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

Abstract

The invention discloses a lithium iron phosphate positive electrode material, a preparation method and a lithium ion battery, and relates to the technical field of lithium ion positive electrode materials. The preparation method comprises the following steps: mixing a lithium source, an iron source, a phosphorus source and a carbon source, adding a graphene dispersion liquid, and performing high-speed sanding and drying to obtain intermediate powder; and carrying out compression molding on the intermediate powder, and carrying out microwave heat treatment to obtain the lithium iron phosphate positive electrode material. Graphene and a mixture containing a lithium source, an iron source, a phosphorus source and a carbon source are subjected to high-speed sanding in a high-speed sanding mode, the dispersity among particles is enhanced, the lithium iron phosphate positive electrode material is generated after microwave heat treatment, the carbon source forms a carbon coating layer on the surface of LiFePO4, graphene is staggered in gaps of LiFePO4 particles, a unique three-dimensional conductive network structure is formed, and the lithium iron phosphate positive electrode material is prepared. Meanwhile, pore structures exist among LiFePO4 particles, so that the interface contact area between the LiFePO4 positive electrode material and an electrolyte is greatly expanded, the migration process of lithium ions is effectively accelerated, and the low-temperature performance is remarkably improved.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion positive electrode materials, and in particular to a lithium iron phosphate positive electrode material, a preparation method and a lithium ion battery. Background Art

[0003] As the penetration rate of electric vehicles increases year by year, people are calling for "high safety" and "low cost" in electric vehicles. The selection of cathode materials is directly related to the safety and cost-effectiveness of the product, so the selection of cathode materials is particularly important. Compared with other cathode materials (lithium cobalt oxide, lithium manganese oxide, nickel cobalt manganese oxide, etc.), lithium iron phosphate has the highest comprehensive advantages in safety, cost and stability. This cathode material has become the dominant choice in the new energy vehicle market and occupies the highest share.

[0004] However, the conductivity of lithium iron phosphate cathode materials is relatively poor, and this characteristic is particularly evident in low temperature environments. When the temperature drops, the diffusion rate of ions in the lithium iron phosphate cathode slows down significantly, causing the migration of lithium ions during the charge and discharge process to be severely hindered. This is because the activation energy required for ion movement in a low temperature environment increases, which increases the difficulty of ion migration, which is directly manifested as a significant attenuation of the battery capacity. At the same time, the conductivity of lithium iron phosphate is relatively low under low temperature conditions, which aggravates the polarization phenomenon inside the battery and significantly reduces the battery capacity and energy density. In addition, low temperatures may also cause changes in the crystal structure of the lithium iron phosphate cathode material, thereby affecting the stability of the cathode material and the cycle life of the battery cell.

[0005] In response to the phenomenon that the low-temperature performance of lithium iron phosphate cathode materials is poor, researchers have conducted some modification research work. For example, the Chinese patent with publication number CN202111590337.9 discloses a method for preparing an in-situ graphene-doped low-temperature lithium iron phosphate cathode material. By using dicyandiamide and glucose as carbon sources, the surface of the lithium iron phosphate cathode material coated with graphene is obtained through ball milling, sand milling, spray drying, high-temperature sintering, air flow crushing and other processes. The Chinese patent with publication number CN202211485830.9 discloses a high-density low-temperature lithium iron phosphate cathode material and a preparation method thereof. The precursor solution and citrate are mixed and then subjected to a hydrothermal reaction. At the same time, a binder and spray cracking and granulation are combined to mix large and small particles to obtain a target sample. A Chinese patent with publication number CN202410128235.2 discloses a low-temperature resistant lithium iron phosphate positive electrode material and a preparation method thereof, wherein a modified product is obtained by mixing reactants and surfactants, grinding, spray drying, roasting, and then grinding, spray drying, roasting, and the like.

[0006] Although the low-temperature performance of the modified lithium iron phosphate positive electrode materials prepared by the above methods has been improved to a certain extent, they still have defects such as long preparation cycle, relatively high energy consumption, relatively complex process, and difficulty in large-scale mass production. Summary of the invention

[0007] The technical problem to be solved by the present invention is to provide a low-temperature resistant lithium iron phosphate positive electrode material, a preparation method and a lithium ion battery which have a simple preparation process, low energy consumption and are suitable for mass production.

[0008] In order to solve the above problems, the present invention proposes the following technical solutions:

[0009] In a first aspect, the present invention provides a method for preparing a lithium iron phosphate positive electrode material, comprising the following steps:

[0010] S1, mixing a lithium source, an iron source, a phosphorus source and a carbon source to obtain a mixture;

[0011] S2, adding graphene dispersion to the mixture, high-speed sand milling, and drying to obtain an intermediate powder, wherein the particle size distribution of the intermediate powder is 60 to 100 nm;

[0012] S3, pressing the intermediate powder into a shape to obtain a green body, and subjecting the green body to microwave heat treatment to obtain a lithium iron phosphate positive electrode material;

[0013] The particle size of the lithium iron phosphate positive electrode material is D10 ≥ 0.42 μm, D90 ≤ 4 μm, and the tap density is ≥ 0.9 g / cm 3 , compacted density ≥2.5g / cm 3 .

[0014] A further technical solution is that the rotation speed of the high-speed sand mill is 1200-2000 r / min and the time is 0.5-1.5 h.

[0015] The present invention adopts high-speed sand milling technology to mix the mixture added with graphene dispersion liquid with high-speed sand milling. The material and the sand milling medium collide, impact, shear and squeeze at high speed, and deformation, fracture and welding occur continuously. The high-intensity grinding for a long time makes the powder fully uniform and refined, and the particle size and distribution range of the material can be accurately regulated. The process of high-speed sand milling not only efficiently refines large-sized particles into nanometer level, but also cleverly utilizes the gap effect generated in the refinement process, that is, while crushing large particles, it promotes the newly generated small particles to naturally form gaps between larger particles. This structural characteristic significantly enhances the dispersibility between particles. At the same time, the dispersibility of graphene in the mixture is improved. In this way, in the subsequent microwave heat treatment stage, these pre-generated gap structures can effectively hinder the aggregation of LiFePO4 particles. At the same time, graphene is interspersed therebetween, and a unique three-dimensional conductive network structure can be formed to avoid the agglomeration of LiFePO4 particles, ensuring the uniformity and stability of material performance.

[0016] A further technical solution is that the temperature of the microwave heat treatment is 600-700° C. and the time is 30-60 minutes.

[0017] The present invention adopts microwave heat treatment to sinter the material. The advantage of microwave uniformly heating the material ensures that the material can be heated uniformly during the heat treatment synthesis process, which can greatly shorten the reaction heating time, make the synthesis process more efficient, and effectively reduce the energy consumption of the entire process.

[0018] A further technical solution is that the mixing in step S1 is ball milling mixing, the rotation speed of the ball mill is 150-250 r / min, and the time is 2.0-8.0 h.

[0019] Compared with the high-speed sand milling in step S2, step S1 of the present invention only uses conventional ball milling to mix the lithium source, iron source, phosphorus source and carbon source thoroughly.

[0020] A further technical solution is that in the mixture of step S1, the mass fraction of the carbon source is 5-15%.

[0021] A further technical solution is that in step S2, the amount of graphene dispersion added is calculated based on the mass fraction of graphene in the generated lithium iron phosphate positive electrode material being 1 to 10%.

[0022] A further technical solution is that the concentration of the graphene dispersion is 50-250 mg / L.

[0023] A further technical solution is that the amounts of the lithium source, iron source and phosphorus source are measured according to the atomic ratio of LiFePO4, that is, the amounts of the lithium source, iron source and phosphorus source are measured according to the atomic ratio of Li, Fe and P = 1:1:1.

[0024] In the present invention, the iron source includes any one of ferric chloride, ferric phosphate, ferrous oxalate, ferrous sulfate, ferrous nitrate, ferrous acetate or ferric hydroxide, or a combination of at least two thereof.

[0025] The phosphorus source includes any one of lithium dihydrogen phosphate, ammonium dihydrogen phosphate, triammonium phosphate, phosphorus pentoxide and phosphoric acid, or a combination of at least two of them.

[0026] The lithium source includes any one of lithium chloride, lithium bromide, lithium phosphate, lithium dihydrogen phosphate, lithium hydrogen phosphate, lithium sulfate, lithium carbonate, lithium hydroxide, lithium acetate, lithium nitrate, lithium oxalate, lithium formate, lithium tert-butoxide, lithium benzoate or lithium citrate, or a combination of at least two of them, preferably any one of lithium carbonate, lithium hydroxide, lithium acetate, lithium nitrate, lithium oxalate, lithium formate, lithium citrate or lithium tert-butoxide, or a combination of at least two of them, and further preferably any one of lithium carbonate, lithium hydroxide, lithium acetate or lithium oxalate, or a combination of at least two of them.

[0027] For example, when the lithium source is selected from lithium carbonate (Li2CO3), the iron source is selected from ferrous oxalate (FeC2O4·2H2O) and the phosphorus source is selected from ammonium dihydrogen phosphate (NH4H2PO4), they are weighed respectively according to a molar ratio of 1:2:2 to ensure that the atomic ratio of lithium, iron and phosphorus elements in the final product strictly conforms to the stoichiometric ratio of LiFePO4.

[0028] In the present invention, the carbon source is a conventional non-doped carbon source, and the carbon source is added to the raw material. During the heat treatment, the carbon source is carbonized to form a carbon coating layer on the surface of the generated LiFePO4, thereby improving the conductivity and stability of LiFePO4. Suitable carbon sources include any one or a combination of at least two of glucose, sucrose, cellulose, polyethylene glycol, polyvinyl alcohol, soluble starch, monocrystalline rock sugar, polycrystalline rock sugar, fructose, citric acid, phenolic resin, vinyl pyrrolidone, epoxy resin, polysaccharide alcohol, polyvinylidene fluoride, polyvinyl chloride, urea-formaldehyde resin, polymethacrylate or furan resin.

[0029] In a second aspect, the present invention provides a lithium iron phosphate positive electrode material prepared by the preparation method described in the first aspect.

[0030] In a third aspect, the present invention provides a lithium-ion battery, comprising the lithium iron phosphate positive electrode material described in the second aspect, or comprising the lithium iron phosphate positive electrode material prepared by the preparation method described in the first aspect.

[0031] Compared with the prior art, the technical effects achieved by the present invention include:

[0032] The present invention adopts high-speed sand milling to grind graphene and a mixture containing a lithium source, an iron source, a phosphorus source and a carbon source at high speed, so as to enhance the dispersibility between particles. After microwave heat treatment, the lithium source, the iron source and the phosphorus source generate LiFePO4 through chemical reaction, and the carbon source forms a carbon coating layer on the surface of LiFePO4. Microscopically, graphene is interlaced between the LiFePO4 particles to form a unique three-dimensional conductive network structure. At the same time, there is a pore structure between the LiFePO4 particles, which greatly expands the interface contact area between the LiFePO4 positive electrode material and the electrolyte. In addition, these pores also serve as high-speed channels for lithium ion transmission, effectively accelerating the migration process of lithium ions, thereby significantly reducing the electrochemical polarization phenomenon, and significantly improving the low-temperature performance.

[0033] The preparation method of the invention is simple, has a short production cycle, low energy consumption, excellent low-temperature performance, and is suitable for industrial large-scale production. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings required for use in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other accompanying drawings can be obtained based on these accompanying drawings without paying any creative work.

[0035] Figure 1 This is a SEM photograph of the lithium iron phosphate positive electrode material obtained in Example 1.

[0036] Figure 2 These are the test results for each group of materials in the low temperature performance test experiment. DETAILED DESCRIPTION

[0037] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments. Obviously, the embodiments described below are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0038] It should be understood that when used in this specification and the appended claims, the terms "include" and "comprises" indicate the presence of described features, integers, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, components and / or combinations thereof.

[0039] It should also be understood that the terms used in this description of the embodiments of the present invention are only for the purpose of describing specific embodiments and are not intended to limit the embodiments of the present invention. As used in the description of the embodiments of the present invention and the appended claims, unless the context clearly indicates otherwise, the singular forms of "a", "an" and "the" are intended to include plural forms.

[0040] The present invention provides a lithium iron phosphate positive electrode material and a preparation method thereof. The specific preparation process is described as follows:

[0041] (1) The lithium source, iron source and phosphorus source are weighed so that the atomic ratio of lithium, iron and phosphorus in the final product strictly conforms to the stoichiometric ratio of LiFePO4, and an appropriate amount of carbon source (5-15wt%) is added, and the mixture is ball-milled to obtain a mixture.

[0042] The addition of the carbon source is intended to form a carbon coating of LiFePO4 during the subsequent microwave heat treatment process to improve the conductivity and stability of the material. The ball milling mixing of the present invention specifically uses anhydrous ethanol as a dispersion medium, and is ball milled at a speed of 150 to 250 rpm under the protection of an inert gas (such as nitrogen or argon) for 2 to 8 hours to ensure uniform mixing and preliminary refinement of the raw materials. In other embodiments, the dispersion medium can also be deionized water, ether, acetone, etc.

[0043] (2) Slowly add the pre-prepared graphene dispersion to the ball-milled mixture, stirring while adding, the amount of graphene dispersion added is preferably 1-10% of the mass of the final product, preferably 4-7%. After adding the graphene dispersion, use anhydrous ethanol as a dispersion medium, and perform high-speed sand milling at a speed of 1200-2000 rpm for 0.5-1.5 hours to further refine the material particles and promote the uniform dispersion of graphene in the material. After completing the sand milling, remove the excess ethanol medium by drying to obtain a dry intermediate product powder, the particle size distribution of the intermediate powder is 60-100 nm.

[0044] In other embodiments, the dispersion medium for high-speed sand milling can also be deionized water, ether, acetone, etc. Conventional drying techniques can be used to remove excess ethanol medium.

[0045] The concentration of the graphene dispersion used in this embodiment is 50-250 mg / L. The graphene dispersion can be obtained in a commercially available manner.

[0046] (3) The intermediate product powder is pressed into a shape to obtain a green body, which is placed in a corundum crucible in a microwave oven under the protection of an inert atmosphere (such as nitrogen or argon), heated to 600-700 degrees Celsius at a heating rate of 60°C / min, and maintained at this temperature for heat treatment for 30-60 minutes. It is then cooled to room temperature and crushed to obtain a black lithium iron phosphate positive electrode material. The particle size of the lithium iron phosphate positive electrode material is D10 ≥ 0.42 μm, D90 ≤ 4 μm, and the tap density is ≥ 0.9 g / cm 3 , compacted density ≥2.5g / cm 3 .

[0047] In a specific embodiment, the intermediate product is made into a sheet-like body by dry pressing to facilitate subsequent heat treatment.

[0048] The following specific examples are used to verify that the lithium iron phosphate positive electrode material obtained by the preparation method of the present invention has good low temperature resistance.

[0049] Example 1

[0050] Li2CO3, FeC2O4·2H2O and NH4H2PO4 are weighed and mixed in a molar ratio of 1:2:2, and then 10wt% of glucose is added as a carbon source. The materials are ball-milled at 200r / min for 2h in an argon atmosphere with anhydrous ethanol as a ball-milling medium to obtain a mixture; a graphene dispersion with a concentration of 50mg / L is added to the mixture while stirring, wherein the amount of the graphene dispersion added is based on the proportion of graphene in the product being 5wt%, and after stirring evenly, the mixture containing graphene is milled with anhydrous ethanol as a medium at 1500rmp·s -1 High-speed sand grinding for 1 hour, then drying to remove excess ethanol, to obtain an intermediate powder; the particle size distribution of the intermediate powder is 60-100nm. The intermediate powder is pressed into a sheet-like body, and the sheet-like body is placed in a corundum crucible in a microwave oven. The temperature is rapidly increased to 650°C at a heating rate of 60°C / min under an argon atmosphere, and the reaction is continued for 45 minutes to obtain a product. The product is crushed to obtain a black lithium iron phosphate positive electrode material product. According to actual measurements, the tap density of the material in this embodiment is 0.91g / cm 3 , suitable for high-end applications with strict requirements on volume energy density and compaction performance.

[0051] When the positive electrode material obtained in this embodiment is coated on the surface of the current collector to make a positive electrode sheet, the compaction density of the material can be ≥2.5g / cm by controlling the pressing rate. 3 , for example 2.6 g / cm 3 , 2.7g / cm 3 .

[0052] The lithium iron phosphate cathode material obtained in this example was characterized by SEM. Figure 1 It can be seen that the particle size distribution of the lithium iron phosphate positive electrode material prepared by the method of the embodiment of the present invention is relatively uniform, about 100 to 200 nm, where D10 ≥ 0.42 μm and D90 ≤ 4 μm. The nanostructured lithium iron phosphate greatly increases the specific surface area of ​​the material, providing more active sites for electrochemical reactions. At the same time, nano-sizing effectively shortens the lithium ion (Li + ) during the charge and discharge cycle, the migration path through the interior of the positive electrode material significantly reduces the electrochemical polarization phenomenon. Furthermore, the lithium iron phosphate particles present a spherical morphology, and the carbon layer uniformly coated on the surface is intertwined with the flaky graphene, together constructing an efficient three-dimensional conductive network, thereby exhibiting better low-temperature performance. In addition, some small particles of lithium iron phosphate melt and bond together to form a unique structure with a mixed distribution of large and small particles, which significantly improves the compaction density of the material. It can be understood that the higher the compaction density, the more uniform the distribution of the pore size and pores of the electrode, the more uniform the distribution of the conductive agent and the binder, and the more conducive to reducing the contact resistance and charge transfer impedance of the electrode, thereby having higher low-temperature resistance.

[0053] The lithium iron phosphate positive electrode material of this embodiment adopts high-speed sand grinding technology, which can make the distribution of graphene in the raw material more uniform. At the same time, after high-speed sand grinding, the particle size distribution of the raw material is 60 to 100 nm, which is conducive to the formation of lithium iron phosphate particles of different nanometer levels, which are then interwoven with graphene to form a unique microstructure, showing excellent low-temperature resistance.

[0054] Example 2

[0055] Li2CO3, FeC2O4·2H2O and NH4H2PO4 are weighed and mixed in a molar ratio of 1:2:2, and then 10wt% of sucrose is added as a carbon source, the materials are ball-milled and mixed, and anhydrous ethanol is used as a ball-milling medium, and the ball-milling is carried out at 200r / min for 3h in a nitrogen atmosphere to obtain a mixture; a graphene dispersion with a concentration of 50mg / L is added to the mixture while stirring, wherein the amount of the graphene dispersion added is based on the proportion of graphene in the product being 2wt%, and after stirring evenly, the mixture containing graphene is subjected to ball-milling at 1500rmp·s with anhydrous ethanol as a medium. -1 High-speed sand milling for 1 hour, then drying to remove excess ethanol, to obtain an intermediate powder, the particle size distribution of the intermediate powder is 60 to 100 nm. The intermediate powder is pressed into a sheet-like body, the sheet-like body is placed in a corundum crucible in a microwave oven, and the temperature is rapidly increased to 600°C at a heating rate of 60°C / min under a nitrogen atmosphere, and the reaction is continued for 30 minutes to obtain a product, which is crushed to obtain a black lithium iron phosphate positive electrode material product. According to actual measurements, the tap density of the material in this embodiment is 0.94g / cm3 .

[0056] Example 3

[0057] Li2CO3, FeC2O4·2H2O and NH4H2PO4 are weighed and mixed in a molar ratio of 1:2:2, and then 8wt% of sucrose is added as a carbon source. The materials are ball-milled at 200r / min for 1.5h in an argon atmosphere with anhydrous ethanol as a ball-milling medium to obtain a mixture; a graphene dispersion with a concentration of 50mg / L is added to the mixture while stirring, wherein the amount of the graphene dispersion added is based on the proportion of graphene in the product being 1wt%, and after stirring evenly, the mixture containing graphene is ball-milled at 1500rmp·s with anhydrous ethanol as a medium. -1 High-speed sand milling for 1 hour, then drying to remove excess ethanol, to obtain an intermediate powder, the particle size distribution of the intermediate powder is 60 to 100 nm. The intermediate powder is pressed into a sheet-like body, the sheet-like body is placed in a corundum crucible in a microwave oven, and the temperature is rapidly increased to 700°C at a heating rate of 60°C / min under an argon atmosphere, and the reaction is continued for 60 minutes to obtain a product, which is crushed to obtain a black lithium iron phosphate positive electrode material product. According to actual measurements, the tap density of the material in this embodiment is 0.93g / cm 3 .

[0058] Comparative Example 1

[0059] Li2CO3, FeC2O4·2H2O and NH4H2PO4 were weighed and mixed in a molar ratio of 1:2:2, and then 10wt% sucrose was added as a carbon source. The material was ball-milled at 200r / min for 4h in a nitrogen atmosphere with anhydrous ethanol as a ball-milling medium to obtain a mixture; the mixture was dried and transferred to a quartz crucible, and then the quartz crucible was placed in a muffle furnace and pre-sintered at 400℃ for 10h in a nitrogen atmosphere; the sample was fully ground after natural cooling; the ground sample was placed in a muffle furnace and sintered at 750℃ for 20h; the reactants were then crushed to obtain a black lithium iron phosphate cathode material product. According to actual measurements, the tap density of the comparative material is 0.75g / cm 3 .

[0060] Comparative Example 2

[0061] Li2CO3, FeC2O4·2H2O and NH4H2PO4 are weighed and mixed in a molar ratio of 1:2:2, and then 10wt% of sucrose is added as a carbon source. The material is ball-milled at 200r / min for 4h in a nitrogen atmosphere with anhydrous ethanol as a ball-milling medium to obtain a mixture; a graphene dispersion with a concentration of 50mg / L is added to the mixture while stirring, wherein the amount of graphene dispersion added is calculated based on the proportion of graphene in the product as 5wt%, and after stirring evenly, the mixture is dried, transferred to a quartz crucible, and then placed in a muffle furnace, and pre-sintered at 400℃ for 10h in a nitrogen atmosphere; the sample is fully ground after natural cooling; the ground sample is then placed in a muffle furnace and sintered at 750℃ for 20h; the reactants are then crushed to obtain a black lithium iron phosphate positive electrode material product. According to actual measurements, the tap density of the material in this comparative example is 0.73g / cm 3 .

[0062] Comparative Example 3

[0063] Li2CO3, FeC2O4·2H2O and NH4H2PO4 were weighed and mixed in a molar ratio of 1:2:2, and then 10wt% sucrose was added as a carbon source. The material was ball-milled at 200r / min for 2h in a nitrogen atmosphere with anhydrous ethanol as a medium, and then subjected to 1500rmp·s -1 High-speed sand milling for 1 hour to obtain a mixture; after drying the mixture, transfer it to a quartz crucible, place the quartz crucible in a muffle furnace, and pre-sinter it at 400°C for 4 hours under a nitrogen atmosphere; after the sample is cooled naturally, grind it thoroughly; the ground sample is placed in a muffle furnace and sintered at 750°C for 10 hours; then the reactant is crushed to obtain a black lithium iron phosphate cathode material product. According to actual measurement, the tap density of the material in this comparative example is 0.82g / cm 3 .

[0064] Low temperature performance test experiment

[0065] The lithium iron phosphate positive electrode material synthesized by the method of comparative example 1-3 was used as the control group, and the lithium iron phosphate positive electrode material synthesized by the method of embodiment 1-3 was used as the experimental group. By strictly controlling the variables (only changing the positive electrode material), they were assembled into 2Ah soft-pack batteries. Subsequently, without any external insulation measures, the two groups of battery cells were placed in a -20°C constant temperature environment box for 4 hours to simulate a low temperature environment. Afterwards, using the battery testing system in the constant temperature box, the two groups of battery cells were subjected to a 1C rate discharge test (voltage range: 2.0~3.75V) at -20°C and the discharge capacity was recorded internally. The discharge capacity divided by the rated capacity of the battery cell gives the discharge capacity retention rate at 1C rate at -20°C. The test results of each group are shown in the attached figure. Figure 2 shown.

[0066] Depend on Figure 2The results show that under the same conditions, the capacity retention rate of the lithium iron phosphate positive electrode material obtained by the method of the embodiment of the present invention in an environment of -20°C is about 40%, which is significantly higher than that of the lithium iron phosphate positive electrode material obtained by the comparative example method. This indicates that the lithium iron phosphate positive electrode material obtained by the preparation method of the present invention has good low-temperature performance and can withstand ultra-low temperatures of -20°C.

[0067] In the above embodiments, the description of each embodiment has its own emphasis. For parts that are not described in detail in a certain embodiment, reference can be made to the relevant descriptions of other embodiments.

[0068] The above is a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can easily think of various equivalent modifications or replacements within the technical scope disclosed by the present invention, and these modifications or replacements should be included in the protection scope of the present invention. Therefore, the protection scope of the present invention shall be based on the protection scope of the claims.

Claims

1. A method for preparing a lithium iron phosphate positive electrode material, characterized in that: The following steps are involved: S1, mixing a lithium source, an iron source, a phosphorus source and a carbon source to obtain a mixture, S2, adding graphene dispersion to the mixture, high-speed sand milling, and drying to obtain an intermediate powder, wherein the particle size distribution of the intermediate powder is 60 to 100 nm; S3, pressing the intermediate powder into a shape, and subjecting the mixture to microwave heat treatment to obtain a lithium iron phosphate positive electrode material; The particle size of the lithium iron phosphate positive electrode material is D10 ≥ 0.42 μm, D90 ≤ 4 μm, and the tap density is ≥ 0.9 g / cm 3 , compacted density ≥2.5g / cm 3 .

2. The preparation method according to claim 1, characterized in that The rotation speed of the high-speed sand mill is 1200-2000 r / min, and the time is 0.5-1.5 h.

3. The preparation method according to claim 1, characterized in that: The temperature of the microwave heat treatment is 600-700° C., and the time is 30-60 minutes.

4. The preparation method according to claim 1, characterized in that: The mixing in step S1 is ball milling mixing, the rotation speed of the ball mill is 150-250 r / min, and the time is 2.0-8.0 h.

5. The preparation method according to claim 1, characterized in that: In the mixture of step S1, the mass fraction of the carbon source is 5-15%.

6. The preparation method according to claim 1, characterized in that: In the step S2, the amount of graphene dispersion added is based on the mass fraction of graphene in the generated lithium iron phosphate positive electrode material being 1 to 10%.

7. The preparation method according to claim 1, characterized in that: The concentration of the graphene dispersion is 50-250 mg / L.

8. The preparation method according to claim 1, characterized in that: The amounts of the lithium source, iron source and phosphorus source are measured according to the atomic ratio of generating LiFePO4.

9. A lithium iron phosphate positive electrode material, characterized in that: The method is prepared according to any one of claims 1 to 8.

10. A lithium ion battery, characterized in that: The invention comprises the lithium iron phosphate positive electrode material as claimed in claim 9, or comprises the lithium iron phosphate positive electrode material prepared by the preparation method as claimed in any one of claims 1 to 8.

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