Lithium iron phosphate positive electrode material and preparation method thereof, positive electrode plate and lithium ion battery

Abstract

The invention provides a lithium iron phosphate positive electrode material and a preparation method thereof, a positive electrode plate and a lithium ion battery, and belongs to the technical field of secondary batteries. The lithium iron phosphate positive electrode material comprises a first lithium iron phosphate positive electrode material and a second lithium iron phosphate positive electrode material, and the first lithium iron phosphate positive electrode material comprises solid primary particles A and hollow secondary particles formed by encircling a plurality of solid primary particles A; the second lithium iron phosphate positive electrode material comprises solid primary particles B and solid secondary particles formed by melting the solid primary particles B, and the Dv50 of the first lithium iron phosphate positive electrode material is greater than the Dv50 of the second lithium iron phosphate positive electrode material. The lithium iron phosphate positive electrode material has high compaction density and high rate capability, so that the lithium ion battery has good energy density and exerting capacity.

Description

Technical Field

[0001] The present invention belongs to the technical field of secondary batteries, and particularly relates to a lithium iron phosphate cathode material, a preparation method thereof, a cathode electrode sheet, and a lithium ion battery. Background Art

[0002] Lithium ion batteries are widely used in new energy electric vehicles and large-scale energy storage devices due to their high energy density, clean and green, high specific capacity, long cycle stability and other characteristics. As a common cathode material for lithium ion batteries, lithium iron phosphate is easy to obtain raw materials, low in price, has excellent chemical and thermal stability, and will not deteriorate significantly after thousands of charge and discharge cycles. Compared with ternary cathode materials, it has the advantages of safety, stability and low cost. However, the conductivity of lithium iron phosphate is relatively low, and it is a one-dimensional Li + movement path, so its diffusion rate must be relatively slow. Secondly, lithium iron phosphate with low tap density cannot meet the design requirements of battery energy density. To meet the requirements of energy density design, it is necessary to increase the tap density of the lithium iron phosphate cathode material and increase the active substance of the cathode electrode sheet (increase the areal density / thickness of the electrode sheet). However, as the tap density of the electrode sheet increases, the wettability between the positive active material and the electrolyte will become worse, resulting in an increase in battery polarization and internal resistance, affecting the capacity of the battery.

[0003] The tap density of lithium iron phosphate is mainly related to the particle size distribution. In the prior art, usually the particle size of small primary particles is reduced while the particle size of large primary particles is increased, and the tap density of lithium iron phosphate is increased through the above grading regulation. However, increasing the particle size of large primary particles will inevitably lead to a decrease in the rate performance of lithium iron phosphate, an increase in battery polarization, and the discharged capacity being lower than the designed capacity.

[0004] Therefore, there is an urgent need to develop a lithium iron phosphate cathode material with high tap density and high rate performance. Summary of the Invention

[0005] The present invention aims to solve at least one of the technical problems in the related art to some extent. For this reason, an embodiment of the present invention provides a lithium iron phosphate cathode material, a preparation method thereof, a cathode electrode sheet, and a lithium ion battery. The lithium iron phosphate cathode material has a high tap density and high rate performance, so that good energy density and discharged capacity can be taken into account.

[0006] In a first aspect, an embodiment of the present invention provides a lithium iron phosphate cathode material, including a first lithium iron phosphate cathode material and a second lithium iron phosphate cathode material. The first lithium iron phosphate cathode material includes solid primary particles A and hollow secondary particles formed by surrounding multiple solid primary particles A. The primary particle size of the first lithium iron phosphate cathode material is 50 - 200 nm, the secondary particle size of the first lithium iron phosphate cathode material is below 10 μm, and the Dv50 of the first lithium iron phosphate cathode material is 0.5 - 6 μm. The second lithium iron phosphate cathode material includes solid primary particles B and solid secondary particles formed by melting the solid primary particles B. The primary particle size of the second lithium iron phosphate cathode material is 100 - 400 nm, the secondary particle size of the second lithium iron phosphate cathode material is 0.2 - 5 μm, and the Dv50 of the second lithium iron phosphate cathode material is 0.6 - 1.2 μm. The Dv50 of the first lithium iron phosphate cathode material is greater than the Dv50 of the second lithium iron phosphate cathode material.

[0007] The advantages and technical effects brought by the lithium iron phosphate cathode material of the embodiment of the present invention are as follows:

[0008] (1) The primary particle size of the first lithium iron phosphate cathode material is 50 - 200 nm, the secondary particle size of the first lithium iron phosphate cathode material is below 10 μm, and the Dv50 of the first lithium iron phosphate cathode material is 0.5 - 6 μm. It can be seen that the first lithium iron phosphate cathode material is mainly composed of hollow secondary particles. The primary particle size of the second lithium iron phosphate cathode material is 100 - 400 nm, the secondary particle size of the second lithium iron phosphate cathode material is 0.2 - 5 μm, and the Dv50 of the second lithium iron phosphate cathode material is 0.6 - 1.2 μm. It can be seen that the second lithium iron phosphate cathode material is mainly composed of solid primary particles.

[0009] (2) The Dv50 of the first lithium iron phosphate cathode material is greater than the Dv50 of the second lithium iron phosphate cathode material. It can be seen that the particles of the first lithium iron phosphate cathode material are relatively large, while the particles of the second lithium iron phosphate cathode material are relatively small. Therefore, the second lithium iron phosphate cathode material mainly composed of hollow secondary particles can well fill the gaps in the first lithium iron phosphate cathode material mainly composed of solid primary particles, improving the overall tap density of the lithium iron phosphate cathode material, thereby ensuring the design of high energy density of the positive electrode sheet.

[0010] (3) There are voids inside the hollow secondary particles of the first lithium iron phosphate cathode material, which can ensure sufficient infiltration between the lithium iron phosphate cathode material of the embodiment of the present invention and the electrolyte, reduce battery polarization, reduce battery internal resistance, and thus improve the rate performance of the lithium-ion battery.

[0011] (4)In summary, the lithium iron phosphate cathode material of the embodiments of the present invention has a high tap density and high rate performance, so that good energy density and capacity can be balanced.

[0012] In some embodiments, the mass ratio of the first lithium iron phosphate cathode material to the second lithium iron phosphate cathode material is (0.2-1.5):(8.5-9.8).

[0013] In some embodiments, the first lithium iron phosphate cathode material and / or the second lithium iron phosphate cathode material further includes a doped metal ion, and the doped metal ion is at least one of Mg, Mo, Ti, V, Mn, and Nb.

[0014] Second, the embodiments of the present invention provide a preparation method of a lithium iron phosphate cathode material, including the following steps:

[0015] S1. Mix a phosphorus source, an iron source, a lithium source, a carbon source, and a solvent to obtain a first mixed slurry; grind, demagnetize, and perform first spray drying on the first mixed slurry to obtain a first powder; perform first high-temperature sintering on the first powder to prepare the first lithium iron phosphate cathode material; wherein, the temperature of the first spray drying is 160-175°C;

[0016] S2. Mix a phosphorus source, an iron source, a lithium source, a carbon source, and a solvent to obtain a second mixed slurry; grind, demagnetize, and perform second spray drying on the second mixed slurry to obtain a second powder; perform second high-temperature sintering on the second powder and then perform air jet milling to prepare the second lithium iron phosphate cathode material;

[0017] S3. Mix the first lithium iron phosphate cathode material and the second lithium iron phosphate cathode material to obtain the lithium iron phosphate cathode material of the first aspect.

[0018] The advantages and technical effects brought by the preparation method of the embodiments of the present invention are as follows:

[0019] (1) The temperature of the first spray drying is 160-175°C. The surface of the secondary particles of the first powder obtained by spray drying is dry, but the inside is not completely dry, and the solvent remains in the voids inside. After the first powder is subjected to first high-temperature sintering, the carbon source inside the secondary particles is carbonized and the solvent volatilizes, forming a hollow inside the secondary particles. Therefore, the secondary particles of the prepared first lithium iron phosphate cathode material are a hollow structure formed by multiple solid primary particles A surrounding, and in addition, there are inevitably a small amount of solid primary particles A.

[0020] (2) After the second powder is subjected to second high-temperature sintering and then air jet milling, the prepared second lithium iron phosphate cathode material is mainly solid primary particles B, and in addition, there are inevitably a small amount of solid secondary particles formed by the melting of solid primary particles B.

[0021] (3) By the reasonable grading of the first lithium iron phosphate cathode material and the second lithium iron phosphate cathode material, the obtained lithium iron phosphate cathode material has both high tap density and high rate performance.

[0022] (4) The preparation method of the embodiment of the present invention has a simple process and convenient operation, and is suitable for industrial promotion.

[0023] In some embodiments, the secondary particle size of the first powder is 2 - 4 μm.

[0024] In some embodiments, the holding temperature of the first high-temperature sintering is 780 - 810 °C, and the holding time is 4 - 6 h.

[0025] In some embodiments, the temperature of the second spray drying is greater than or equal to the temperature of the first spray drying, and the secondary particle size of the second powder is 3 - 5 μm.

[0026] In some embodiments, the holding temperature of the second high-temperature sintering is 780 - 850 °C, and the holding time is 5 - 8 h.

[0027] In a third aspect, the embodiment of the present invention provides a positive electrode tab, including a positive electrode current collector and a positive electrode active layer located on at least one surface of the positive electrode current collector, and the positive electrode active layer includes the lithium iron phosphate cathode material, a conductive agent, and a binder of the first aspect.

[0028] The advantages and technical effects brought by the positive electrode tab of the embodiment of the present invention are as follows:

[0029] Due to the adoption of the lithium iron phosphate cathode material of the first aspect, the positive electrode tab of the embodiment of the present invention has high tap density and high rate performance.

[0030] In a fourth aspect, the embodiment of the present invention provides a lithium-ion battery, including the positive electrode tab of the third aspect.

[0031] The advantages and technical effects brought by the lithium-ion battery of the embodiment of the present invention are as follows:

[0032] Due to the adoption of the positive electrode tab of the third aspect, the lithium-ion battery of the embodiment of the present invention can have both high energy density and high discharge capacity. Description of the Drawings

[0033] Figure 1 is the SEM image of the first lithium iron phosphate cathode material of Example 1;

[0034] Figure 2 is the SEM image of the first lithium iron phosphate cathode material of Example 1;

[0035] Figure 3It is the SEM image of the positive electrode sheet prepared from the lithium iron phosphate positive electrode material of Example 1. Detailed implementation manners

[0036] The embodiments of the present invention will be described in detail below. The examples are shown in the accompanying drawings. The embodiments described by referring to the accompanying drawings are exemplary and are intended to explain the present invention, and should not be construed as a limitation to the present invention.

[0037] A lithium iron phosphate positive electrode material includes a first lithium iron phosphate positive electrode material and a second lithium iron phosphate positive electrode material. The first lithium iron phosphate positive electrode material includes solid primary particles A and hollow secondary particles formed by surrounding a plurality of the solid primary particles A. The primary particle size of the first lithium iron phosphate positive electrode material is 50 - 200 nm, the secondary particle size of the first lithium iron phosphate positive electrode material is below 10 μm, the Dv50 of the first lithium iron phosphate positive electrode material is 0.5 - 6 μm. The second lithium iron phosphate positive electrode material includes solid primary particles B and solid secondary particles formed by melting the solid primary particles B. The primary particle size of the second lithium iron phosphate positive electrode material is 100 - 400 nm, the secondary particle size of the second lithium iron phosphate positive electrode material is 0.2 - 5 μm, the Dv50 of the second lithium iron phosphate positive electrode material is 0.6 - 1.2 μm, and the Dv50 of the first lithium iron phosphate positive electrode material is greater than the Dv50 of the second lithium iron phosphate positive electrode material.

[0038] The particles of the first lithium iron phosphate positive electrode material are relatively large, while the particles of the second lithium iron phosphate positive electrode material are relatively small. Therefore, the second lithium iron phosphate positive electrode material can well fill the gaps between the first lithium iron phosphate positive electrode materials, improve the tap density of the lithium iron phosphate positive electrode material, and thus ensure the design of high energy density of the positive electrode sheet. In addition, there are voids inside the first lithium iron phosphate positive electrode material, which can ensure sufficient infiltration between the lithium iron phosphate positive electrode material and the electrolyte, reduce battery polarization, reduce battery internal resistance, and thus improve the rate performance of the lithium-ion battery. In summary, the lithium iron phosphate positive electrode material of the embodiment of the present invention has a high tap density and a high rate performance, and thus can take into account good energy density and capacity performance.

[0039] In the lithium iron phosphate positive electrode material of the embodiment of the present invention, the Dv50 of the first lithium iron phosphate positive electrode material is 0.5 - 6 μm, such as 0.5 μm, 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, etc. When the Dv50 of the first lithium iron phosphate positive electrode material is too large, the wettability between the first lithium iron phosphate positive electrode material and the electrolyte is poor, which will lead to poor rate performance of the lithium iron phosphate positive electrode material.

[0040] In some embodiments, the secondary particle size of the first lithium iron phosphate cathode material is below 10 μm, such as 1 μm, 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, 10 μm, etc. When the secondary particle size of the first lithium iron phosphate cathode material is too large, there are relatively large gaps between the particles, which is not conducive to improving the energy density of the battery cell.

[0041] In some embodiments, the primary particle size of the first lithium iron phosphate cathode material is 50 - 200 nm, such as 50 nm, 80 nm, 100 nm, 120 nm, 150 nm, 180 nm, 200 nm, etc. When the primary particle size of the first lithium iron phosphate cathode material is too small, it is not easy to form hollow secondary particles. When the primary particle size of the first lithium iron phosphate cathode material is too large, it is not conducive to further improving the wettability of the first lithium iron phosphate cathode material with the electrolyte, thus not conducive to further improving the overall rate performance of the lithium iron phosphate cathode material.

[0042] In some embodiments, the Dv50 of the second lithium iron phosphate cathode material is 0.6 - 1.2 μm, such as 0.6 μm, 0.7 μm, 0.8 μm, 0.9 μm, 1 μm, 1.1 μm, 1.2 μm, etc. When the Dv50 of the second lithium iron phosphate cathode material is too large, it is not conducive to improving the wettability of the second lithium iron phosphate cathode material, thus not conducive to improving the overall rate performance of the lithium iron phosphate cathode material.

[0043] In some embodiments, the secondary particle size of the second lithium iron phosphate cathode material is 0.2 - 5 μm, such as 0.2 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, etc. When the secondary particle size of the second lithium iron phosphate cathode material is too large, it is not conducive to the second lithium iron phosphate cathode material filling the gaps in the first lithium iron phosphate cathode material, and is not conducive to improving the overall tap density of the lithium iron phosphate cathode material.

[0044] In some embodiments, the mass ratio of the first lithium iron phosphate cathode material to the second lithium iron phosphate cathode material is (0.2 - 1.5):(8.5 - 9.8), such as 0.2:9.8, 0.5:9.8, 1:9.8, 1.5:9.8, 1.5:9, 1.5:9.5, 1.5:8.5, etc. When this mass ratio is too small, it is not conducive to improving the tap density of the lithium iron phosphate cathode material. When this mass ratio is too large, it is not conducive to improving the rate performance of the lithium iron phosphate cathode material.

[0045] In some embodiments, the first lithium iron phosphate cathode material and / or the second lithium iron phosphate cathode material further includes a doped metal ion, and the doped metal ion is at least one of Mg, Mo, Ti, V, Mn, and Nb. The above doped metal ions can effectively improve the conductivity of the lithium iron phosphate cathode material, thereby improving the electrochemical performance.

[0046] Second, an embodiment of the present invention provides a method for preparing a lithium iron phosphate cathode material, including the following steps:

[0047] S1. Mix a phosphorus source, an iron source, a lithium source, a carbon source, and a solvent to obtain a first mixed slurry; grind, demagnetize by electricity, and perform first spray drying on the first mixed slurry to obtain a first powder; perform first high-temperature sintering on the first powder to prepare the first lithium iron phosphate cathode material; wherein, the temperature of the first spray drying is 160 - 175 °C;

[0048] S2. Mix a phosphorus source, an iron source, a lithium source, a carbon source, and a solvent to obtain a second mixed slurry; grind, demagnetize by electricity, and perform second spray drying on the second mixed slurry to obtain a second powder; perform second high-temperature sintering on the second powder, and then perform air jet milling to prepare the second lithium iron phosphate cathode material;

[0049] S3. Mix the first lithium iron phosphate cathode material and the second lithium iron phosphate cathode material to obtain the lithium iron phosphate cathode material of the first aspect.

[0050] The temperature of the first spray drying is 160 - 175 °C. The surface of the secondary particles of the first powder obtained by spray drying is dry, but the inside is not completely dry, and the solvent remains in the voids inside. After the first powder undergoes first high-temperature sintering, the carbon source inside the secondary particles is carbonized and the solvent volatilizes, forming a hollow inside the secondary particles. Therefore, the secondary particles of the prepared first lithium iron phosphate cathode material are a hollow structure formed by multiple solid primary particles A surrounding it, and inevitably, there are also a small amount of solid primary particles A. After the second powder undergoes second high-temperature sintering and then air jet milling, the prepared second lithium iron phosphate cathode material is mainly solid primary particles B, and inevitably, there are also a small amount of solid secondary particles formed by the melting of solid primary particles B. Through the reasonable grading of the first lithium iron phosphate cathode material and the second lithium iron phosphate cathode material, the obtained lithium iron phosphate cathode material has both high tap density and high rate performance.

[0051] In some embodiments, the secondary particle size of the first powder is 2 - 4 μm, such as 2 μm, 2.2 μm, 2.4 μm, 2.6 μm, 2.8 μm, 3 μm, 3.2 μm, 3.4 μm, 3.6 μm, 3.8 μm, 4 μm, etc. Spray drying forms spherical secondary particles, and the particle size of the secondary particles is within the above range to control the secondary particle size of the first lithium iron phosphate cathode material within a range below 10 μm.

[0052] In some embodiments, the heat preservation temperature of the first high-temperature sintering is 780 - 810 °C, such as 780 °C, 785 °C, 790 °C, 795 °C, 800 °C, 805 °C, 810 °C, etc., and the heat preservation time is 4 - 6 h, such as 4 h, 4.5 h, 5 h, 5.5 h, 6 h, etc. During high-temperature sintering, a particle melting state will occur. The higher the heat preservation temperature and the longer the heat preservation time, the larger the particle size formed by the melting of small particles. Therefore, controlling the heat preservation temperature and heat preservation time of the first high-temperature sintering within the above range is to avoid too large secondary particle size of the first lithium iron phosphate cathode material.

[0053] In some embodiments, the temperature of the second spray drying is greater than or equal to the temperature of the first spray drying, and the secondary particle size of the second powder is 3 - 5 μm, such as 3 μm, 3.5 μm, 4 μm, 4.5 μm, 5 μm, etc. When the secondary particle size of the second powder is too large, the time required for the second high-temperature sintering is too long, which is not conducive to reducing production costs. In addition, the time required for airflow comminution is also too long, which is also not conducive to reducing production costs.

[0054] In some embodiments, the heat preservation temperature of the second high-temperature sintering is 780 - 850 °C, such as 780 °C, 790 °C, 800 °C, 810 °C, 820 °C, 830 °C, 840 °C, 850 °C, etc., and the heat preservation time is 5 - 8 h, such as 5 h, 5.5 h, 6 h, 6.5 h, 7 h, 7.5 h, 8 h, etc. The heat preservation temperature and heat preservation time of the second high-temperature sintering within the above range are conducive to the doping of metal elements.

[0055] In a third aspect, an embodiment of the present invention provides a positive electrode plate, including a positive electrode current collector and a positive electrode active layer located on at least one surface of the positive electrode current collector, and the positive electrode active layer includes the lithium iron phosphate cathode material of the first aspect, a conductive agent, and a binder.

[0056] Due to the use of the lithium iron phosphate cathode material of the first aspect, the positive electrode plate of the embodiment of the present invention has a high tap density and a high rate performance.

[0057] In a fourth aspect, an embodiment of the present invention provides a lithium-ion battery, including the positive electrode plate of the third aspect.

[0058] Due to the adoption of the positive electrode tab of the third party, the lithium-ion battery of the embodiment of the present invention can have both high energy density and high discharge capacity.

[0059] The present invention will be described in detail below with reference to the embodiments and the accompanying drawings.

[0060] Embodiment 1

[0061] 1) Preparation of the first lithium iron phosphate positive electrode material: Mix a phosphorus source, an iron source, a lithium source, a vanadium source, a titanium source, a carbon source, and water. The molar ratio of P, Fe, and Li in the phosphorus source, iron source, and lithium source is 1:1:1.3. After rough grinding, fine grinding, electromagnetic removal, spray drying at 170 °C (particle size after drying: 2 - 4 μm), and high-temperature sintering at 800 °C for 5 h, the first lithium iron phosphate positive electrode material is obtained. The primary particle size of the first lithium iron phosphate positive electrode material is 50 - 200 nm, the secondary particle size is ≤ 8 μm, and the Dv50 of the first lithium iron phosphate positive electrode material is 3 μm;

[0062] 2) Preparation of the second lithium iron phosphate positive electrode material: Mix a phosphorus source, an iron source, a lithium source, a vanadium source, a titanium source, a carbon source, and water. The molar ratio of P, Fe, and Li in the phosphorus source, iron source, and lithium source is 1:1:3. After rough grinding, fine grinding, electromagnetic removal, spray drying at 170 °C (particle size after drying: 3 - 5 μm), high-temperature sintering at 840 °C for 7 h, and air flow pulverization, the second lithium iron phosphate positive electrode material is obtained. The primary particle size of the second lithium iron phosphate positive electrode material is 100 - 400 nm, the secondary particle size is 0.2 - 5 μm, and the Dv50 of the second lithium iron phosphate positive electrode material is 1.0 μm);

[0063] 3) By adjusting the mass ratio of the first lithium iron phosphate positive electrode material to the second lithium iron phosphate positive electrode material, the ratio is 1:9, and uniformly mixing through a batch mixer to obtain the lithium iron phosphate positive electrode material.

[0064] Embodiment 2

[0065] The difference between this embodiment and Embodiment 1 is that the mass ratio of the first lithium iron phosphate positive electrode material to the second lithium iron phosphate positive electrode material is 0.5:9.5.

[0066] Embodiment 3

[0067] The difference between this embodiment and Embodiment 1 is that the mass ratio of the first lithium iron phosphate positive electrode material to the second lithium iron phosphate positive electrode material is 1.5:8.5.

[0068] Embodiment 4

[0069] The difference between this embodiment and Embodiment 1 is that the mass ratio of the first lithium iron phosphate positive electrode material to the second lithium iron phosphate positive electrode material is 2:8.

[0070] Comparative Example 1

[0071] This comparative example is different from Example 1 in that it consists entirely of the first lithium iron phosphate cathode material.

[0072] Comparative Example 2

[0073] This comparative example is different from Example 1 in that it consists entirely of the second lithium iron phosphate cathode material.

[0074] Performance Test

[0075] (1) The tap density of the lithium iron phosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-2 was tested, and the results are shown in Table 1. The test method for tap density is as follows: Weigh 1 g ± 0.0100 g of the sample, pour it into a metal sleeve, and test the tap density of its powder under a pressure of 30 KN.

[0076] (2) The lithium iron phosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-2 were assembled into lithium-ion coin cells and full cells, and their discharge specific capacity was measured at 25°C. The results are shown in Table 1. The test steps for coin cell discharge: First, charge at a constant current and voltage of 0.1C until full charge, then discharge at a constant current of 0.1C, with a voltage range of 2.0 - 3.75V. The test steps for full cell: First, charge at a constant current and voltage of 1C until full charge, then discharge at a constant current of 1C, with a voltage range of 2.0 - 3.65V.

[0077] Table 1. Tap density and specific capacity of the lithium iron phosphate cathode materials prepared in Examples 1-4 and Comparative Examples 1-2

[0078]

[0079] Figure 1 is the SEM image of the first lithium iron phosphate cathode material of Example 1; Figure 2 is the SEM image of the first lithium iron phosphate cathode material of Example 1; Figure 3 is the SEM image of the positive electrode sheet prepared from the lithium iron phosphate cathode material of Example 1. It can be seen from Figure 1 that the first lithium iron phosphate cathode material includes solid primary particles A and hollow secondary particles formed by surrounding multiple solid primary particles A, and the hollow secondary particles formed by surrounding multiple solid primary particles A are the main component. It can be seen from Figure 2 that the second lithium iron phosphate cathode material includes primary solid particles B and secondary solid particles, and primary solid particles B are the main component. It can be seen from Figure 3 that in the lithium iron phosphate cathode material, the first lithium iron phosphate cathode material and the second lithium iron phosphate cathode material are evenly mixed, and the second lithium iron phosphate cathode material can well fill the gaps between the first lithium iron phosphate cathode materials, improving the tap density of the powder.

[0080] From the comparison between Example 1 and Comparative Examples 1-2, it can be seen that: for the lithium iron phosphate cathode material of Comparative Example 1, only the first lithium iron phosphate cathode material is used, and the tap density is significantly lower. For the lithium iron phosphate cathode material of Comparative Example 2, only the second lithium iron phosphate cathode material is used, and the discharge specific capacity of the battery is significantly lower. While for the lithium iron phosphate cathode material of Example 1, a compound of the first lithium iron phosphate cathode material and the second lithium iron phosphate cathode material is adopted. Through the mixing of powders and particle size grading, the tap density of the overall lithium iron phosphate cathode material is improved; in addition, since the first lithium iron phosphate cathode material is mainly composed of hollow secondary particles, the hollow secondary particles are evenly distributed, and the secondary particles are all composed of solid primary particles A, and there are voids inside the secondary particles, which ensures good wettability of the positive electrode sheet, thereby improving the rate performance of the lithium iron phosphate cathode material and thus increasing the specific capacity of the battery.

[0081] Through the comparison between Examples 1-3 and Example 4, it can be seen that when the mass ratio of the first lithium iron phosphate cathode material to the second lithium iron phosphate cathode material is too large, it is not conducive to improving the capacity of the battery.

[0082] In the present invention, the terms "one embodiment", "some embodiments", "example", "specific example", or "some examples", etc. mean that the specific features, structures, materials, or characteristics described in connection with the embodiment or example are included in at least one embodiment or example of the present invention. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific features, structures, materials, or characteristics described can be combined in a suitable manner in any one or more embodiments or examples. In addition, without contradiction, those skilled in the art can combine and combine the different embodiments or examples described in this specification and the features of different embodiments or examples.

[0083] Although the embodiments of the present invention have been shown and described above, it can be understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those of ordinary skill in the art can make changes, modifications, substitutions, and variations to the above embodiments within the scope of the present invention.

Claims

1. A lithium iron phosphate positive electrode material, characterized in that: It includes a first lithium iron phosphate positive electrode material and a second lithium iron phosphate positive electrode material, the first lithium iron phosphate positive electrode material includes solid primary particles A and hollow secondary particles surrounded by a plurality of the solid primary particles A, the primary particle size of the first lithium iron phosphate positive electrode material is 50-200nm, the secondary particle size of the first lithium iron phosphate positive electrode material is less than 10μm, the Dv50 of the first lithium iron phosphate positive electrode material is 0.5-6μm, the second lithium iron phosphate positive electrode material includes solid primary particles B and solid secondary particles formed by melting the solid primary particles B, the primary particle size of the second lithium iron phosphate positive electrode material is 100-400nm, the secondary particle size of the second lithium iron phosphate positive electrode material is 0.2-5μm, the Dv50 of the second lithium iron phosphate positive electrode material is 0.6-1.2μm, and the Dv50 of the first lithium iron phosphate positive electrode material is greater than the Dv50 of the second lithium iron phosphate positive electrode material.

2. The lithium iron phosphate positive electrode material according to claim 1, characterized in that: The mass ratio of the first lithium iron phosphate positive electrode material to the second lithium iron phosphate positive electrode material is (0.2-1.5):(8.5-9.8).

3. The lithium iron phosphate positive electrode material according to claim 1 or 2, characterized in that: The first lithium iron phosphate positive electrode material and / or the second lithium iron phosphate positive electrode material further include doped metal ions, and the doped metal ions are at least one of Mg, Mo, Ti, V, Mn and Nb.

4. The method for preparing the lithium iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The following steps are involved: S1. Mixing a phosphorus source, an iron source, a lithium source, a carbon source and a solvent to obtain a first mixed slurry; grinding, electromagnetizing and spray drying the first mixed slurry to obtain a first powder; sintering the first powder at a first high temperature to obtain the first lithium iron phosphate positive electrode material; wherein the temperature of the first spray drying is 160-175°C; S2. Mixing a phosphorus source, an iron source, a lithium source, a carbon source and a solvent to obtain a second mixed slurry; grinding the second mixed slurry, performing electromagnetization and a second spray drying to obtain a second powder; performing a second high-temperature sintering on the second powder, and then performing air flow pulverization to obtain the second lithium iron phosphate positive electrode material; S3. Mixing the first lithium iron phosphate positive electrode material and the second lithium iron phosphate positive electrode material to obtain the lithium iron phosphate positive electrode material.

5. The preparation method according to claim 4, characterized in that: The secondary particle size of the first powder is 2-4 μm.

6. The preparation method according to claim 4, characterized in that: The first high-temperature sintering has a holding temperature of 780-810° C. and a holding time of 4-6 hours.

7. The preparation method according to claim 4, characterized in that: The temperature of the second spray drying is greater than or equal to the temperature of the first spray drying, and / or the secondary particle size of the second powder is 3-5 μm.

8. The preparation method according to claim 4, characterized in that: The second high temperature sintering has a holding temperature of 780-850° C. and a holding time of 5-8 hours.

9. A positive electrode sheet, characterized in that: It comprises a positive electrode current collector and a positive electrode active layer located on at least one side of the surface of the positive electrode current collector, wherein the positive electrode active layer comprises the lithium iron phosphate positive electrode material according to any one of claims 1 to 3, a conductive agent and a binder.

10. A lithium ion battery, characterized in that: Including the positive electrode sheet as described in claim 9.

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