Preparation method of high-capacity lithium iron manganese phosphate cathode material

Abstract

The invention discloses a preparation method of a high-capacity lithium iron manganese phosphate cathode material. In the method of the invention, by doping nickel and modifying polyethylene glycol, the intrinsic conductivity of the material is improved. At the same time, the lattice distortion caused by doping inhibits the growth of grains, improves the grading of particles, shortens the diffusion path of lithium ions. Meanwhile, nickel in nickel and fluorine modified polyethylene glycol can form nickel oxide on the surface of carbon sources such as carbon black or carbon fiber, which promotes the electron and ion transport between the cathode material and the electrolyte to a certain extent, improves the energy density and cycle performance of the battery. Fluorine can improve the interfacial composition between the carbon source and the cathode material, promote the transfer and diffusion of lithium ions, and improve the charge / discharge rate and capacity retention of the battery. Therefore, doping nickel and modifying polyethylene glycol in the invention have a synergistic effect. Especially when the molar amount of doped nickel is 0.02, the prepared battery has a high capacity and excellent rate performance.

Description

Technical Field

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

[0002] Lithium iron manganese phosphate is a cathode active material for lithium-ion batteries belonging to phosphate salts, and its theoretical specific capacity can reach 170 mAh·g -1 , and it has advantages such as good cycle performance, excellent safety performance, wide raw material sources, low cost, and environmental friendliness. In addition, compared with lithium iron phosphate, lithium iron manganese phosphate has a higher discharge voltage, and there are certain advantages in specific energy and specific power. Therefore, developing lithium iron manganese phosphate with excellent electrochemical performance as a cathode active material has become one of the research hotspots of lithium-ion batteries.

[0003] The patent application with the application number CN202211040153.X provides a lithium iron manganese phosphate composite material, its preparation method, a cathode material, and a lithium battery, relating to the battery field. The lithium iron manganese phosphate composite material includes lithium iron manganese phosphate particles and a coating layer wrapped on the surface of the lithium iron manganese phosphate particles, and the coating layer contains carbon and lithium iron borate. The coating layer of the lithium iron manganese phosphate composite material does not affect the electrical properties of the lithium iron manganese phosphate particles, and at the same time can reduce the manganese dissolution rate in the lithium iron manganese phosphate particles and reduce the surface impedance of the composite material. Using the lithium iron manganese phosphate composite material in the embodiments of this application as a cathode material and a lithium battery can improve the cycle life of the lithium battery and increase the energy density of the lithium battery.

[0004] The patent application with the application number CN202210996177.6 provides a lithium iron manganese phosphate cathode composite material, including lithium iron manganese phosphate particles, a silver coating layer wrapping the lithium iron manganese phosphate particles, and a carbon coating layer wrapping the silver coating layer; the particle size of the lithium iron manganese phosphate particles is 50 - 500 nm; the thickness of the silver coating layer is 5 - 50 nm, and the thickness of the carbon coating layer is 1 - 20 nm; the molar ratio of manganese to iron in the lithium iron manganese phosphate particles is 1∶(0.5 - 2). The LiMn x Fe y PO4 / Ag / C composite cathode material prepared by this invention adopts a double-coating layer heterogeneous coating structure, which can in-situ coat silver on the surface of the lithium iron manganese phosphate particles to achieve uniform coating of silver on the surface of the lithium iron manganese phosphate, and plays a key protective role in the dissolution of Mn 3+ ions, and can effectively improve the cycle performance of the composite cathode material. However, the cost of using silver is relatively high and it is not very suitable for industrial production.

[0005] Due to the characteristics of the olivine structure of lithium manganese iron phosphate materials, the materials have low electronic and ionic conductivities, showing poor electrical conductivity and poor rate performance, which hinders the application of the materials in actual production and life. During the charge and discharge process of LMFP materials, the dissolution of transition metal manganese inevitably occurs. On the one hand, the dissolution of manganese will affect the structure of the cathode material and the lithium storage capacity of the cathode. On the other hand, the dissolved manganese deposits on the surface of the anode, further damaging the SEI film, causing the SEI film to continuously regenerate and repair, consuming a large amount of active lithium, resulting in capacity loss, and also affecting the diffusion of lithium ions. At the same time, the deposited manganese will catalyze the decomposition of the electrolyte, leading to the consumption of active lithium and the formation of a thicker solid electrolyte interface film. To realize the large-scale production and application of lithium manganese iron phosphate cathode materials in commercialization, comprehensively considering, improving the ionic and electronic conductivities of the materials is the top priority for improving the electrochemical activity of the materials in research.

[0006] A large number of studies have shown that cation doping can effectively improve the intrinsic conductivity of materials. Numerous experimental cases have shown that the doping of nickel ions in phosphates is feasible. However, for the doping amount and doping method of nickel ions in phosphates to achieve the technical effects of high battery capacity and excellent rate performance, further research is needed. In addition, the electrochemical performance and battery life of the electrode materials prepared by the existing experimental schemes need to be further improved. Summary of the Invention

[0007] In view of at least one of the technical problems or defects in the above background technology, the present invention provides a preparation method of a modified high-capacity lithium manganese iron phosphate cathode material. By exploring the effects of different doping amounts of nickel and the synergistic effect of modified polyethylene glycol on the electrical properties of lithium manganese iron phosphate cathode materials, the prepared batteries have high capacity and excellent rate performance.

[0008] The technical solution of the present invention is as follows:

[0009] The present invention provides a preparation method of a high-capacity lithium manganese iron phosphate cathode material, and the general formula of the high-capacity lithium manganese iron phosphate cathode material is LiFe 1-x-y Mn y Ni x PO4@C1@C2, where 0.01 ≤ x ≤ 0.03, 0.61 ≤ x + y ≤ 0.63, the carbon source of C1 is the first organic carbon source, and the carbon source of C2 is the second organic carbon source.

[0010] The preparation method includes the following steps:

[0011] (1) According to the high-capacity lithium manganese iron phosphate cathode material LiFe 1-x-y Mn y Ni xPO4@C1@C2 Add the iron source, manganese source, lithium source, phosphorus source, nickel source and the first organic carbon source into a reaction vessel according to a certain molar stoichiometric coefficient ratio, grind and mix them evenly, add water, and mix evenly again to obtain the lithium iron manganese phosphate precursor slurry;

[0012] The first organic carbon source is nickel and fluorine modified polyethylene glycol;

[0013] (2) After drying the lithium iron manganese phosphate precursor slurry obtained in step (1), sinter it in a protective atmosphere to obtain the once-coated nickel-doped lithium iron manganese phosphate cathode material;

[0014] (3) Mix the once-coated nickel-doped lithium iron manganese phosphate cathode material obtained in step (2) with the second organic carbon source, and sinter it in a protective atmosphere to obtain the high-capacity lithium iron manganese phosphate cathode material.

[0015] The preparation method of the nickel and fluorine modified polyethylene glycol includes the following steps:

[0016] Nickel acrylate, hexafluorobutyl acrylate and polyethylene glycol bis(amine) react by addition in a basic medium to obtain the nickel and fluorine modified polyethylene glycol.

[0017] Preferably, the mass ratio of nickel acrylate, hexafluorobutyl acrylate to polyethylene glycol bis(amine) is 1:20:266.67 - 500.

[0018] The temperature of the reaction is 60 - 70 °C, and the reaction time is 100 - 150 minutes.

[0019] Specifically, the preparation method of the modified polyethylene glycol includes the following steps:

[0020] By weight, mix 0.1 - 0.3 parts of nickel acrylate, 2 - 6 parts of hexafluorobutyl acrylate, 50 - 80 parts of polyethylene glycol bis(amine), 300 - 500 parts of DMF and 1 - 4 parts of sodium tert-butoxide evenly, heat and stir to react, and after the reaction, remove DMF by vacuum distillation to obtain the modified polyethylene glycol.

[0021] Nickel and fluorine modified polyethylene glycol is obtained by the addition reaction of nickel acrylate, hexafluorobutyl acrylate and polyethylene glycol bis(amine).

[0022] The carbon source of nickel and fluorine modified polyethylene glycol can improve the conductivity and structural stability of the lithium iron manganese phosphate cathode material.

[0023] Nickel can form nickel oxide on the surface of carbon sources such as carbon black or carbon fiber, which can promote the electron and ion transport between the cathode material and the electrolyte to a certain extent, improve the energy density and cycle performance of the battery. It helps to improve the performance of lithium iron manganese phosphate batteries, especially in high-power and high-temperature applications.

[0024] Fluorine can improve the structure and chemical properties of the lithium iron manganese phosphate cathode material, thereby enhancing the performance and cycle life of the battery. Fluorine can improve the interfacial composition between the carbon source and the cathode material, promote the transfer and diffusion of lithium ions, and increase the charge / discharge rate and capacity retention of the battery. It has a positive impact on the electrochemical performance and cycle life of the lithium iron manganese phosphate battery.

[0025] The iron source is at least one of iron oxide, ferrous sulfate, ferric sulfate, iron phosphate, and ferrous oxalate;

[0026] The manganese source is at least one of manganese acetate, manganese dioxide, manganese oxalate, and manganese carbonate;

[0027] The lithium source is at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate;

[0028] The phosphorus source is at least one of iron phosphate, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate;

[0029] The nickel source is at least one of nickelous oxide, nickel sesquioxide, nickel tetroxide, and nickel oxalate.

[0030] Specifically, the first organic carbon source accounts for 5.4% of the total mass of the iron source, manganese source, lithium source, phosphorus source, nickel source, and the reaction raw materials of the first organic carbon source, and the second organic carbon source accounts for 3.6% of the total mass of the nickel-doped lithium iron manganese phosphate cathode material with carbon coating and the reaction raw materials of the second organic carbon source.

[0031] The second organic carbon source is polyvinyl alcohol.

[0032] The sintering temperature in step (2) is 600 - 800 °C, and the sintering time is 6 - 10 h;

[0033] The sintering temperature in step (3) is 300 - 800 °C, and the sintering time is 3 - 5 h.

[0034] The present invention also provides a high-capacity lithium iron manganese phosphate cathode material prepared by the described preparation method.

[0035] The present invention also provides the application of the high-capacity lithium iron manganese phosphate cathode material in the preparation of lithium-ion batteries.

[0036] The beneficial effects of the present invention:

[0037] In the method of the present invention, by doping nickel and modifying polyethylene glycol, not only the intrinsic conductivity of the material is improved, but also the lattice distortion caused by doping inhibits the growth of grains, improves the grading of particles, shortens the diffusion path of lithium ions. At the same time, nickel in nickel- and fluorine-modified polyethylene glycol can form nickel oxide on the surface of carbon sources such as carbon black or carbon fiber, promoting the electron and ion transport between the cathode material and the electrolyte to a certain extent, improving the energy density and cycle performance of the battery. Fluorine can improve the interfacial composition between the carbon source and the cathode material, promote the transfer and diffusion of lithium ions, and improve the charge / discharge rate and capacity retention of the battery. Therefore, doping nickel and modifying polyethylene glycol in the present invention have a synergistic effect. Especially when the molar amount of doped nickel is 0.02, the prepared battery has a high capacity and excellent rate performance. Description of the Drawings

[0038] Figure 1 The first charge-discharge curve of the battery assembled with the lithium iron manganese phosphate cathode material prepared in Example 5 at 1C. Detailed Description of the Invention

[0039] Example 1

[0040] The general formula of the high-capacity lithium iron manganese phosphate cathode material is LiFe 1-x-y Mn y Ni x PO4@C1@C2, where x = 0.01, y = 0.6, the carbon source for C1 is glucose, and the carbon source for C2 is polyvinyl alcohol.

[0041] (1) Mixing and grinding: Mix FePO4, MnC2O4, Li2CO3, NH4H2PO4 and NiO in a molar ratio of 0.39∶0.6∶0.5∶1∶0.01 to obtain a mixed powder. Using glucose as the primary carbon source, the addition amount is 5.4% of the total mass of the above raw materials. Add the mixed powder to deionized water and mix and grind it by ball milling or other methods for 5 h to obtain a lithium iron manganese phosphate precursor slurry.

[0042] (2) Drying: Dry and granulate the lithium iron manganese phosphate precursor slurry obtained in step (1) by spray drying (inlet air temperature 280 °C, outlet air temperature 145 °C) to obtain a precursor powder.

[0043] (3) Sintering: Sinter the precursor powder obtained in step (2) in a protective atmosphere (700 °C, 8 h) to obtain a sintered material; naturally cool it to room temperature to obtain a carbon-coated nickel-doped lithium iron manganese phosphate material.

[0044] (4) Secondary Coating: The carbon-coated lithium nickel-doped manganese iron phosphate material described in step (3) and polyvinyl alcohol (PVA) (3.6% of the total mass of the raw materials (carbon-coated lithium nickel-doped manganese iron phosphate material and PVA)) as the secondary carbon source are added to deionized water. After ultrasonic oscillation for 1 h, suction filtration is carried out, and drying is performed in a vacuum drying oven at 120 °C for 4 h. The dried mixture is sintered in a protective atmosphere, and the temperature is raised to 350 °C at a rate of 10 °C min -1 and sintered for 1 h, then the temperature is raised to 740 °C at a rate of 15 °C min -1 and sintered for 4 h, and then cooled to room temperature to obtain the secondary-coated lithium manganese iron phosphate cathode material.

[0045] Example 2

[0046] The general formula of the high-capacity lithium manganese iron phosphate cathode material is LiFe 1-x-y Mn y Ni x PO4@C1@C2, where x = 0.02, y = 0.6, the carbon source for C1 is glucose, and the carbon source for C2 is polyvinyl alcohol.

[0047] (1) Mixing and Grinding: FePO4, MnC2O4, Li2CO3, NH4H2PO4, and NiO are mixed in a molar ratio of 0.38∶0.6∶0.5∶1∶0.02 to obtain a mixed powder. Glucose is used as the primary carbon source, and the addition amount is 5.4% of the total mass of the above raw materials. The mixed powder is added to deionized water and mixed and ground by ball milling and other methods for 5 h to obtain a lithium manganese iron phosphate precursor slurry.

[0048] (2) Drying: The lithium manganese iron phosphate precursor slurry obtained in step (1) is dried and granulated by spray drying (inlet air temperature 280 °C, outlet air temperature 145 °C) to obtain a precursor powder.

[0049] (3) Sintering: The precursor powder obtained in step (2) is sintered in a protective atmosphere (700 °C, 8 h) to obtain a sintered material; it is naturally cooled to room temperature to obtain a carbon-coated lithium nickel-doped manganese iron phosphate material.

[0050] (4) Secondary Coating: The carbon-coated lithium nickel-doped manganese iron phosphate material obtained in step (3) and PVA (3.6% of the total mass of the raw materials (lithium nickel-doped manganese iron phosphate material and PVA)) as the secondary carbon source are added to deionized water. After ultrasonic oscillation for 1 h, suction filtration is carried out, and drying is performed in a vacuum drying oven at 120 °C for 4 h. The dried mixture is sintered in a protective atmosphere, and the temperature is raised to 350 °C at a rate of 10 °C min -1 and sintered for 1 h, then the temperature is raised to 740 °C at a rate of 15 °C min -1The rate is increased to 740 °C and sintered for 4 h, then cooled to room temperature to obtain the secondary-coated lithium iron manganese phosphate cathode material.

[0051] Example 3

[0052] The general formula of the high-capacity lithium iron manganese phosphate cathode material is LiFe 1-x-y Mn y Ni x PO4@C1@C2, where x = 0.03, y = 0.6, the carbon source for C1 is glucose, and the carbon source for C2 is polyvinyl alcohol.

[0053] (1) Mixing and grinding: FePO4, MnC2O4, Li2CO3, NH4H2PO4 and NiO are mixed in a molar ratio of 0.37:0.6:0.5:1:0.03 to obtain a mixed powder. Using glucose as the primary carbon source, the addition amount is 5.4% of the total mass of the raw materials. The mixed powder is added to deionized water and mixed and ground by ball milling or other methods for 4 h to obtain a lithium iron manganese phosphate precursor slurry.

[0054] (2) Drying: The slurry obtained in step (1) is dried and granulated by spray drying (inlet air temperature 280 °C, outlet air temperature 145 °C) to obtain a precursor powder.

[0055] (3) Sintering: The precursor powder described in (2) is sintered in a protective atmosphere (700 °C, 8 h) to obtain a sintered material; it is naturally cooled to room temperature to obtain a carbon-coated lithium iron manganese phosphate material.

[0056] (4) Secondary coating: The carbon-coated lithium iron manganese phosphate material described in (3) and PVA as the secondary carbon source (3.6% of the total mass of the raw materials) are added to deionized water, ultrasonically oscillated for 1 h, then filtered, and dried in a vacuum drying oven at 120 °C for 4 h. The dried mixture is sintered in a protective atmosphere, and the rate is increased to 350 °C at a rate of 10 °C / min and sintered for 1 h, and then the rate is increased to 740 °C at a rate of 15 °C / min and sintered for 4 h, and then cooled to room temperature to obtain the secondary-coated lithium iron manganese phosphate cathode material. -1 The rate is increased to 350 °C and sintered for 1 h, and then the rate is increased to 740 °C at a rate of 15 °C / min -1 The rate is increased to 740 °C and sintered for 4 h, then cooled to room temperature to obtain the secondary-coated lithium iron manganese phosphate cathode material.

[0057] Example 4

[0058] The general formula of the high-capacity lithium iron manganese phosphate cathode material is LiFe 1-x-y Mn y Ni x PO4@C1@C2, where x = 0.02, y = 0.6, the carbon source for C1 is glucose, and the carbon source for C2 is polyvinyl alcohol.

[0059] (1) 0.1 g of nickel acrylate, 2 g of hexafluorobutyl acrylate, 50 g of diaminopolyethylene glycol, 300 g of DMF, and 1 g of sodium tert-butoxide were heated to 60 °C and stirred for reaction for 150 minutes. After the reaction, DMF was removed by distillation under reduced pressure to obtain nickel- and fluorine-modified polyethylene glycol.

[0060] (2) Mixing and grinding: FePO4, MnC2O4, Li2CO3, NH4H2PO4, and NiO were mixed in a molar ratio of 0.38∶0.6∶0.5∶1∶0.02. Nickel- and fluorine-modified polyethylene glycol was used as the primary carbon source, and the addition amount was 5.4% of the total mass of the raw materials. The mixed powder was added to deionized water and mixed and ground by ball milling or other methods for 5 h to obtain a lithium iron manganese phosphate precursor slurry.

[0061] (3) Drying: The slurry obtained in step (2) was dried and granulated by spray drying (inlet air temperature 280 °C, outlet air temperature 145 °C) to obtain a precursor powder.

[0062] (4) Sintering: The precursor powder described in (3) was sintered in a protective atmosphere (700 °C, 8 h) to obtain a sintered material; it was naturally cooled to room temperature to obtain a carbon-coated lithium iron manganese phosphate material.

[0063] (5) Secondary coating: The carbon-coated lithium iron manganese phosphate material described in (4) and PVA as the secondary carbon source (3.6% of the total mass of the raw materials) were added to deionized water, ultrasonically oscillated for 1 h, then filtered by suction, and dried in a vacuum drying oven at 120 °C for 4 h. The dried mixture was sintered in a protective atmosphere, heated to 350 °C at a rate of 10 °C / min and sintered for 1 h, then heated to 740 °C at a rate of 15 °C / min and sintered for 4 h, and then cooled to room temperature to obtain a secondary-coated lithium iron manganese phosphate cathode material. -1 of the rate to 350 °C and sintered for 1 h, and then at a rate of 15 °C / min -1 of the rate to 740 °C and sintered for 4 h, and then cooled to room temperature to obtain a secondary-coated lithium iron manganese phosphate cathode material.

[0064] Example 5

[0065] The general formula of the high-capacity lithium iron manganese phosphate cathode material is LiFe 1-x-y Mn y Ni x PO4@C1@C2, where x = 0.02, y = 0.6, the carbon source for C1 is glucose, and the carbon source for C2 is polyvinyl alcohol.

[0066] (1) 0.3 g of nickel acrylate, 6 g of hexafluorobutyl acrylate, 80 g of diaminopolyethylene glycol, 500 g of DMF, and 4 g of sodium tert-butoxide were heated to 70 °C and stirred for reaction for 100 minutes. After the reaction, DMF was removed by distillation under reduced pressure to obtain nickel- and fluorine-modified polyethylene glycol.

[0067] (2) Hybrid grinding: Mix FePO4, MnC2O4, Li2CO3, NH4H2PO4, and NiO in a molar ratio of 0.38∶0.6∶0.5∶1∶0.02. Use nickel- and fluorine-modified polyethylene glycol as the primary carbon source, with an addition amount of 5.4% of the total mass of the raw materials. Add the mixed powder to deionized water and mix and grind it for 5 h by means of ball milling or the like to obtain a lithium iron manganese phosphate precursor slurry.

[0068] (3) Drying: Dry and granulate the slurry obtained in step (2) by spray drying (inlet air temperature 280 °C, outlet air temperature 145 °C) to obtain a precursor powder.

[0069] (4) Sintering: Sinter the precursor powder described in (3) in a protective atmosphere (700 °C, 8 h) to obtain a sintered material; cool it naturally to room temperature to obtain a carbon-coated lithium iron manganese phosphate material.

[0070] (5) Secondary coating: Add the carbon-coated lithium iron manganese phosphate material described in (4) and PVA as the secondary carbon source (3.6% of the total mass of the raw materials) to deionized water, ultrasonically vibrate for 1 h, then perform suction filtration, and dry it in a vacuum drying oven at 120 °C for 4 h. Sinter the dried mixture in a protective atmosphere, raise the temperature to 350 °C at a rate of 10 °C / min and sinter for 1 h, then raise the temperature to 740 °C at a rate of 15 °C / min -1 and sinter for 4 h, and then cool to room temperature to obtain a secondary-coated lithium iron manganese phosphate cathode material. -1

[0071] Comparative Example 1

[0072] (1) Hybrid grinding: Mix FePO4, MnC2O4, NH4H2PO4, and Li2CO3 in a molar ratio of 0.4∶0.6∶0.5∶1. Use glucose as the primary carbon source, with an addition amount of 5.4% of the total mass of the raw materials. Add the mixed powder to deionized water and mix and grind it for 5 h by means of ball milling or the like to obtain a lithium iron manganese phosphate precursor slurry.

[0073] (2) Drying: Dry and granulate the slurry obtained in step (1) by spray drying (inlet air temperature 280 °C, outlet air temperature 145 °C) to obtain a precursor powder.

[0074] (3) Sintering: Sinter the precursor powder described in (2) in a protective atmosphere (700 °C, 8 h) to obtain a sintered material; cool it naturally to room temperature to obtain a carbon-coated lithium iron manganese phosphate material.

[0075] (4) Secondary Coating: The carbon-coated lithium iron phosphate manganese material described in (3) and PVA (3.6% of the total raw material mass) as the secondary carbon source are added to deionized water. After ultrasonic oscillation for 1 h, suction filtration is carried out, and then dried in a vacuum drying oven at 120 °C for 4 h. The dried mixture is sintered in a protective atmosphere, heated to 350 °C at a rate of 10 °C / min and sintered for 1 h, then heated to 740 °C at a rate of 15 °C / min and sintered for 4 h, and then cooled to room temperature to obtain the secondary-coated lithium iron phosphate manganese cathode material. -1 at a rate of -1 to sinter for 1 h, and then heated to 740 °C at a rate of 15 °C / min

[0076] Test Example 1

[0077] (1) Weigh the polyvinylidene fluoride binder (PVDF) with an analytical balance (accuracy 0.0001 g) into N-methylpyrrolidone (NMP), stir and dissolve completely; then add the cathode materials prepared in Examples 1-5 and the carbon black conductive agent (SP), and stir evenly to obtain the cathode slurry. Among them, the mass ratio of the composite material, PVDF, and SP is 8:1:1.

[0078] (2) Use a coater to evenly coat the cathode slurry on the aluminum foil, and dry it in a vacuum drying oven. After removing the solvent NMP, it is further roll-pressed and punched to obtain a disc with a diameter of 16.0 mm as the positive electrode plate.

[0079] (3) Using the above positive electrode plate as the positive electrode, a lithium metal sheet as the negative electrode, a PEPP composite membrane as the battery separator, and 1.0 mol·L -1 of LiPF6 / (EC + DMC) as the electrolyte, where the volume ratio of EC to DMC is 1:1, assemble a CR2032 button cell.

[0080] (4) Perform cyclic charge and discharge tests on the above button cell at a charge and discharge rate of 1C, the test temperature is 25.0 °C, and the charge and discharge voltage is 2.5 V - 4.5 V. The test results are shown in Table 1 below.

[0081] Table 1

[0082]

[0083] From Figure 1 and Table 1, it can be concluded that the lithium iron phosphate manganese cathode material prepared in Example 5, that is, the lithium iron phosphate manganese cathode material with a nickel doping amount of 0.02 mol and coated with modified polyethylene glycol, has the highest initial discharge efficiency and specific capacity, and at the same time has the highest cycle stability.

Claims

1. A preparation method of a high-capacity lithium iron manganese phosphate cathode material, characterized in that, The general formula of the high-capacity lithium iron manganese phosphate cathode material is LiFe 1-x-y Mn y Ni x PO4@C1@C2, where 0.01 ≤ x ≤ 0.03, 0.61 ≤ x + y ≤ 0.63, the carbon source for C1 is the first organic carbon source, and the carbon source for C2 is the second organic carbon source. The preparation method comprises the following steps: (1) For the high-capacity lithium iron manganese phosphate cathode material LiFe 1-x-y Mn y Ni x PO4@C1@C2, add iron source, manganese source, lithium source, phosphorus source, nickel source and the first organic carbon source into the reaction vessel according to a certain molar stoichiometric coefficient ratio, grind and mix evenly, add water, and mix evenly again to obtain the lithium iron manganese phosphate precursor slurry; The first organic carbon source is nickel and fluorine modified polyethylene glycol; The preparation method of the nickel and fluorine modified polyethylene glycol comprises the following steps: Nickel acrylate, hexafluorobutyl acrylate and polyethylene glycol with double-terminal amino groups undergo an addition reaction in an alkaline medium to obtain the nickel and fluorine modified polyethylene glycol; (2) After drying the lithium iron manganese phosphate precursor slurry obtained in step (1), sintering is carried out in a protective atmosphere to obtain a nickel-doped lithium iron manganese phosphate cathode material with a primary coating; (3) After mixing the nickel-doped lithium iron manganese phosphate cathode material with a primary coating obtained in step (2) with a second organic carbon source, sintering is carried out in a protective atmosphere to obtain the high-capacity lithium iron manganese phosphate cathode material.

2. The preparation method according to claim 1, wherein The mass ratio of nickel acrylate, hexafluorobutyl acrylate and polyethylene glycol with double-terminal amino groups is 1:20:266.67 - 500.

3. The preparation method according to claim 1, wherein The temperature of the addition reaction is 60 - 70 °C, and the time of the addition reaction is 100 - 150 minutes.

4. The preparation method according to claim 1, characterized in that, The iron source is at least one of iron oxide, ferrous sulfate, ferric sulfate, iron phosphate, and ferrous oxalate; The manganese source is at least one of manganese acetate, manganese dioxide, manganese oxalate, and manganese carbonate; The lithium source is at least one of lithium carbonate, lithium hydroxide, and lithium dihydrogen phosphate; The phosphorus source is at least one of iron phosphate, lithium dihydrogen phosphate, ammonium dihydrogen phosphate, and diammonium hydrogen phosphate; The nickel source is at least one of nickel oxide, nickel sesquioxide, nickel tetroxide, and nickel oxalate.

5. The preparation method according to claim 1, characterized in that, The first organic carbon source accounts for 5.4% of the total mass of the iron source, manganese source, lithium source, phosphorus source, nickel source and the reaction raw materials of the first organic carbon source, and the second organic carbon source accounts for 3.6% of the total mass of the nickel-doped lithium iron manganese phosphate cathode material with a primary coating and the reaction raw materials of the second organic carbon source.

6. The preparation method according to claim 1, wherein The second organic carbon source is polyvinyl alcohol.

7. The preparation method according to claim 1, characterized in that, In step (2), the sintering temperature is 600 - 800 °C, and the sintering time is 6 - 10 h; In step (3), the sintering temperature is 300 - 800 °C, and the sintering time is 3 - 5 h.

8. A high-capacity lithium iron manganese phosphate cathode material prepared by the preparation method according to any one of claims 1 - 7.

9. Application of the high-capacity lithium iron manganese phosphate cathode material according to claim 8 in the preparation of a lithium ion battery.

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