Lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a lithium iron phosphate positive electrode material, which comprises a large particle material and a small particle material in a mass ratio of 1: 9-5: 5, the particle size range of the large particle material is 200-400 meshes, and the large particle material is prepared from a lithium iron phosphate precursor with the particle size range of 100-200 nm; the particle size range of the small-particle material is 0.2-2 microns, the small-particle material is prepared from a lithium iron phosphate precursor with the particle size range of 300-500 nm, and the lithium iron phosphate precursor comprises the following components: 30-50% of a phosphorus source, 40-50% of an iron source, 20-30% of a lithium source, 5-24% of a carbon source and 0-0.03% of a doped metal source. In the invention, the precursor with smaller particle size is used for preparing the large-particle raw material, the precursor with larger particle size is used for preparing the small-particle raw material, and the problem that the discharge capacity and the energy density are incompatible is solved through large and small particle grading.

Description

Technical Field

[0001] The invention relates to a phosphate, in particular to a lithium iron phosphate positive electrode material, and also to a preparation method of the lithium iron phosphate positive electrode material. Background Art

[0002] In addition to structural stability, the positive electrode materials used in lithium batteries also need to have high energy density, that is, to provide more active Li that can be deintercalated / embedded in a smaller mass or volume. To increase the energy density of the battery while keeping the volume unchanged, it is necessary to increase the compaction density of the LiFePO4 positive electrode. The commonly used improvement methods currently include: raw material process, adjustment of sintering system and size particle grading, etc. (Li Miao et al. Design of high energy density lithium iron phosphate positive electrode. Energy Storage Science and Technology 12.7 (2023): 2045-2058.). In the size particle grading of lithium iron phosphate materials, large particles play a role in increasing compaction density, and small particles contribute to high capacity. Therefore, the pursuit of high compaction density requires the increase of lithium iron phosphate primary particles. However, after the particles are enlarged, lithium ions are difficult to deintercalate, and it is difficult to balance high compaction and high capacity. The invention patent with the publication number CN 118373398A discloses a method for preparing ultra-high compacted lithium iron phosphate positive electrode materials. In the example, an iron-containing precursor is prepared separately to obtain a high-purity iron-containing precursor, and the morphology, crystal form and particle size of the iron-containing precursor are controlled by adjusting the synthesis conditions, the ratio of reactants and the precipitation process. Then, a first precursor with a larger particle size coated with carbon is prepared based on the iron-containing precursor, and a second precursor with a small particle size doped with titanium nitrogen and carbon coated is prepared based on the iron-containing precursor and amino acid chelated titanium. Then, the first precursor with a large particle size and the second precursor with a small particle size are mixed and sintered. The size and ratio of the first precursor with large particles and the second precursor with small particles form a lithium iron phosphate positive electrode material with an ultra-high compacted density with double-doped carbon coating under the densest packing. The current scheme ensures the compaction density and discharge capacity, but requires the pretreatment of the precursor, which prolongs the process flow and is costly. Summary of the invention

[0003] Purpose of the invention: The purpose of the present invention is to provide a lithium iron phosphate positive electrode material to solve the problem of incompatibility between discharge capacity and compaction density of lithium iron phosphate positive electrode materials; another purpose of the present invention is to provide a lithium iron phosphate positive electrode material to simplify the preparation process of iron phosphate materials.

[0004] Technical solution: The lithium iron phosphate positive electrode material described in the present invention includes large-particle material and small-particle material in a mass ratio of 1:9 to 5:5. The large-particle material has a particle size range of 200 to 400 mesh and is made from a lithium iron phosphate precursor with a particle size range of 100 to 200 nm; the small-particle material has a particle size range of 0.2 to 2 μm and is made from a lithium iron phosphate precursor with a particle size range of 300 to 500 nm. The lithium iron phosphate precursor includes the following components: 30% to 50% phosphorus source, 40% to 50% iron source, 20%-30% lithium source, 5% to 24% carbon source, and 0 to 0.03% doped metal source.

[0005] Preferably, the mass ratio of the large particle material to the small particle material is 3:7 to 5:5.

[0006] Preferably, the lithium source content of the large particle material is higher than that of the small particle material.

[0007] Preferably, the lithium source is at least one of lithium carbonate, lithium dihydrogen phosphate, and lithium hydroxide, the iron source is at least one of iron phosphate, red iron oxide, elemental iron, and ferrous oxalate, the carbon source is at least one of inorganic carbon black, sucrose, glucose, polyvinyl alcohol, and polyethylene glycol, and the doped metal source is at least one of titanium dioxide, magnesium acetate, and ammonium vanadate.

[0008] The above-mentioned method for preparing lithium iron phosphate positive electrode material comprises the following steps:

[0009] (1) mixing a phosphorus source, an iron source, a carbon source, a doping metal source, and a solvent, and grinding to obtain a precursor slurry;

[0010] (2) spray drying the precursor slurry to obtain a first spray material and a second spray material;

[0011] (3) sintering the first spray material and the second spray material to obtain a first sintered material and a second sintered material;

[0012] (4) sieving the first sintered material to obtain large particle material, and crushing the second sintered material to obtain small particle material;

[0013] (5) Mix large particle materials and small particle materials to obtain lithium iron phosphate positive electrode materials.

[0014] Preferably, in step (2), the median diameter of the first spray material particles is in the range of 8 to 10 μm, and the median diameter of the second spray material particles is in the range of 20 to 25 μm.

[0015] Preferably, the sintering procedure is: heating at a rate of 2-5°C / min to a final temperature of 700-800°C, keeping the temperature for not less than 10 hours, and cooling at a rate of 5-10°C / min after the temperature is kept.

[0016] Preferably, the final sintering temperature of the first spray material is 700-750° C., and the final sintering temperature of the second spray material is 750-800° C. If the temperature is too high, the density of the first spray material will be too high, resulting in reduced capacity.

[0017] Preferably, in step (1), the solvent is at least one of water, methanol and ethanol.

[0018] Preferably, in step (5), the mixing time is 30 to 60 minutes.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: 1. Taking both discharge capacity and energy density into account: using a precursor with a smaller particle size to make a large-particle raw material, and using a precursor with a larger particle size to make a small-particle raw material, the grading of large and small particles solves the problem of incompatibility between discharge capacity and energy density. The energy density of the obtained lithium iron phosphate material is higher than 111.25mwh / g, and the 1C discharge capacity is greater than 145.24mAh / g; 2. Shortening the process flow, simplifying processing, and reducing costs: No additional pre-treatment is required, and the preparation of large and small particle raw materials can be carried out simultaneously, which helps to shorten the process flow and simplify processing; 3. Reducing the amount of doped metal: When preparing large particles, Ti doping can be eliminated, which can generally reduce the amount of Ti by 30-50%, and can reduce the amount of Ti by up to 80%, further reducing costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a flow chart for preparing lithium iron phosphate according to the present invention;

[0021] Figure 2 This is an electron microscope image of a large particle raw material prepared in the first embodiment of the present invention;

[0022] Figure 3 This is an electron microscope image of a cross-section of a large particle raw material prepared in the first embodiment of the present invention;

[0023] Figure 4 This is an electron microscope image of the surface structure of the large particle raw material prepared in the first embodiment of the present invention;

[0024] Figure 5 This is an electron microscope image of a small particle raw material prepared in the first embodiment of the present invention;

[0025] Figure 6 This is an electron microscope image of lithium iron phosphate prepared in the first embodiment of the present invention; DETAILED DESCRIPTION

[0026] The technical solution of the present invention is further described below in conjunction with the accompanying drawings.

[0027] Example 1: In this example, lithium iron phosphate is prepared according to the following method:

[0028] Step 1: Add 250kg of lithium carbonate, 1000kg of iron phosphate, 50kg of glucose, 41kg of sucrose, 30kg of polyvinyl alcohol, and 8kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 150nm, and spray dry and granulate it with a spray tower, and the granulation particle size is 8μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 740℃, a constant temperature time of 10h, a constant temperature of 740℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. Pass the sintered material through a 200-400 mesh sieve and cache it in the material tank for standby use.

[0029] Step 2: Add 235kg of lithium carbonate, 1000kg of iron phosphate, 90kg of glucose, 30kg of polyethylene glycol, and 2kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 500nm, and spray dry and granulate it with a spray tower, and the granulation particle size is 25μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 770℃, a constant temperature time of 10h, a constant temperature of 770℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. After spray sintering, the material is crushed to a particle size of 0.5-2.0μm and cached in the material tank.

[0030] Step 3: Send the large and small particles to the ribbon mixer in a mass ratio of 3:7 and mix horizontally for 30 minutes.

[0031] Example 2: In this example, lithium iron phosphate is prepared according to the following method:

[0032] Step 1: Add 250kg of lithium carbonate, 1000kg of iron phosphate, 50kg of glucose, 41kg of sucrose, 30kg of polyvinyl alcohol, 8kg of TiO2, etc. into the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 180nm, and spray dry and granulate it with a spray tower, with a particle size of 7μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 740℃, a constant temperature time of 10h, a constant temperature of 740℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. Pass the sintered material through a 200-400 mesh sieve and cache it in the material tank for standby use.

[0033] Step 2: Add 240kg of lithium carbonate, 1000kg of iron phosphate, 70kg of glucose, 70kg of polyethylene glycol, and 5kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 400nm and spray dry it with a spray tower to granulate it with a particle size of 23μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 760℃, a constant temperature time of 10h, a constant temperature of 760℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. After spray sintering, the material is crushed to a particle size of 0.5-2.0μm and cached in the material tank.

[0034] Step 3: Blending of large and small particles: Send large and small particles to the ribbon mixer in a mass ratio of 4:6 and mix horizontally for 40 minutes.

[0035] Example 3 In this example, lithium iron phosphate was prepared according to the following method:

[0036] Step 1: Add 250kg of lithium carbonate, 1000kg of iron phosphate, 50kg of glucose, 41kg of sucrose, 30kg of polyvinyl alcohol, and 8kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 200nm and spray dry it with a spray tower to granulate it with a particle size of 6μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 740℃, a constant temperature time of 10h, a constant temperature of 740℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. Pass the sintered material through a 200-400 mesh sieve and cache it in the material tank for standby use.

[0037] Step 2: Add 245kg of lithium carbonate, 1000kg of iron phosphate, 90kg of glucose, 30kg of polyethylene glycol, and 8kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 300nm and spray dry it with a spray tower to granulate it with a particle size of 20μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 750℃, a constant temperature time of 10h, a constant temperature of 750℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. After spray sintering, the material is crushed to a particle size of 0.5-2.0μm and cached in the material tank.

[0038] Step 3: Send the large and small particles to the ribbon mixer in a mass ratio of 5:5 and mix horizontally for 40 minutes.

[0039] Example 4: In this example, lithium iron phosphate is prepared according to the following method:

[0040] Step 1: Add 250kg of lithium carbonate, 1000kg of iron phosphate, 50kg of glucose, 41kg of sucrose, and 30kg of polyvinyl alcohol into a material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 200nm, and spray dry and granulate it with a spray tower, and the granulation particle size is 8μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 740℃, a constant temperature time of 10h, a constant temperature of 740℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. Pass the sintered material through a 200-400 mesh sieve and cache it in the material tank for standby use.

[0041] Step 2: Add 235kg of lithium carbonate, 1000kg of iron phosphate, 90kg of glucose, 30kg of polyethylene glycol, and 2kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 500nm, and spray dry and granulate it with a spray tower, and the granulation particle size is 25μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 770℃, a constant temperature time of 10h, a constant temperature of 770℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. After spray sintering, the material is crushed to a particle size of 0.5-2.0μm and cached in the material tank.

[0042] Step 3: Send the large and small particles to the ribbon mixer in a mass ratio of 3:7 and mix horizontally for 30 minutes.

[0043] Comparative Example 1: In this comparative example, lithium iron phosphate was prepared according to the following method:

[0044] Step 1: Add 250kg of lithium carbonate, 1000kg of iron phosphate, 90kg of glucose, 30kg of polyethylene glycol, and 8kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 500nm and spray dry it with a spray tower to granulate it, with a particle size of 8μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 740℃, a constant temperature time of 10h, a constant temperature of 740℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. Pass the sintered material through a 200-400 mesh sieve and cache it in the material tank for standby use.

[0045] Step 2: Add 235kg of lithium carbonate, 1000kg of iron phosphate, 50kg of glucose, 41kg of sucrose, 30kg of polyvinyl alcohol, and 8kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 150nm and spray dry it with a spray tower to granulate it with a particle size of 25μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 770℃, a constant temperature time of 10h, a constant temperature of 770℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. After spray sintering, the material is crushed to a particle size of 0.5-2.0μm and cached in the material tank.

[0046] Step 3, blending of large and small particles: blending of large and small particles, sending large and small particles in a mass ratio of 3:7 to a ribbon mixer for horizontal mixing for 40 minutes.

[0047] Comparative Example 2: In this comparative example, lithium iron phosphate was prepared according to the following method:

[0048] Step 1: Add 250kg of lithium carbonate, 1000kg of iron phosphate, 90kg of glucose, 30kg of polyethylene glycol, and 8kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 400nm and spray dry it with a spray tower to granulate it with a particle size of 7μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 740℃, a constant temperature time of 10h, a constant temperature of 740℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. Pass the sintered material through a 200-400 mesh sieve and cache it in the material tank for standby use.

[0049] Step 2: Add 240kg of lithium carbonate, 1000kg of iron phosphate, 50kg of glucose, 41kg of sucrose, 30kg of polyvinyl alcohol, and 8kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 180nm and spray dry it with a spray tower to granulate it with a particle size of 23μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 760℃, a constant temperature time of 10h, a constant temperature of 760℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. After spray sintering, the material is crushed to a particle size of 0.5-2.0μm and cached in the material tank.

[0050] Step 3: Blend large and small particles. Send the large and small particles to the ribbon mixer in a mass ratio of 4:6 and mix horizontally for 40 minutes.

[0051] Comparative Example 3: In this comparative example, lithium iron phosphate was prepared according to the following method:

[0052] Step 1: Add 250kg of lithium carbonate, 1000kg of iron phosphate, 90kg of glucose, 30kg of polyethylene glycol, and 8kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 300nm and spray dry it with a spray tower to granulate it with a particle size of 6μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 740℃, a constant temperature time of 10h, a constant temperature of 740℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. Pass the sintered material through a 200-400 mesh sieve and cache it in the material tank for standby use.

[0053] Step 2: Add 245kg of lithium carbonate, 1000kg of iron phosphate, 50kg of glucose, 41kg of sucrose, 30kg of polyvinyl alcohol, and 8kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 200nm and spray dry it with a spray tower to granulate it with a particle size of 20μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 750℃, a constant temperature time of 10h, a constant temperature of 750℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. After spray sintering, the material is crushed to a particle size of 0.5-2.0μm and cached in the material tank.

[0054] Step 3: Blend large and small particles. Send the large and small particles to the ribbon mixer in a mass ratio of 5:5 and mix horizontally for 40 minutes.

[0055] Comparative Example 4 This comparative example only has small particles prepared, and the specific steps are as follows:

[0056] Add 235kg of lithium carbonate, 1000kg of iron phosphate, 50kg of glucose, 41kg of sucrose, 30kg of polyvinyl alcohol, and 8kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 400nm, and spray dry and granulate it with a spray tower, with a particle size of 25μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 770℃, a constant temperature time of 10h, a constant temperature of 770℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. After spray sintering, the material is crushed to a particle size of 0.5-2.0μm and cached in the material tank. Send the material to the ribbon mixer for mixing for 40 minutes.

[0057] Comparative Example 5: This comparative example only involves the preparation of small particles, and the specific steps are as follows:

[0058] Add 235kg of lithium carbonate, 1000kg of iron phosphate, 90kg of glucose, 30kg of polyethylene glycol, and 2kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 500nm, and spray dry and granulate it with a spray tower, with a particle size of 25μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 770℃, a constant temperature time of 10h, a constant temperature of 770℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. After spray sintering, the material is crushed to a particle size of 0.5-2.0μm and cached in the material tank. Send the material to the ribbon mixer for mixing for 40 minutes.

[0059] Comparative Example 6: This comparative example only involves the preparation of large particles, and the specific steps are as follows:

[0060] Add 250kg of lithium carbonate, 1000kg of iron phosphate, 50kg of glucose, 41kg of sucrose, 30kg of polyvinyl alcohol, and 8kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 180nm, and spray dry and granulate it with a spray tower, with a particle size of 8μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 740℃, a constant temperature time of 10h, a constant temperature of 740℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. Pass the sintered material through a 200-400 mesh sieve and cache it in the material tank for use. Send the material to the ribbon mixer for mixing for 50 minutes.

[0061] Comparative Example 7: This comparative example only involves the preparation of large particles, and the specific steps are as follows:

[0062] Add 250kg of lithium carbonate, 1000kg of iron phosphate, 90kg of glucose, 30kg of polyethylene glycol, and 8kg of TiO2 to the material tank, and add 1000kg of pure water to it to make a lithium iron phosphate precursor. Grind the precursor slurry to 200nm, and spray dry and granulate it with a spray tower, with a particle size of 8μm. Sinter the spray material in three stages, with a heating rate of 2.5℃ / min, a heating time of 8h, a heating temperature of 740℃, a constant temperature time of 10h, a constant temperature of 740℃, a cooling rate of 5℃ / min in the cooling stage, and a cooling time of 3h. Pass the sintered material through a 200-400 mesh sieve and cache it in the material tank for use. Send the material to the ribbon mixer for mixing for 50 minutes.

[0063] The compaction density of the lithium iron phosphate prepared in Examples 1 to 4 and Comparative Examples 1 to 7 was measured, and button-type half-cells were made therefrom to measure the discharge capacity, and the data shown in Table 1 were obtained.

[0064] Discharge gram capacity test method

[0065] 1. Pole piece production: LFP (lithium iron phosphate): PVDF (polyvinylidene fluoride): SP (carbon black) are mixed in a ratio of 90:5:5, evenly coated on aluminum foil, and then taken out after being dried at 110°C for 30 minutes. After roller pressing, slicing, and weighing, they are dried in a vacuum drying oven at 110°C for 3 hours and then taken out for use. Weigh 4 pole pieces with a mass difference of <0.5mg, and then bake them in a vacuum drying oven at 110°C for 3 hours before taking out to assemble the battery.

[0066] 2. Button battery assembly: negative electrode shell - electrolyte - positive electrode sheet - electrolyte - diaphragm - electrolyte - lithium sheet - gasket - spring - positive electrode shell - seal, assemble into CR2032 button battery, assemble 4 batteries.

[0067] 3. Data processing: Power-off test: test voltage 2.0-3.75V, nominal capacity 155mAh / g, take the average value of 4 battery data with relative error ≤0.4%, the test data is shown in Table 1 below.

[0068] Table 1: Test data of lithium iron phosphate obtained in Examples 1 to 3 and Comparative Examples 1 to 7.

[0069]

[0070] The main difference between Comparative Examples 1 to 3 and Examples 1 to 3 is that the large particles of Comparative Examples 1 to 3 are made from precursors with larger particle sizes (300 to 500 nm), and the small particles are made from precursors with smaller particle sizes (100 to 200 nm). Under the same blending conditions, the compaction density of lithium iron phosphate prepared in Comparative Examples 1 to 3 is slightly higher than that in Examples 1 to 3, but the discharge capacity of Comparative Examples 1 to 3 is significantly reduced, and the energy density is also reduced, indicating that the energy density and discharge capacity are incompatible.

[0071] In the present invention, the large particle raw material is made from a precursor with a smaller particle size (100-200nm), and the small particles are made from a precursor with a larger particle size (300-500nm), which solves the problem of incompatibility between discharge capacity and energy density, and there is no need to introduce doping materials into the large particles, which helps to reduce costs. The single use of small particles made from a larger particle size precursor (Comparative Examples 4 and 5) is beneficial to the compaction density, but not to the discharge capacity improvement; the single use of large particles made from a smaller particle size precursor (Comparative Examples 6 and 7) is beneficial to the discharge capacity but not to the compaction.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: The invention comprises large particle material and small particle material in a mass ratio of 1:9 to 5:5, wherein the large particle material has a particle size range of 200 to 400 meshes and is prepared from a lithium iron phosphate precursor with a particle size range of 100 to 200 nm; the small particle material has a particle size range of 0.2 to 2 μm and is prepared from a lithium iron phosphate precursor with a particle size range of 300 to 500 nm, wherein the lithium iron phosphate precursor comprises the following components: 30 to 50% phosphorus source, 40 to 50% iron source, 20 to 30% lithium source, 5 to 24% carbon source, and 0 to 0.03% doped metal source.

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: The mass ratio of the large particle material to the small particle material is 3:7 to 5:

5.

3. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: The lithium source content of the large particle material is higher than that of the small particle material.

4. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: The lithium source is at least one of lithium carbonate, lithium dihydrogen phosphate, and lithium hydroxide; the iron source is at least one of iron phosphate, red iron oxide, elemental iron, and ferrous oxalate; the carbon source is at least one of inorganic carbon black, sucrose, glucose, polyvinyl alcohol, and polyethylene glycol; and the doped metal source is at least one of titanium dioxide, magnesium acetate, and ammonium vanadate.

5. The method for preparing the lithium iron phosphate positive electrode material according to any one of claims 1 to 4, characterized in that: The following steps are involved: (1) mixing a phosphorus source, an iron source, a carbon source, a doping metal source, and a solvent, and grinding to obtain a precursor slurry; (2) spray drying the precursor slurry to obtain a first spray material and a second spray material; (3) sintering the first spray material and the second spray material to obtain a first sintered material and a second sintered material; (4) sieving the first sintered material to obtain large particle material, and crushing the second sintered material to obtain small particle material; (5) Mix large particle materials and small particle materials to obtain lithium iron phosphate positive electrode materials.

6. The preparation method according to claim 5, characterized in that: In step (2), the median diameter of the first spray material particles is in the range of 8 to 10 μm, and the median diameter of the second spray material particles is in the range of 20 to 25 μm.

7. The preparation method according to claim 5, characterized in that: In step (3), the sintering procedure is: heating at a rate of 2 to 5°C / min to a final temperature of 700 to 800°C, keeping the temperature for not less than 10 hours, and cooling at a rate of 5 to 10°C / min after the heat preservation.

8. The preparation method according to claim 7, characterized in that: The final temperature of the first spray material is 700-750°C, and the final temperature of the second spray material is 750-800°C.

9. The preparation method according to claim 7, characterized in that: In step (1), the solvent is at least one of water, methanol and ethanol.

10. The preparation method according to claim 6, characterized in that: In step (5), the mixing time is 30 to 60 minutes.

Images