacity is strong, and the specific capacity is not deteriorated, but the compaction cannot meet the requirements of the third generation of lithium iron phosphate, and the comprehensive performance is not as good as that of Examples 1, 2 and 3. Comparative Example 3 has a large difference in specific surface area, iron-phosphorus ratio and morphology of iron phosphate A, and the activity is low, which affects the compaction of the product. The lithium ion migration and lithium storage capacity of the blocky morphology is strong, and the capacity is not deteriorated. Comparative Example 4 has different iron-phosphorus ratio, specific surface area and morphology of iron phosphate A and iron phosphate B compared with Example 1. The blocky iron phosphate is ground to small particle size, and the flaky iron phosphate is ground to large particle size. Since the blocky iron phosphate is not easy to grow, and the flaky iron phosphate is easy to grow, it will cause the filling of pores between large and small particles, resulting in low compaction. However, since the lithium ion migration and lithium storage capacity of the blocky iron phosphate is strong, the specific capacity is not significantly deteriorated. The sintering temperature of Example 7 is relatively high, the compaction performance is high, and the discharge performance is low. At the same time, the sintering temperature is too high, which is easy to generate iron phosphide impurities, affecting the electrochemical performance of the lithium iron phosphate material. The electrical performance results of the lithium iron phosphate cathode material prepared by the present application show that by matching different indicators of iron phosphate, different dimensional grading regulation can be realized, and compared with the comparative examples, the compaction and capacity performance of the lithium iron phosphate material can be improved.

Claims

1. A method of producing lithium iron phosphate, characterized by: The method comprises the following steps: (1) mixing iron phosphate A, a lithium source, a carbon source and water to obtain slurry C, the iron phosphate A has a molar ratio of iron to phosphorus of 0.950-0.980, a specific surface area of 7-15 m 2 / g; (2) mixing iron phosphate B with a lithium source, a carbon source and water to obtain slurry D, the iron phosphate B has a molar ratio of iron to phosphorus of 0.970-0.998, a specific surface area of 1-7 m 2 / g; (3) mixing the slurry C with the slurry D to obtain a slurry E, spray drying the slurry E, calcining, and crushing to obtain lithium iron phosphate; the particle size D50 of the phosphoric acid iron A is 1-6 μm, the particle size D50 of the phosphoric acid iron B is 6-15 μm; the morphology of the primary particles of the phosphoric acid iron A is lamellar; the morphology of the primary particles of the phosphoric acid iron B is massive.

2. The method of claim 1, wherein the lithium iron phosphate is prepared by the steps of: The iron-phosphorus molar ratio of the iron phosphate A is 0.960-0.975, and the specific surface area is 7-11 m 2 / g; the iron-phosphorus molar ratio of the iron phosphate B is 0.980-0.995, and the specific surface area is 3-7 m 2 / g. ​ 3. The method of claim 1, wherein the lithium iron phosphate is prepared by the steps of: The molar ratio of Fe, P and Li in the slurry C is 1:(1.025-1.040):(1.010-1.070), and the molar ratio of Fe, P and Li in the slurry D is 1:(1.005-1.020):(1.010-1.070). ​ 4. The method of claim 1, wherein the lithium iron phosphate is prepared by the steps of: The slurry C and the slurry D are mixed according to a mass ratio of (0.5-0.8):(0.2-0.5) to obtain the slurry E. ​ 5. The method of claim 1, wherein the lithium iron phosphate is prepared by the steps of: The particle size D50 of the slurry C is 0.38-0.60 μm, and the particle size D50 of the slurry D is 0.80-1.40 μm. ​ 6. The method of claim 1, wherein the lithium iron phosphate is prepared by the steps of: The calcining comprises two-stage calcining or three-stage calcining; the two-stage calcining refers to first calcining at a temperature rising rate of 5-15 ℃ / min to 300-500 ℃ for 2-6 h, and then calcining at a temperature rising rate of 10-30 ℃ / min to 700-850 ℃ for 8-16 h; ​ The three-stage calcining refers to first calcining at a temperature rising rate of 5-15 ℃ / min to 300-500 ℃ for 2-6 h, then calcining at a temperature rising rate of 5-15 ℃ / min to 500-700 ℃ for 4-10 h, and finally calcining at a temperature rising rate of 10-30 ℃ / min to 700-850 ℃ for 8-16 h.

7. A lithium iron phosphate characterized by: The lithium iron phosphate prepared by the method of any one of claims 1-6.

8. The lithium iron phosphate of claim 7, wherein: The lithium iron phosphate has a D50 of 0.8-1.5 μm, a BET of 11-14 m 2 / g, a compaction density of 2.50-2.60 g / cc, and a discharge capacity at 3.75 V / 0.1 C of 155-161 mAh / g.

9. A lithium-ion battery, characterized by: The lithium iron phosphate of any one of claims 7-8.

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