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

Abstract

The invention provides a lithium iron phosphate positive electrode material, a preparation method thereof and a lithium ion battery. The preparation method comprises the following steps: carrying out first sanding treatment on a metal-based dopant to obtain a primary sanding material; mixing the primary sand grinding material, a carbon source and a water solvent to obtain first slurry; adding a lithium source, a ferrous source and a phosphorus source into the first slurry, and carrying out second sanding treatment to obtain second slurry; and calcining the second slurry to obtain the lithium iron phosphate positive electrode material. The preparation process provided by the invention has no potential safety hazard, and the high-compaction lithium iron phosphate positive electrode material can be obtained.

Description

Technical Field

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

[0002] Currently, in lithium ion batteries, choosing high tap density lithium iron phosphate as the cathode material not only has excellent cycling performance, high energy density, but also high safety, low cost, and good environmental protection performance. The production and preparation of high tap density lithium iron phosphate mainly rely on the two-burn process of the iron phosphate method and the ferrous oxalate process to achieve. However, the two-burn process of the iron phosphate method requires two high-temperature sintering processes, during which the energy consumption is huge and the equipment loss is serious, resulting in high production costs; the ferrous oxalate process also faces cost problems, and methanol is used as a solvent in the production process. Since methanol is a flammable and explosive organic solvent, fires, explosions and other serious safety accidents may occur with a slight carelessness during storage, transportation and use, bringing great safety hazards to enterprises, which to a certain extent restricts the efficient and safe development of the high tap density lithium iron phosphate industry.

[0003] Therefore, it is of great research significance to provide a safe and reliable preparation process to obtain a high tap density lithium iron phosphate cathode material and improve its electrochemical performance. Summary of the Invention

[0004] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a lithium iron phosphate cathode material, a preparation method thereof, and a lithium ion battery. The preparation process provided by the present invention has no safety hazards and can obtain a high tap density lithium iron phosphate cathode material.

[0005] To achieve the purpose of this invention, the following technical solutions are adopted:

[0006] In the first aspect, the present invention provides a preparation method of a lithium iron phosphate cathode material, and the preparation method includes the following steps:

[0007] Perform a first sanding treatment on the metal-based dopant to obtain a primary sanded material.

[0008] Mix the primary sanded material, a carbon source, and an aqueous solvent to obtain a first slurry.

[0009] Add a lithium source, a ferrous source, and a phosphorus source to the first slurry, and perform a second sanding treatment to obtain a second slurry.

[0010] Calcine the second slurry to obtain a lithium iron phosphate cathode material.

[0011] The present invention first performs sanding treatment on the metal-based dopant, which is beneficial to the nanosizing of the metal-based catalyst, thereby improving the doping uniformity and electrical properties. Subsequently, the obtained primary abrasive is mixed with the carbon source in an aqueous solvent, which can improve the dispersion performance and reduce local agglomeration. Then, a lithium source, an iron(II) source, and a phosphorus source are added and sanding treatment is continued, followed by calcination to obtain the lithium iron phosphate cathode material. In summary, this preparation process has no safety hazards, and a high-compactness lithium iron phosphate cathode material can be obtained, and its electrochemical performance is also improved.

[0012] Preferably, the metal-based dopant includes any one or a combination of at least two of a titanium-based dopant, a magnesium-based dopant, or a boron-based dopant, preferably a combination of a titanium-based dopant and a boron-based dopant.

[0013] In the present invention, the titanium-based dopant and the boron-based dopant are co-doped, and they interact with each other, which helps to reduce the subsequent calcination temperature and improve the compaction and cycling performance of the material.

[0014] Preferably, the titanium-based dopant includes any one or a combination of at least two of TiO2, H2TiO3, or H4TiO4.

[0015] Preferably, the magnesium-based dopant includes any one or a combination of at least two of magnesium carbonate, magnesium hydroxide, or magnesium oxide.

[0016] Preferably, the boron-based dopant includes boric acid and / or boron oxide.

[0017] Preferably, for the combination of the titanium-based dopant and the boron-based dopant, the mass ratio of the titanium-based dopant to the boron-based dopant is 1:(0.2 - 0.5), for example, it can be 1:0.2, 1:0.3, 1:0.4, or 1:0.5, etc.

[0018] In the present invention, if the mass ratio of the titanium-based dopant to the boron-based dopant is too small, the subsequent calcination temperature will be relatively high, resulting in poor cycling performance of the material; if the mass ratio of the titanium-based dopant to the boron-based dopant is too large, the capacity performance of the material will be reduced.

[0019] Preferably, during the first sanding treatment, the solid content of the sanding system is 10 - 40 wt%, for example, it can be 10 wt%, 20 wt%, 30 wt%, or 40 wt%, etc.

[0020] In the present invention, during the process of controlling the first sanding treatment, the solid content of the sanding system is 10-40 wt%, which can enable the particles in the sanding system to maintain an appropriate collision frequency and intensity under the action of the sanding medium. Moreover, the particles will neither reduce the collision opportunities due to being too sparse nor interfere with each other due to being too dense. As a result, the grinding medium can more effectively impact, shear, and grind the particles, accelerating the refinement process of the particles and improving the grinding efficiency. Additionally, the particle size distribution of the material is more uniform.

[0021] Preferably, during the process of the first sanding treatment, the sanding speed is 1500-3000 rpm, for example, it can be 1500, 2000, 2200, 2500, 2800, 3000

[0022] In the present invention, an appropriate sanding speed helps to generate an appropriate shearing force and impact force on the material particles, enabling larger particle materials to be more effectively broken and refined, achieving a better grinding effect and obtaining a finer particle size distribution. Moreover, it can avoid the occurrence of particle agglomeration phenomenon, thereby ensuring that the ground material has good stability and uniformity and improving the grinding quality.

[0023] Preferably, the particle size D99 of the primary sanded material ≤ 300 nm, for example, it can be 300 nm, 200 nm, 100 nm, or 50 nm, etc.

[0024] In the present invention, the particle size D99 ≤ 300 nm means that 99% of the material particles have reached a size of ≤ 300 nm and are more uniformly distributed. The nano-sized and uniform material particles are conducive to more sufficient dispersion of subsequent doping, making the performance of each part of the cathode material consistent, avoiding local performance weak points caused by uneven particle sizes, and enhancing the overall stability and reliability of the cathode material.

[0025] Preferably, the carbon source includes any one or a combination of at least two of glucose, sucrose, starch, citric acid, stearic acid, or polyethylene glycol.

[0026] Preferably, the water solvent includes pure water and / or deionized water.

[0027] Preferably, the mixing method includes:

[0028] Perform a first mixing of the carbon source and the water solvent to obtain a carbon-containing solution, and then add the primary sanded material to the carbon-containing solution for a second mixing.

[0029] In the present invention, in the above mixing method, the carbon-containing solution provides a uniform dispersion medium for the primary sanded material, which can enable the primary sanded material to more fully contact with the carbon particles, achieve a uniform doping effect, and is beneficial to the consistency and stability of the subsequent cathode material performance.

[0030] Preferably, during the second mixing process, washing water is also added, and the washing water is the washing water of the sand mill used in the first sanding process.

[0031] In the present invention, during the second mixing process, the purpose of adding washing water is to ensure that there is no loss of the metal-based additive in lithium iron phosphate during the preparation process and the doping ratio is accurate.

[0032] Preferably, the volume ratio of the water solvent used in the first mixing process to the washing water added in the second mixing process is (50 - 70):(30 - 50). Among them, the selection range of the water solvent used in the first mixing process, "50 - 70", can be, for example, 50, 55, 60, 65, or 70, etc.; the selection range of the washing water added in the second mixing process, "30 - 50", can be, for example, 30, 35, 40, 45, or 50, etc.

[0033] Preferably, the lithium source includes Li3PO4 and / or Li2CO3.

[0034] Preferably, the ferrous source includes any one or a combination of at least two of FeC2O4, FePO3, or Fe(OH)3.

[0035] Preferably, the phosphorus source includes iron phosphate.

[0036] Preferably, the molar ratio of the lithium source, ferrous source, and phosphorus source is (0.95 - 1.02):(0.98 - 1.2):(1.9 - 2.2). Among them, the selection range of the lithium source, "0.95 - 1.02", can be, for example, 0.95, 0.96, 0.97, 0.98, 0.99, 1, 1.01, or 1.02, etc.; the selection range of the ferrous source, "0.98 - 1.2", can be, for example, 0.98, 0.99, 1, 1.05, 1.1, 1.15, or 1.2, etc.; the selection range of the phosphorus source, "1.9 - 2.2", can be, for example, 1.9, 1.95, 2, 2.05, 2.1, 2.15, or 2.2, etc.

[0037] Preferably, the second sanding process includes rough grinding and fine grinding.

[0038] In the present invention, coarse grinding can initially break large particle substances in the slurry, reducing the particle size range of the slurry particles. Subsequently, fine grinding further refines them, making the particle sizes more uniform and the particle size distribution more concentrated. Coarse grinding increases the specific surface area of the slurry particles, and fine grinding further increases the specific surface area of the slurry. A larger specific surface area can provide more active sites for the electrochemical reaction between the electrode material and the electrolyte, improving the charge-discharge performance of the battery. In summary, since the second slurry particles have uniform particle sizes, regular shapes, and a large specific surface area after coarse grinding and fine grinding, during the calcination process, the contact between particles is closer and the mass transfer is more uniform, which is beneficial to further improving the density and mechanical properties of the cathode material.

[0039] Preferably, the D50 of the particles after coarse grinding is 2 - 10 μm, for example, it can be 2 μm, 3 μm, 4 μm, 5 μm, 6 μm, 7 μm, 8 μm, 9 μm, or 10 μm, etc.

[0040] Preferably, the D50 of the particles after fine grinding is 0.3 - 3 μm, for example, it can be 0.3 μm, 0.5 μm, 1 μm, 1.5 μm, 2 μm, 2.5 μm, or 3 μm, etc.

[0041] Preferably, the solid content of the second slurry is 20 - 50 wt%, for example, it can be 20 wt%, 30 wt%, 40 wt%, or 50 wt%, etc.

[0042] Preferably, the calcination is a single calcination.

[0043] In the present invention, only a single calcination is required to make the tap density of lithium iron phosphate reach 2.6 g / cm 3 As mentioned above, it shows the high efficiency of the preparation process provided by the present invention. A higher tap density is beneficial to improving the volumetric energy density of the material. In battery applications, more active substances can be filled in a limited battery volume, so that the battery can have better electrochemical performance during charge and discharge, such as higher charge-discharge efficiency, better cycle stability, and rate performance, etc.

[0044] Preferably, the single calcination includes two-stage calcination, and the two-stage calcination includes the first-stage calcination and the second-stage calcination.

[0045] Preferably, the atmosphere of the single calcination is an inert atmosphere. Exemplarily, for example, it can be nitrogen or argon, etc.

[0046] Preferably, the temperature of the first-stage calcination is lower than that of the second-stage calcination.

[0047] In the present invention, the first-stage calcination is carried out at a lower temperature, which can cause a preliminary chemical reaction of the material and lay a foundation for the subsequent second-stage calcination.

[0048] Preferably, the temperature of the first-stage calcination is 350-500 °C, such as 350 °C, 400 °C, 450 °C or 500 °C, etc., and the time is 3-5 h, such as 3 h, 4 h or 5 h, etc.

[0049] Preferably, the temperature of the second-stage calcination is 750-790 °C, such as 750 °C, 760 °C, 770 °C, 780 °C or 790 °C, etc., and the time is 5-8 h, such as 5 h, 6 h, 7 h or 8 h, etc.

[0050] Preferably, the preparation method includes the following steps:

[0051] (1) Perform the first sanding treatment on the metal-based dopant at a sanding speed of 1500-3000 rpm to obtain a primary abrasive with a particle size D99 ≤ 300 nm; wherein, during the first sanding treatment, the solid content of the sanding system is 10-40 wt%.

[0052] (2) Wash the sand mill used in the first sanding treatment with a part of the water solvent to obtain washing water;

[0053] First mix the carbon source and another part of the water solvent to obtain a carbon-containing solution, and then add the primary abrasive and the washing water to the carbon-containing solution for a second mixing to obtain a first slurry; wherein, the washing water accounts for 30-50% of the total mass of the water solvent.

[0054] (3) Add Li3PO4, FeC2O4 and iron phosphate to the first slurry and perform a second sanding treatment to obtain a second slurry with a solid content of 20-50 wt%; wherein, the second sanding treatment includes rough grinding and fine grinding, the speed of the rough grinding is 500-2000 rpm (such as 500 rpm, 1000 rpm, 1500 rpm or 2000 rpm), the speed of the fine grinding is 1500-3000 rpm (such as 1500 rpm, 2000 rpm, 2500 rpm or 3000 rpm, etc.), the particle size D50 of the particles after rough grinding is 2-10 μm, and the particle size D50 of the particles after fine grinding is 0.3-3 μm.

[0055] (4) First dry the second slurry, and then perform a first-stage calcination in an inert atmosphere. The first-stage calcination includes a first-stage calcination and a second-stage calcination. The temperature of the first-stage calcination is 350-500 °C, the time is 3-5 h, the temperature of the second-stage calcination is 750-950 °C, and the time is 5-8 h to obtain a lithium iron phosphate cathode material.

[0056] In a second aspect, the present invention provides a lithium iron phosphate cathode material, which is prepared by using the preparation method of the lithium iron phosphate cathode material as described in the first aspect.

[0057] Preferably, the tap density of the lithium iron phosphate cathode material is greater than 2.6 g / cm 3 , for example, it can be 2.65 g / cm 3 , 2.7 g / cm 3 or 2.72 / cm 3 and so on.

[0058] Preferably, in the lithium iron phosphate cathode material, the mass ratio of the metal doping element is 0.1 - 0.3%, for example, it can be 0.1%, 0.2% or 0.3% etc.

[0059] In the present invention, a metal doping element with an appropriate mass ratio in the lithium iron phosphate cathode material helps to enhance the electron transport inside the material, making the electrode reaction easier to proceed during the charge and discharge process of the battery, and improving the charge and discharge performance of the battery; it helps to improve the diffusion kinetics of lithium ions, enabling the battery to still maintain good charge and discharge performance at high current densities; it helps to enhance the structural stability of the lithium iron phosphate cathode material and improve the safety performance of the battery.

[0060] Preferably, in the lithium iron phosphate cathode material, the carbon content is 1.2 - 2.5 wt%, for example, it can be 1.2 wt%, 1.4 wt%, 1.6 wt%, 1.8 wt%, 2 wt%, 2.1 wt%, 2.2 wt%, 2.3 wt%, 2.4 wt% or 2.5 wt% etc.

[0061] In a third aspect, the present invention provides a lithium ion battery, and the positive electrode of the lithium ion battery includes the lithium iron phosphate cathode material as described in the second aspect.

[0062] The numerical ranges described in the present invention not only include the above-listed point values, but also include any point values between the above numerical ranges that are not listed. Due to space limitations and for the sake of brevity, the present invention does not exhaustively list the specific point values included in the range.

[0063] Compared with the prior art, the present invention has the following beneficial effects:

[0064] The present invention first performs sanding treatment on the metal-based dopant, which is beneficial to the nanometerization of the metal-based catalyst, thereby improving the doping uniformity and electrical performance. Subsequently, the obtained primary sanded material is mixed with the carbon source in an aqueous solvent, which can improve the dispersion performance and reduce local agglomeration. Then, a lithium source, an iron(II) source, and a phosphorus source are added and sanding treatment is continued, and then calcined to obtain the lithium iron phosphate cathode material. In summary, this preparation process has no safety hazards, and a high-tap-density lithium iron phosphate cathode material can be obtained, and its electrochemical performance is also improved. Brief Description of the Drawings

[0065] Figure 1 It is a schematic process flow diagram provided by the present invention. Detailed Embodiments

[0066] The technical solution of the present invention will be further described below through specific embodiments. Those skilled in the art should understand that the embodiments are only for helping to understand the present invention and should not be regarded as specific limitations on the present invention.

[0067] Embodiment 1

[0068] This embodiment provides a preparation method of a lithium iron phosphate cathode material, and its schematic process flow diagram is as Figure 1 shown. The preparation method includes the following steps:

[0069] (1) Using a sand mill, perform a first sanding treatment on the titanium-based dopant at a sanding speed of 2000 rpm to obtain a primary sanded material with a particle size D99 ≤ 300 nm; wherein, during the first sanding treatment, the solid content of the sanding system is 25 wt%, and the primary sanded material is H2TiO3.

[0070] (2) Wash the sand mill used in the first sanding treatment with a part of pure water to obtain washing water;

[0071] Mix glucose and another part of pure water for the first time to obtain a carbon-containing solution, and then add the primary sanded material and the washing water to the carbon-containing solution for the second mixing to obtain a first slurry; wherein, the washing water accounts for 40% of the total mass of the pure water.

[0072] (3) Add Li3PO4, FeC2O4, and iron phosphate in a molar ratio of 1:1:2 to the first slurry, and perform a second sanding treatment to obtain a second slurry with a solid content of 35 wt%; wherein, the second sanding treatment includes rough grinding and fine grinding, the speed of the rough grinding is 1000 rpm, the speed of the fine grinding is 2000 rpm, the particle size D50 after rough grinding is 6 μm, and the particle size D50 after fine grinding is 1.5 μm.

[0073] (4) First perform spray drying on the second slurry to control the moisture content to be less than 4%, and then perform a first calcination in a nitrogen atmosphere. The first calcination includes a first-stage calcination and a second-stage calcination. The temperature of the first-stage calcination is 400 °C and the time is 4 h. The temperature of the second-stage calcination is 800 °C and the time is 5 h to obtain a lithium iron phosphate cathode material.

[0074] This embodiment also provides a lithium iron phosphate cathode material, which is prepared by the above preparation method; in the lithium iron phosphate cathode material, the mass ratio of titanium element is 0.2%, and the carbon content is 2 wt%.

[0075] Example 2

[0076] This embodiment provides a preparation method of a lithium iron phosphate cathode material, and the preparation method includes the following steps:

[0077] (1) Using a sand mill, perform the first sanding treatment on the magnesium-based dopant at a sanding speed of 1500 rpm to obtain a primary sanded material with a particle size D99 ≤ 300 nm; wherein, during the first sanding treatment, the solid content of the sanding system is 10 wt%, and the primary sanded material is magnesium carbonate.

[0078] (2) Wash the sand mill used in the first sanding treatment with a part of pure water to obtain washing water;

[0079] Mix sucrose and another part of pure water for the first time to obtain a carbon-containing solution, and then add the primary sanded material and the washing water to the carbon-containing solution for the second mixing to obtain a first slurry; wherein, the washing water accounts for 30% of the total mass of the pure water.

[0080] (3) Add Li3PO4, FeC2O4, and iron phosphate in a molar ratio of 1.02:1.2:1.9 to the first slurry, and perform the second sanding treatment to obtain a second slurry with a solid content of 20 wt%; wherein, the second sanding treatment includes rough grinding and fine grinding, the rotation speed of the rough grinding is 500 rpm, the rotation speed of the fine grinding is 1500 rpm, the particle size D50 after rough grinding is 2 μm, and the particle size D50 after fine grinding is 0.3 μm.

[0081] (4) First perform spray drying on the second slurry to control the moisture to be less than 4%, and then perform a first calcination in a nitrogen atmosphere. The first calcination includes a first-stage calcination and a second-stage calcination. The temperature of the first-stage calcination is 350 °C and the time is 5 h. The temperature of the second-stage calcination is 750 °C and the time is 8 h to obtain a lithium iron phosphate cathode material.

[0082] This embodiment also provides a lithium iron phosphate cathode material, which is prepared by the above preparation method; in the lithium iron phosphate cathode material, the mass ratio of magnesium element is 0.1%, and the carbon content is 2.5 wt%.

[0083] Example 3

[0084] This embodiment provides a preparation method of a lithium iron phosphate cathode material, and the preparation method includes the following steps:

[0085] (1) Use a sand mill to perform a first sanding treatment on the boron-based dopant at a sanding speed of 3000 rpm to obtain a primary sanded material with a particle size D99 ≤ 300 nm; wherein, during the first sanding treatment, the solid content of the sanding system is 40 wt%, and the primary sanded material is boron oxide.

[0086] (2) Wash the sand mill used in the first sanding treatment with a portion of pure water to obtain washing water.

[0087] Mix starch with another portion of pure water for the first time to obtain a carbon-containing solution, and then add the primary sanded material and the washing water to the carbon-containing solution for a second mixing to obtain a first slurry; wherein, the washing water accounts for 50% of the total mass of the pure water.

[0088] (3) Add Li3PO4, FeC2O4, and iron phosphate in a molar ratio of 0.95:0.98:2.2 to the first slurry and perform a second sanding treatment to obtain a second slurry with a solid content of 50 wt%; wherein, the second sanding treatment includes rough grinding and fine grinding, the rotation speed of the rough grinding is 2000 rpm, the rotation speed of the fine grinding is 3000 rpm, the particle size D50 after rough grinding is 10 μm, and the particle size D50 after fine grinding is 3 μm.

[0089] (4) First perform spray drying on the second slurry to control the moisture content to be less than 4%, and then perform a first calcination in a nitrogen atmosphere. The first calcination includes a first-stage calcination and a second-stage calcination. The temperature of the first-stage calcination is 500 °C and the time is 3 h. The temperature of the second-stage calcination is 950 °C and the time is 5 h to obtain a lithium iron phosphate cathode material.

[0090] This example also provides a lithium iron phosphate cathode material, which is prepared by the above preparation method; in the lithium iron phosphate cathode material, the mass ratio of titanium element is 0.3%, and the carbon content is 1.2 wt%.

[0091] Example 4

[0092] The difference between this example and Example 1 is that the titanium-based dopant is replaced with a combination of H2TiO3 and boron oxide, and the mass ratio of the two is 1:0.3.

[0093] The remaining preparation methods and parameters are the same as those in Example 1.

[0094] Example 5

[0095] The difference between this example and Example 4 is that the mass ratio of H2TiO3 and boron oxide is 1:0.2.

[0096] The remaining preparation methods and parameters are the same as those in Example 4.

[0097] Example 6

[0098] The difference between this example and Example 4 is that the mass ratio of H2TiO3 to boron oxide is 1:0.5.

[0099] The remaining preparation methods and parameters are the same as those in Example 4.

[0100] Example 7

[0101] The difference between this example and Example 4 is that the mass ratio of H2TiO3 to boron oxide is 1:0.1.

[0102] The remaining preparation methods and parameters are the same as those in Example 1.

[0103] Example 8

[0104] The difference between this example and Example 4 is that the mass ratio of H2TiO3 to boron oxide is 1:0.6.

[0105] The remaining preparation methods and parameters are the same as those in Example 4.

[0106] Example 9

[0107] The difference between this example and Example 1 is that the particle size D99 of the primary abrasive in step (1) is 500 nm.

[0108] The remaining preparation methods and parameters are the same as those in Example 1.

[0109] Example 10

[0110] The difference between this example and Example 1 is that in step (2), glucose, the primary abrasive, washing water, and another part of pure water are blended together to obtain the first slurry.

[0111] The remaining preparation methods and parameters are the same as those in Example 1.

[0112] Example 11

[0113] The difference between this example and Example 1 is that no washing water is added during the second mixing in step (2).

[0114] The remaining preparation methods and parameters are the same as those in Example 1.

[0115] Example 12

[0116] The difference between this example and Example 1 is that rough grinding is not performed in step (3).

[0117] The remaining preparation methods and parameters are the same as those in Example 1.

[0118] Comparative Example 1

[0119] The difference between this comparative example and Example 1 is that step (1) is not carried out, that is, the titanium-based dopant is not subjected to the first sanding treatment.

[0120] The remaining preparation methods and parameters are the same as those in Example 1.

[0121] Comparative Example 2

[0122] The difference between this comparative example and Example 1 is that both the first sanding treatment and the second sanding treatment described in step (1) are replaced with ball milling treatment.

[0123] The remaining preparation methods and parameters are the same as those in Example 1.

[0124] Comparative Example 3

[0125] The difference between this comparative example and Example 1 is that steps (2) and (3) are combined, that is, the carbon-containing solution, the primary abrasive, the washing water, Li3PO4, FeC2O4 and iron phosphate are blended together.

[0126] The remaining preparation methods and parameters are the same as those in Example 1.

[0127] Comparative Example 4

[0128] The difference between this comparative example and Example 1 is that pure water in step (2) is replaced with methanol.

[0129] The remaining preparation methods and parameters are the same as those in Example 1.

[0130] Performance Test

[0131] I. The tap density of the lithium iron phosphate cathode material provided in the above examples and comparative examples was tested. The test method is as follows: Weigh 1 g of the sample and apply a pressure of 20 MP.

[0132] II. Using the lithium iron phosphate cathode material provided in the above examples and comparative examples as the cathode active material, it was mixed evenly with a carbon black conductive agent, a binder PVDF and a solvent NMP in a mass ratio of 95:2.5:2.5:5 to prepare a cathode slurry for the battery; the cathode slurry was coated on an aluminum foil with a thickness of 30 μm, and after vacuum drying and rolling, a cathode electrode sheet was made. Using a lithium metal sheet as the anode and an electrolyte of 1.15 mol / L LiPF6 carbonate solution (the solvent is a 1:1 volume ratio of EC and DMC), a button battery was assembled and its first discharge capacity at 1C was detected.

[0133] The above test results are shown in Table 1.

[0134] Table 1

[0135]

[0136]

[0137] Analysis:

[0138] As can be seen from Table 1, in the present invention, the metal-based dopant is first subjected to sanding treatment, which is beneficial to the nanometerization of the metal-based catalyst, thereby improving the doping uniformity and electrical properties. Subsequently, the obtained primary sanded material is mixed with the carbon source in an aqueous solvent, which can improve the dispersion performance and reduce local agglomeration. Then, a lithium source, an iron(II) source, and a phosphorus source are added and sanding treatment is continued, followed by calcination to obtain the lithium iron phosphate cathode material. In summary, this preparation process has no safety hazards, and a high-compactness lithium iron phosphate cathode material can be obtained, and its electrochemical performance is also improved.

[0139] As can be seen from the comparison between Example 1 and Example 4, the synergistic doping of H2TiO3 and boron oxide interacts with each other, which helps to improve the compaction and cycling performance of the material.

[0140] As can be seen from Example 4 and Examples 7-8, if the mass ratio of H2TiO3 to boron oxide is too large, the capacity performance of the material will decrease; if the mass ratio of H2TiO3 to boron oxide is too small, the cycling performance of the material will deteriorate.

[0141] As can be seen from Example 1 and Example 9, if the particle size D99 of the primary sanded material is too large, it is difficult to uniformly enter the crystal lattice structure of lithium iron phosphate, more lattice defects are likely to occur, and local enrichment is likely to occur, thereby triggering the phase separation phenomenon, seriously affecting the overall structure and performance consistency of the cathode material, and resulting in a decrease in the tap density and capacity performance of the cathode material.

[0142] As can be seen from Example 1 and Example 10, washing the sand mill used in the first sanding treatment is to ensure that all titanium-based dopants are added to the product in proportion; then, the carbon-containing solution, the primary sanded material, and the washing water are mixed and dispersed evenly, which is more conducive to improving the doping uniformity and the compaction and electrical properties of the cathode material.

[0143] As can be seen from Example 1 and Example 11, if washing water is not added during the second mixing process, there will be losses of titanium-based dopants during the preparation process, and the loss amount is uncertain, resulting in unstable product batches and affected performance.

[0144] As can be seen from Example 1 and Example 12, if rough grinding is not carried out in step (3), firstly, the overall production cycle will be prolonged, reducing production efficiency. Secondly, large particles in the slurry may not be fully dispersed and evenly ground during fine grinding, easily resulting in the situation that some particles are too fine while some particles are still large, leading to uneven particle size distribution of the cathode material and affecting the performance and quality stability of the cathode material. In addition, it should be noted that the purpose of rough grinding includes grinding large particles into fine particles to prevent clogging of the sand mill and affecting subsequent processing.

[0145] As can be seen from Example 1 and Comparative Example 1, if the titanium-based dopant is not subjected to the first sand grinding treatment, it is difficult for the titanium-based dopant with too large a particle size to achieve a uniform doping effect, and local stress concentration may be formed in the crystal lattice of the cathode material, resulting in distortion, deformation or even defects in the crystal structure and damaging the integrity of the crystal structure. Moreover, the titanium-based dopant with too large a particle size will occupy a large space inside the cathode and it is difficult to obtain a high tap density through the compaction process, while the lower tap density will lead to a decrease in the specific capacity of the electrode and affect the energy density of the battery.

[0146] As can be seen from Example 1 and Comparative Example 2, if both the first sand grinding treatment and the second sand grinding treatment are replaced by ball milling treatment, since it is difficult to precisely control the movement trajectory and collision force of the grinding medium during ball milling, the grinding action on different particles is uneven, resulting in some particles being over-ground while some particles are under-ground, leading to a relatively wide final particle size distribution and being unfavorable for improving the tap density and capacity performance of the cathode material. Moreover, the efficiency of ball milling is relatively low and the equipment wear is relatively serious, which is not conducive to industrial production.

[0147] As can be seen from Example 1 and Comparative Example 3, if the carbon-containing solution, the sand-ground titanium-based dopant, washing water, Li3PO4, FeC2O4 and iron phosphate are blended, the carbon particles in the carbon-containing solution may not be evenly dispersed in the slurry, resulting in uneven distribution of carbon in the cathode material and uneven local adsorption, thus affecting the overall charge-discharge efficiency and consistency of the battery; the sand-ground titanium-based dopant may agglomerate due to direct mixing and cannot be evenly distributed in the crystal lattice of the cathode material in the form of small particles, which will lead to poor doping effect and unable to fully exert the improvement effect on the cathode material.

[0148] As can be seen from Example 1 and Comparative Example 4, if pure water in step (2) is replaced by methanol, although there is little difference in tap density and capacity performance when compared with water as the solvent, the safety hazards increase sharply, which is not conducive to industrial production.

[0149] It should be noted that the present invention illustrates the process method of the present invention through the above embodiments, but the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvement to the present invention, the equivalent substitution of the raw materials selected for the present invention, the addition of auxiliary components, the selection of specific methods, etc. all fall within the protection scope and the disclosure scope of the present invention.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The preparation method comprises the following steps: The metal-based dopant is subjected to a first sand-grinding treatment to obtain a primary sand-grinding material; Mixing the primary abrasive, a carbon source and an aqueous solvent to obtain a first slurry; Adding a lithium source, a ferrous source and a phosphorus source into the first slurry, and performing a second sand milling process to obtain a second slurry; The second slurry is calcined to obtain a lithium iron phosphate positive electrode material.

2. The method for preparing the lithium iron phosphate positive electrode material according to claim 1, characterized in that: The metal-based dopant includes any one of a titanium-based dopant, a magnesium-based dopant or a boron-based dopant, or a combination of at least two of them, preferably a combination of a titanium-based dopant and a boron-based dopant; Preferably, the titanium-based dopant includes any one of TiO2, H2TiO3 or H4TiO4, or a combination of at least two thereof; Preferably, the magnesium-based dopant includes any one or a combination of at least two of magnesium carbonate, magnesium hydroxide or magnesium oxide; Preferably, the boron-based dopant comprises boric acid and / or boron oxide; Preferably, in the combination of the titanium-based dopant and the boron-based dopant, the mass ratio of the titanium-based dopant to the boron-based dopant is 1:(0.2-0.5).

3. The method for preparing the lithium iron phosphate positive electrode material according to claim 1 or 2, characterized in that: During the first sand milling process, the solid content of the sand milling system is 10-40wt%; Preferably, during the first sanding process, the sanding speed is 1500-3000 rpm; Preferably, the particle size D99 of the primary abrasive is ≤300 nm.

4. The method for preparing a lithium iron phosphate positive electrode material according to claim 1, characterized in that: The carbon source includes any one of glucose, sucrose, starch, citric acid, stearic acid or polyethylene glycol, or a combination of at least two thereof; Preferably, the aqueous solvent comprises pure water and / or deionized water; Preferably, the mixing method includes: The carbon source and the water solvent are first mixed to obtain a carbon-containing solution, and then the primary abrasive is added to the carbon-containing solution to perform a second mixing.

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 lithium source includes Li3PO4 and / or Li2CO3; Preferably, the ferrous source comprises any one or a combination of at least two of FeC2O4, FePO3 or Fe(OH)3; Preferably, the phosphorus source comprises iron phosphate; Preferably, the molar ratio of the lithium source, the ferrous source and the phosphorus source is (0.95-1.02):(0.98-1.2):(1.9-2.2).

6. The method for preparing the lithium iron phosphate positive electrode material according to any one of claims 1 to 5, characterized in that: The second sanding process includes coarse grinding and fine grinding; Preferably, the particle size D50 of the coarsely ground particles is 2-10 μm; Preferably, the particle size D50 of the finely ground particles is 0.3-3 μm.

7. The method for preparing the lithium iron phosphate positive electrode material according to any one of claims 1 to 6, characterized in that: The calcination is a single calcination; Preferably, the primary calcination includes two-stage calcination, and the two-stage calcination includes a first-stage calcination and a second-stage calcination; Preferably, the temperature of the first stage calcination is lower than the temperature of the second stage calcination.

8. The method for preparing the lithium iron phosphate positive electrode material according to any one of claims 1 to 7, characterized in that: The preparation method comprises the following steps: (1) subjecting the metal-based dopant to a first sand milling treatment at a sand milling speed of 1500-3000 rpm to obtain a primary sand mill material with a particle size D99≤300 nm; wherein during the first sand milling treatment, the solid content of the sand milling system is 10-40 wt %; (2) using a portion of the aqueous solvent to wash the sand mill used in the first sand milling process to obtain washing water; The carbon source and another part of the aqueous solvent are first mixed to obtain a carbon-containing solution, and then the primary sand abrasive and washing water are added to the carbon-containing solution for second mixing to obtain a first slurry; wherein the washing water accounts for 30-50% of the total mass of the aqueous solvent; (3) adding Li3PO4, FeC2O4 and iron phosphate to the first slurry, and performing a second sand milling treatment to obtain a second slurry with a solid content of 20-50wt%; wherein the second sand milling treatment includes coarse grinding and fine grinding, the rotation speed of the coarse grinding is 500-2000rpm, the rotation speed of the fine grinding is 1500-3000rpm, the particle size D50 of the particles after the coarse grinding is 2-10μm, and the particle size D50 of the particles after the fine grinding is 0.3-3μm; (4) The second slurry is first dried and then calcined once in an inert atmosphere. The first calcination includes a first calcination and a second calcination. The first calcination temperature is 350-500°C and the time is 3-5 hours. The second calcination temperature is 750-950°C and the time is 5-8 hours to obtain a lithium iron phosphate positive electrode material.

9. A lithium iron phosphate positive electrode material, characterized in that: The lithium iron phosphate positive electrode material is prepared by the method for preparing the lithium iron phosphate positive electrode material according to any one of claims 1 to 8; Preferably, the compaction density of the lithium iron phosphate positive electrode material is greater than 2.6 g / cm 3 ; Preferably, in the lithium iron phosphate positive electrode material, the mass proportion of the metal doping element is 0.1-0.3%; Preferably, the carbon content in the lithium iron phosphate positive electrode material is 1.2-2.5wt%.

10. A lithium ion battery, characterized in that: The positive electrode of the lithium-ion battery comprises the lithium iron phosphate positive electrode material as claimed in claim 9.

Images