A Lithium Iron Phosphate Cathode Material, Preparation Method and Lithium Ion Battery

Abstract

The present invention discloses a lithium iron phosphate cathode material, a preparation method thereof and a lithium ion battery. The lithium iron phosphate cathode material is a carbon-coated lithium iron phosphate LiFePO4 / C cathode material with a nano-sheet structure; the lithium iron phosphate in the carbon-coated lithium iron phosphate cathode material has a hollow sphere structure; the preparation method includes the following steps: preparation of nano-lithium phosphate and preparation of carbon-coated hollow lithium iron phosphate material. The present invention prepares a carbon-coated hollow lithium iron phosphate material with a nano-sheet structure by depositing ferrous ions on the surface of lithium phosphate particles and carrying out an ion migration reaction under high temperature and high pressure; the hollow structure can not only accommodate more electrolyte to increase the electrochemical life of the material, but also improve the tap density of the material; in addition, the homogeneous carbon coating layer formed by the self-polymerization of organic monomers directly coating the lithium iron phosphate particles has good electronic conductivity and ionic conductivity.

Description

Technical Field

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

[0002] Due to its advantages such as high working voltage (3.4V), high safety, and low cost, lithium iron phosphate materials are widely used in the new energy market. However, the inherently low electronic conductivity (10 -19 S / cm) and ionic conductivity (1.8x10 -14 cm 2 / S) of lithium iron phosphate materials lead to increased polarization, increased impedance, and capacity attenuation during use, manifested as insufficient low-temperature and rate performance, and a significant drop in cycle life.

[0003] To address the above deficiencies and further expand the application field of lithium iron phosphate cathode materials, researchers mainly start from aspects such as bulk doping, surface coating, and morphology and size control of the materials, continuously improving the kinetic and electrochemical properties of the materials, aiming to improve the low-temperature, rate, and cycle life of the battery. Currently, the mainstream process routes include the liquid-phase ferric nitrate process of DFN, the sodium-based iron phosphate process of HNYN, the ammonium-based iron phosphate process of HBWR, the ferrous oxalate process of JXSH, the iron oxide red process of CQTR, and the hydrothermal process of DYWX, etc. However, the above process routes are all limited by the current price pressure of lithium sources in the market, and using secondary lithium sources will seriously affect the performance and consistency of the products.

[0004] Lithium iron phosphate with an olivine crystal structure has good characteristics such as high safety, long life, and low cost due to its stable P-O bonds, wide raw material sources, and low price. However, the current process routes in the market all use iron phosphate as the matrix to prepare lithium iron phosphate products, and there are certain limitations in technical advantages and lithium source selection. Therefore, developing a lithium phosphate process route can not only improve the performance of the material but also relieve the current high cost pressure of lithium sources such as lithium carbonate and lithium hydroxide.

[0005] The invention patent CN111137869A discloses a preparation method of lithium iron phosphate using crude lithium phosphate as a raw material. A pure lithium solution is obtained through stirring and washing, acid dissolution, and impurity removal, and then a lithium source, an iron source, a phosphorus source, and a carbon source are added, mixed, and sintered to obtain a finished product of lithium iron phosphate. This preparation method effectively reduces the cost of the lithium source, but the reaction process is complicated and requires strict requirements for production equipment, which is not conducive to large-scale production. The invention patent CN111732089A uses a lithium-rich waste liquid as the lithium source, supplements the corresponding iron source and carbon source according to the metering ratio, etc., and synthesizes a lithium iron phosphate material. Although this scheme does not require purification and impurity removal and is a one-time sintering and pulverization, the prepared finished product has poor consistency and the residual impurities are not conducive to the safety of the battery. Sun et al. used lithium acetate, iron nitrate, and ammonium dihydrogen phosphate as raw materials, prepared a gel-like precursor through a solvothermal method, and obtained lithium iron phosphate microspheres composed of nanoplates or three-dimensional porous nanoparticles after high-temperature sintering of the precursor material. The conductive performance of the material was improved by coating the surface of the particles with polypyrrole to form a conductive network layer. Although this study prepared high-performance lithium iron phosphate materials, the selection of the lithium source was limited and the internal solid structure extended the migration path of lithium ions. Summary of the Invention

[0006] Aiming at the problems existing in the prior art, the present invention provides a lithium iron phosphate cathode material, a preparation method, and a lithium ion battery, and a lithium iron phosphate cathode material with a hollow structure and carbon-coated nanosheets is prepared by a liquid phase method.

[0007] In order to achieve the above object, the present invention adopts the following technical solutions.

[0008] A lithium iron phosphate cathode material, the lithium iron phosphate cathode material is a carbon-coated lithium iron phosphate LiFePO4 / C cathode material with a nanosheet structure; in the carbon-coated lithium iron phosphate cathode material, the lithium iron phosphate has a hollow sphere structure.

[0009] For the above-mentioned nano lithium iron phosphate cathode material, as a preferred embodiment, the carbon-coated lithium iron phosphate is coated on the surface of the lithium iron phosphate by self-polymerization of an organic monomer.

[0010] For the above-mentioned nano lithium iron phosphate cathode material, as a preferred embodiment, the thickness of the nanosheet is 20-50 nm (for example, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm).

[0011] For the above-mentioned nano lithium iron phosphate cathode material, as a preferred embodiment, the diameter of the hollow sphere structure is 0.5-2 μm (for example, 0.75 μm, 1 μm, 1.5 μm, 1.75 μm).

[0012] For the lithium iron phosphate cathode material according to the present invention, on the one hand, the nanosheets can effectively increase the contact area with the electrolyte, shorten the migration paths of electrons and lithium ions, and improve the migration efficiency of lithium ions; on the other hand, the special hollow spherical structure can accommodate more electrolyte to increase the electrochemical life of the material and improve the tap density of the material; finally, the organic carbon source coated by monomer self-polymerization has good electronic conductivity and ionic conductivity, effectively avoiding side reactions caused by direct contact between the active material and the electrolyte, thereby improving the electrochemical performance and safety performance of the battery.

[0013] The present invention also provides a preparation method of the above lithium iron phosphate cathode material, which is prepared by a liquid phase method, and the preparation method includes the following steps:

[0014] (1) Preparation of nano lithium phosphate by liquid phase method

[0015] Dissolve the lithium source, phosphorus source A and surfactant in water, stir to dissolve it, and adjust the pH value of the solution system to obtain a reaction system marked as system A;

[0016] (2) Preparation of carbon-coated hollow lithium iron phosphate material

[0017] S1: Dissolve the iron source and phosphorus source B in water and stir well at room temperature, adjust the pH value of the system, and name the obtained solution as solution B;

[0018] S2: Add solution B to system A, add the organic monomer, and carry out a constant temperature reaction to obtain a solid-phase precursor;

[0019] S3: Sinter the solid-phase precursor to obtain a carbon-coated hollow lithium iron phosphate material (LiFePO4 / C material).

[0020] In the above preparation method, as a preferred embodiment, in step (1), the stoichiometric ratio of the lithium source to the phosphorus source is 1-3.2:1 (for example, 1:1, 1.5:1, 2:1, 2.5:1, 3:1), and the addition amount of the surfactant is 0.1-5% of the mass of the lithium source (for example, 0.5%, 1%, 2%, 3%, 4%, 4.5%).

[0021] In the above preparation method, as a preferred embodiment, in step (1), the lithium source includes any one or more of lithium oxide, lithium hydroxide, lithium carbonate, lithium acetate, lithium oxalate and lithium chloride; the phosphorus source A includes any one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate and lithium dihydrogen phosphate; preferably, the surfactant includes any one or more of PVP, SDS, CTAB, PEG.

[0022] In the above preparation method, as a preferred embodiment, in step (1), stirring is carried out at a temperature of 50 to 100 °C (for example, 60 °C, 70 °C, 80 °C, 90 °C) to dissolve the lithium source, phosphorus source A and surfactant in water. Preferably, the concentration of the surfactant is 0.2 to 1.2 mol / L (for example, 0.3 mol / L, 0.5 mol / L, 0.8 mol / L, 1.0 mol / L).

[0023] In the above preparation method, as a preferred embodiment, in step (1), ammonia water is used to adjust the pH value of the solution system to 9 to 11 (for example, 9.2, 9.5, 10, 10.5, 10.8) to form lithium phosphate precipitation, and the obtained reaction system is marked as system A.

[0024] In the above preparation method, as a preferred embodiment, in step S1 of step (2), the molar ratio of the iron source to the phosphorus source B is 0.95 to 1.10:1 (for example, 0.98:1, 1:1, 1.02:1, 1.05:1, 1.08:1).

[0025] In the above preparation method, as a preferred embodiment, in step S1 of step (2), the iron source includes any one or more of ferrous sulfate, ferrous chloride, ferrous oxalate, ferrous oxide, etc.; the phosphorus source B includes any one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, lithium dihydrogen phosphate, etc.

[0026] In the above preparation method, as a preferred embodiment, in step S1 of step (2), phosphoric acid is used to adjust the pH value of the system to 1 to 3 (for example, 1.5, 2, 2.5), and the mass concentration of the phosphoric acid is 83 - 98% (for example, 85%, 90%, 95%), preferably 85%.

[0027] In the present invention, phosphoric acid is used to adjust the pH value of the system to 1 to 3. On the one hand, it ensures the subsequent precipitation of lithium phosphate, and on the other hand, it supplements the phosphorus source to ensure the full crystallization of the lithium source.

[0028] In the above preparation method, as a preferred embodiment, in step S2 of step (2), solution B is added to system A, an organic monomer accounting for 5 to 20% (for example, 7%, 10%, 12%, 15%, 17%, 19%) of the mass of the iron source is added, the pH of the system is adjusted, and then the reaction is carried out at a constant temperature of 120 to 200 °C. After cooling to room temperature, filtration, washing, and drying are carried out to obtain a solid-phase precursor (light gray).

[0029] In the above preparation method, as a preferred embodiment, in step S2 of step (2), the organic monomer includes at least one of aniline, pyrrole, acrylic acid, acrylonitrile, acrylamide, dopamine, etc.

[0030] In the above preparation method, as a preferred embodiment, in step S2 of step (2), the molar ratio of lithium element of the lithium source in system A to iron element of the iron source in solution B is 1:1.

[0031] In the above preparation method, as a preferred embodiment, in step S2 of step (2), after adjusting the pH of the system to 5-7 (for example, 5.5, 6, 6.5), the reaction is carried out at a constant temperature of 120-200 °C (for example, 130 °C, 150 °C, 180 °C, 190 °C) for 3-10 h (for example, 4 h, 5 h, 6 h, 7 h, 8 h, 9 h), so that ferrous ions adhere to the surface of lithium phosphate, and an ion migration reaction occurs under high temperature and high pressure conditions to form a lithium iron phosphate cathode material with a hollow structure. Preferably, the pressure of the reaction system is 0.2-0.8 MPa (for example, 0.3 MPa, 0.4 MPa, 0.5 MPa, 0.6 MPa, 0.7 MPa). Preferably, the constant temperature reaction is carried out in a high temperature and high pressure reaction kettle.

[0032] In the present invention, in step S2 of step (2), the organic monomer is dissolved in the solution to ensure that the iron source or iron phosphate is fully in a homogeneous solution, which is beneficial to the uniform coating of the organic matter and restricts the continuous growth of particles during subsequent high temperature sintering.

[0033] The high temperature and high pressure conditions are mainly used for the crystallization of lithium iron phosphate; in addition, high temperature and high pressure can also promote the self-polymerization reaction of the organic monomer, improve the reaction efficiency and shorten the reaction cycle. Too low a temperature will cause incomplete reaction and poor coating effect; too high a temperature will cause too fast a reaction rate and low polymerization analysis amount, which is not conducive to the uniformity of the coating layer. Adjusting the system pH mainly affects the structural stability and charge property of the organic matter, ensures that the solubility of the system and the organic matter is similar, and improves the coating effect.

[0034] In the above preparation method, as a preferred embodiment, in step S3 of step (2), the solid-phase precursor is placed under an inert protective atmosphere, heated to a constant temperature of 500-800 °C, cooled to room temperature and then pulverized to obtain a carbon-coated hollow lithium iron phosphate material (LiFePO4 / C material, black powder).

[0035] In the above preparation method, as a preferred embodiment, in step S3 of step (2), the inert protective atmosphere includes nitrogen, argon or helium, etc.

[0036] In the above preparation method, as a preferred embodiment, in step S3 of step (2), it is heated at a heating rate of 5 to 20 °C / min (for example, 8 °C / min, 10 °C / min, 15 °C / min, 18 °C / min) to 500 to 800 °C (for example, 550 °C, 600 °C, 700 °C, 750 °C) and kept at a constant temperature for 3 to 12 h (for example, 4 h, 5 h, 6 h, 8 h, 10 h).

[0037] In the present invention, in step S3 of step (2), the polymer formed by the self-polymerization of the organic monomer is pyrolyzed at a high temperature to form a homogeneous carbon coating layer on the surface of lithium iron phosphate, which is beneficial to the conductivity of the obtained material. In addition, too high a temperature will cause the particles to continue to grow and is not conducive to the rate performance of the obtained material.

[0038] The design concept of the present invention is as follows:

[0039] Firstly, the nano-lithium phosphate particles prepared by the liquid phase method have regular morphology and uniform size, and effectively expand the types of lithium sources; secondly, by depositing ferrous ions on the surface of lithium phosphate particles, a carbon-coated hollow lithium iron phosphate material with a nano-sheet structure is prepared through an ion migration reaction under high temperature and high pressure conditions, which is beneficial to the service life of the material and can also improve the overall tap density of the material; finally, an organic monomer is introduced to be uniformly coated on the surface of the lithium iron phosphate material, greatly improving the electrochemical performance and safety performance of the material.

[0040] According to the reaction mechanism:

[0041] Li3PO4 + Fe3(PO4)2 → 3LiFePO4,

[0042] Taking ferrous phosphate as the substrate, lithium ions diffuse from the inside to the outside into ferrous phosphate to form a hollow lithium iron phosphate material.

[0043] On the other hand, the present invention also provides a lithium ion battery, including a positive electrode plate, a negative electrode plate and a separator, wherein the positive electrode plate includes a lithium iron phosphate positive electrode material or a lithium iron phosphate positive electrode material prepared according to the preparation method of the above lithium iron phosphate positive electrode material.

[0044] In the present invention, without conflict, the above technical features can be freely combined to form a new technical solution.

[0045] Compared with the prior art, the beneficial technical effects of the present invention are as follows:

[0046] 1. In the present invention, a surfactant is introduced into the liquid phase to effectively adjust the morphology and size of the lithium iron phosphate material, so that it forms sheet-like single crystal particles with a thickness of 20 to 50 nm. The nano-sheet material can effectively increase the contact area with the electrolyte, shorten the electron and lithium ion migration paths, and improve the migration efficiency of lithium ions;

[0047] 2. In the present invention, the lithium phosphate precipitate is precipitated by regulating the liquid-phase pH. The ferrous ions adhere to the surface of the lithium phosphate, and an ion migration reaction is carried out under high temperature and high pressure conditions to form a lithium iron phosphate cathode material with a hollow structure. The whole presents as a regular sphere with a diameter of about 0.5 - 2 μm. The hollow structure can not only accommodate more electrolytes to increase the electrochemical life of the material, but also improve the tap density of the material.

[0048] 3. The self-polymer initiated by the organic monomer in the present invention has higher uniformity compared with the direct coating of the saccharide carbon source. The homogeneous carbon coating layer after pyrolysis has good electronic conductivity and ionic conductivity, effectively avoiding side reactions caused by the direct contact between the active material and the electrolyte, thereby improving the electrochemical performance and safety performance of the battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0049] Figure 1 It is a SEM image of the finished LiFePO4 / C composite material of Example 1 of the present invention.

[0050] Figure 2 is Figure 1 an enlarged view of.

[0051] Figure 3 It is a transmission electron microscope (TEM) image of the finished LiFePO4 / C composite material of Example 1 of the present invention.

[0052] Figure 4 It is a schematic diagram of the expected structure of the finished nano-hollow spherical LiFePO4 / C composite material of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0053] The present invention will be described in detail below in conjunction with the specification drawings and the embodiments of the present invention. Each example is provided by way of explanation of the present invention rather than limitation of the present invention. In fact, those skilled in the art will appreciate that modifications and variations can be made in the present invention without departing from the scope or spirit of the present invention. For example, features shown or described as part of one embodiment can be used in another embodiment to yield yet another embodiment. Accordingly, it is intended that the present invention cover such modifications and variations that fall within the scope of the appended claims and their equivalents.

[0054] In the present invention, the related technical solutions not specifically described can adopt the conventional technologies in the art.

[0055] Examples 1 - 3

[0056] A preparation method of a nano lithium iron phosphate cathode material, comprising the following steps:

[0057] (1) Weigh the lithium source and phosphorus source A according to a molar ratio of 1 - 3.2:1, and add them to an aqueous surfactant solution with a concentration of 0.2 - 1.2 mol / L. The surfactant accounts for 0.1 - 5% of the mass of the lithium source. Stir and dissolve them fully at 50 - 100 °C to prepare. Use ammonia water to adjust the pH value of the system to 9 - 11 to form a white precipitate. This reaction system is defined as system A;

[0058] (2) Weigh the iron source and phosphorus source B according to a molar ratio of 0.95 - 1.10:1, dissolve them in deionized water, and stir fully at room temperature. Use 85% phosphoric acid to adjust the pH value of the system to 1 - 3 to obtain a solution named solution B;

[0059] (3) Slowly add solution B to solution A of system A. Weigh an organic monomer accounting for 5 - 20% of the mass of the iron source, adjust the pH of the system to 5 - 7, and then transfer it to a high-temperature and high-pressure reactor. Keep it at a constant temperature of 120 - 200 °C and a pressure of 0.2 - 0.8 MPa for 3 - 10 h. After cooling to room temperature, filter, wash, and dry to obtain a light gray solid precursor;

[0060] (4) Place the dried solid precursor under an inert protective atmosphere, heat it at a heating rate of 5 - 20 °C / min to 500 - 800 °C, keep it at a constant temperature for 3 - 12 h, and cool it to room temperature and then crush it to obtain a black powder, which is the target product LiFePO4 / C material.

[0061] Specifically, Table 1 shows the raw materials in Examples 1 - 3, and Table 2 shows the reaction conditions in Examples 1 - 3.

[0062] Table 1 Raw materials in Examples 1 - 3

[0063]

[0064]

[0065] Table 2 Reaction conditions in Examples 1 - 3

[0066]

[0067] Comparative Example 1

[0068] This comparative example provides a preparation method of a lithium iron phosphate cathode material, which is different from Example 1 in that the reaction conditions in step (3) are different. Specifically as follows:

[0069] Slowly add solution B to solution A. Weigh 21.9 g of the organic monomer pyrrole, adjust the pH of the system to 6.0, and then transfer it to a high-temperature and high-pressure reactor. Keep it at a constant temperature of 220 °C for 8 h. After cooling to room temperature, filter, wash, and dry to obtain a light gray solid precursor.

[0070] Comparative Example 2

[0071] This comparative example provides a method for preparing a lithium iron phosphate cathode material, which is different from Example 1 in that the reaction conditions in step (2) are different. Specifically as follows:

[0072] Weigh the iron source and phosphorus source B according to a molar ratio of 1.12:1, dissolve them in deionized water, stir well at room temperature, and adjust the pH value of the system to 1-3 with 85% phosphoric acid. The resulting solution is named solution B.

[0073] Comparative Example 3

[0074] This comparative example provides a method for preparing a lithium iron phosphate cathode material, which is different from Example 1 in that the reaction conditions in step (3) are different. Specifically as follows:

[0075] Slowly add solution B to solution A, weigh 4% of the organic monomer pyrrole of the iron source (i.e., 5.836 g), adjust the pH of the system to 6.0, then transfer it to a high-temperature and high-pressure reaction kettle and keep it at a constant temperature of 150 °C for 8 h. After cooling to room temperature, filter, wash, and dry to obtain a light gray solid-phase precursor.

[0076] Comparative Example 4

[0077] This comparative example provides a method for preparing a lithium iron phosphate cathode material, which is different from Example 2 in that no surfactant is added in step (1), and other conditions are the same as those in Example 2.

[0078] Microstructure characterization of the material

[0079] The surface microstructure of the lithium iron phosphate cathode material prepared in Example 1 was characterized by scanning electron microscopy (SEM). The hollow structure of the lithium iron phosphate cathode material prepared in Example 1 was characterized by transmission electron microscopy (TEM).

[0080] Figures 1-3 The SEM and TEM images of the target product LiFePO4 / C composite material of Example 1 of the present invention are respectively shown. According to Figure 3 The TEM shows the hollow structure of the LiFePO4 / C composite material, and the structural schematic diagram of the nano-hollow spherical LiFePO4 / C composite material is drawn, as Figure 4 shown.

[0081] It can be seen from Figures 1-2 that the surface of the LiFePO4 / C composite material prepared in Example 1 has nano-sheet single crystal particles with a thickness of 20-50 nm. It can be seen from Figure 3 that the LiFePO4 / C composite material prepared in Example 1 conforms to the design result, and the composite material as a whole shows a nano-hollow structure, as Figure 4As shown, the diameter of the sphere is about 0.5 - 1.0 μm, and the LiFePO4 / C cathode material has a flaky structure, with the thickness of a single crystal sheet being about 20 - 30 nm.

[0082] Battery Assembly and Performance Testing

[0083] Taking LiFePO4 / C as the active material, it is mixed with polyvinylidene fluoride (PVDF) and SuperP carbon black in a mass ratio of 93.5:4.2:2.3, and ball-milled for 60 min with NMP as the solvent; then the slurry is evenly coated on a metal aluminum foil and vacuum-dried at 80 °C for 2 h, and finally punched into a circular pole piece with a diameter of 14 mm as the working electrode; in a purified glove box filled with Ar (O2 content less than 0.1 ppm, H2O content less than 0.1 ppm), using a lithium metal sheet as the counter electrode, a Celgard 2400 porous polypropylene membrane (PP) as the separator, and the electrolyte is 1M L -1 solution of lithium hexafluorophosphate (LiPF6), and the solvent is a mixed solution of ethylene carbonate (EC): diethyl carbonate (DMC) = 1:1 by volume ratio, and a CR2032 type button battery is prepared according to a certain assembly process.

[0084] Let the assembled CR2032 type button battery stand for 3 h to fully infiltrate the electrolyte and the electrode material. At room temperature (25 °C ± 1), a constant current charge-discharge experiment of the battery is carried out in the voltage range of 2.0 - 3.8V for Li / Li + The performance results are shown in Table 3.

[0085] Table 3 Electrochemical Performance of Batteries Assembled with Cathode Materials Prepared in Examples 1 - 3 and Comparative Examples 1 - 4

[0086]

[0087]

[0088] As can be seen from Table 3, the stoichiometric ratio mainly affects the rate performance of the material, the hydrothermal temperature mainly affects the crystallization nucleation of the material and the polymerization effect of the organic monomer, the monomer molecular weight mainly affects the conductivity of the material and the capacity per gram, and the surfactant mainly affects the microstructure of the material.

[0089] It should be understood that the use of these examples is only for illustrating the present invention and is not intended to limit the protection scope of the present invention. In addition, it should also be understood that after reading the technical content of the present invention, those skilled in the art can make various changes, modifications, and / or variations to the present invention, and all these equivalent forms also fall within the protection scope defined by the appended claims of this application.

Claims

1. A lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate cathode material is a carbon-coated lithium iron phosphate LiFePO4 / C cathode material. The carbon-coated lithium iron phosphate LiFePO4 / C cathode material has a nano-sheet structure, and the thickness of the nano-sheets is 20-50 nm. In the carbon-coated lithium iron phosphate LiFePO4 / C cathode material, lithium iron phosphate has a hollow sphere structure. The carbon-coated lithium iron phosphate LiFePO4 / C cathode material is prepared by a liquid-phase method. The preparation method of the carbon-coated lithium iron phosphate LiFePO4 / C cathode material includes the following steps: (1) Preparation of nano-lithium phosphate by liquid-phase method Dissolve a lithium source, a phosphorus source A, and CTAB in water, stir to dissolve it, and adjust the pH value of the solution system to obtain a reaction system labeled as system A, where the addition amount of CTAB is 0.1-5% of the mass of the lithium source; (2) Preparation of carbon-coated lithium iron phosphate LiFePO4 / C cathode material S1: Dissolve an iron source and a phosphorus source B in water and stir well at room temperature, adjust the pH value of the system, and name the resulting solution solution B; S2: Add solution B to system A, add an organic monomer, and carry out a constant-temperature reaction to obtain a solid-phase precursor; S3: Sinter the solid-phase precursor to obtain a carbon-coated lithium iron phosphate LiFePO4 / C cathode material.

2. The lithium iron phosphate cathode material according to claim 1, wherein In step S3 of step (2), the polymer formed by the self-polymerization of the organic monomer is pyrolyzed at a sintering temperature of 500-800 °C to generate a homogeneous carbon coating layer on the surface of lithium iron phosphate; The diameter of the spheres of the hollow sphere structure is 0.5-2 μm.

3. A method for preparing the lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, The carbon-coated lithium iron phosphate LiFePO4 / C cathode material is prepared by a liquid-phase method. The preparation method of the carbon-coated lithium iron phosphate LiFePO4 / C cathode material includes the following steps: (1) Preparation of nano-lithium phosphate by liquid-phase method Dissolve a lithium source, a phosphorus source A, and CTAB in water, stir to dissolve it, and adjust the pH value of the solution system to obtain a reaction system labeled as system A, where the addition amount of CTAB is 0.1-5% of the mass of the lithium source; (2) Preparation of carbon-coated lithium iron phosphate LiFePO4 / C cathode material S1: Dissolve an iron source and a phosphorus source B in water and stir well at room temperature, adjust the pH value of the system, and name the resulting solution solution B; S2: Add solution B to system A, add an organic monomer, and carry out a constant-temperature reaction to obtain a solid-phase precursor; S3: Sinter the solid-phase precursor to obtain a carbon-coated lithium iron phosphate LiFePO4 / C cathode material.

4. The preparation method of the lithium iron phosphate cathode material according to claim 3, wherein in step S2 of step (2), add solution B to system A, add an organic monomer accounting for 5-20% of the mass of the iron source, adjust the pH of the system, carry out a constant-temperature reaction at 120-200 °C, filter, wash, and dry after cooling to room temperature to obtain a solid-phase precursor.

5. The preparation method of the lithium iron phosphate cathode material according to claim 3, wherein In step S3 of step (2), the solid-phase precursor is placed under an inert protective atmosphere and heated to 500-800 °C for isothermal sintering. After cooling to room temperature, it is pulverized to obtain the carbon-coated lithium iron phosphate LiFePO4 / C cathode material.

6. The method for preparing a lithium iron phosphate cathode material according to claim 5, wherein in step S3 of step (2), the inert protective atmosphere includes nitrogen, argon, and / or helium.

7. The method for preparing a lithium iron phosphate cathode material according to claim 5, wherein in step S3 of step (2), the heating to 500-800 °C for isothermal sintering includes heating to 500-800 °C at a heating rate of 5-20 °C / min and holding for 3-12 h.

8. The method for preparing a lithium iron phosphate cathode material according to any one of claims 3 to 5, wherein in step (1), the lithium source includes any one or more of lithium oxide, lithium hydroxide, lithium carbonate, lithium acetate, lithium oxalate, and lithium chloride.

9. The method for preparing a lithium iron phosphate cathode material according to any one of claims 3 to 5, wherein in step (1), the phosphorus source A includes any one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, and lithium dihydrogen phosphate.

10. The method for preparing a lithium iron phosphate cathode material according to any one of claims 3 to 5, wherein in step S1 of step (2), the iron source includes any one or more of ferrous sulfate, ferrous chloride, ferrous oxalate, and ferrous oxide.

11. The method for preparing a lithium iron phosphate cathode material according to any one of claims 3 to 5, wherein in step S1 of step (2), the phosphorus source B includes any one or more of phosphoric acid, monoammonium phosphate, diammonium phosphate, and lithium dihydrogen phosphate.

12. The method for preparing a lithium iron phosphate cathode material according to any one of claims 3 to 5, wherein in step S2 of step (2), the organic monomer includes at least one of aniline, pyrrole, acrylic acid, acrylonitrile, acrylamide, and dopamine.

13. The method for preparing a lithium iron phosphate cathode material according to any one of claims 3 to 5, wherein in step (1), the stoichiometric ratio of the lithium source to the phosphorus source is 1-3.2:

1.

14. The method for preparing a lithium iron phosphate cathode material according to any one of claims 3 to 5, wherein in step S1 of step (2), the molar ratio of the iron source to the phosphorus source B is 0.95-1.10:

1.

15. The method for preparing a lithium iron phosphate cathode material according to any one of claims 3 to 5, wherein in step S2 of step (2), the molar ratio of the lithium element of the lithium source in system A to the iron element of the iron source in solution B is 1:

1.

16. The method for preparing a lithium iron phosphate cathode material according to any one of claims 3 to 5, wherein in step (1), the concentration of CTAB is 0.2-1.2 mol / L.

17. The preparation method of the lithium iron phosphate cathode material according to any one of claims 3 to 5, characterized in that in the step (1), stirring is carried out at a temperature of 50 to 100 °C, and the pH value of the solution system is adjusted to 9 to 11 by using ammonia water.

18. The preparation method of the lithium iron phosphate cathode material according to any one of claims 3 to 5, characterized in that in the step S1 of the step (2), the pH value of the system is adjusted to 1 to 3 by using phosphoric acid, and the mass concentration of the phosphoric acid is 83-98%.

19. The preparation method of the lithium iron phosphate cathode material according to any one of claims 3 to 5, characterized in that in the step S2 of the step (2), the constant temperature reaction is carried out in a high-temperature and high-pressure reaction kettle; the temperature of the reaction system is 120 to 200 °C, and the pressure of the reaction system is 0.2 to 0.8 MPa.

20. A lithium-ion battery, characterized in that, The lithium ion battery includes a positive electrode plate, a negative electrode plate and a separator, and the positive electrode plate includes the lithium iron phosphate cathode material according to claim 1 or 2 or the lithium iron phosphate cathode material prepared by the preparation method of the lithium iron phosphate cathode material according to any one of claims 3-19.

Images