Lithium iron phosphate positive electrode material, preparation method and application thereof

Abstract

The application discloses a lithium iron phosphate positive electrode material and a preparation method and application thereof, and belongs to the technical field of lithium ion batteries, and comprises the following steps: (1) uniformly mixing a composite iron source, a phosphorus source, a lithium source and a carbon source to obtain a mixture; (2) dividing the mixture into two parts, namely a first part of the mixture and a second part of the mixture; (3) uniformly mixing the first part of the mixture, metal powder and a surfactant to obtain a first precursor; (4) uniformly mixing the second part of the mixture and a carbon material to obtain a second precursor; and (5) uniformly mixing the first precursor and the second precursor, drying, sintering, and crushing to obtain the lithium iron phosphate positive electrode material. The lithium iron phosphate positive electrode material has improved conductivity and density, and the growth range of internal resistance is small during use, so that the electrochemical performance and cycle performance of the lithium iron phosphate battery are improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium ion batteries, in particular to a lithium iron phosphate positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] Environmental pollution and energy shortage have become a problem for the further development of human society. Developing new energy is a "hot" topic worldwide, such as wind energy, electric energy, solar energy, etc. As a kind of energy storage unit, lithium ion battery has the advantages of high energy density and long cycle life, and has become the first choice of energy storage and supply medium. Lithium iron phosphate has become one of the most widely used lithium ion battery positive electrode materials due to its high energy density, low cost, stable charge and discharge platform, environmental friendliness, safety and other advantages.

[0003] The lithium iron phosphate positive electrode material has the disadvantages of low electronic conductivity and small lithium ion diffusion coefficient, which restricts its development in the field of high-rate batteries.

[0004] In view of this, the present application is proposed. SUMMARY

[0005] The present application aims to overcome the shortcomings of the prior art and provide a lithium iron phosphate positive electrode material and a preparation method thereof, which significantly improve the electrochemical performance and cycle life of the battery composed of the lithium iron phosphate positive electrode material.

[0006] To achieve the above-mentioned purpose, the technical solution adopted by the present application is as follows:

[0007] A preparation method of a lithium iron phosphate positive electrode material, comprising the following steps:

[0008] (1) mixing a composite iron source, a phosphorus source, a lithium source and a carbon source uniformly to obtain a mixture;

[0009] (2) dividing the mixture into two parts, namely a first part of the mixture and a second part of the mixture;

[0010] (3) mixing the first part of the mixture, a metal powder and a surfactant uniformly to obtain a first precursor;

[0011] (4) mixing the second part of the mixture and a carbon material uniformly to obtain a second precursor;

[0012] (5) mixing the first precursor and the second precursor uniformly, drying, sintering, and crushing to obtain the lithium iron phosphate positive electrode material.

[0013] The inventors of the present application found in a large number of studies that by uniformly mixing a composite iron source, a phosphorus source, a lithium source and a carbon source to obtain a mixture, dividing the mixture into two parts, mixing each part with metal powder and a surfactant and carbon material respectively, and then blending and heat treating, the conductivity and density of the lithium iron phosphate positive electrode material are significantly improved, the internal resistance growth rate is small during use, and thus the electrochemical performance and cycle performance of the lithium iron phosphate battery are significantly improved.

[0014] By adding metal powder to the first part of the mixture, the crystal lattice volume can be effectively increased, the lithium iron phosphate crystal lattice distortion can be avoided, the Li-O bond energy can be reduced, and the diffusion capacity of Li + can be improved, thereby effectively improving the electrochemical performance and cycle performance; by adding carbon material to the second part of the mixture, the internal resistance can be reduced, the electron transfer between lithium iron phosphate can be improved, and the electrical performance of the material can be improved.

[0015] The inventors found that by using a separate preparation method to mix part of the mixture with metal powder and a surfactant and part of the mixture with carbon material during the preparation of the lithium iron phosphate material, the dispersion performance of the material can be effectively improved, the metal powder and the carbon material will not agglomerate, and the stability of the material can be improved.

[0016] The inventors found that if only metal powder or carbon material is added, the electrical performance will decrease, that is, by adding metal powder and carbon material together, the electrochemical performance and cycle performance are significantly improved under the interaction and mutual influence of the metal powder and the carbon material, the inventors further explored the principle and found that the copper powder and the carbon material both have good electrical conductivity, and the two materials cooperatively construct an electrical conduction network, significantly improve the electrical conductivity and reduce the internal resistance, thereby significantly improving the electrochemical performance and cycle performance of the lithium iron phosphate battery.

[0017] As a preferred embodiment of the present application, the molar ratio of Li:Fe:P in step (1) is (0.9-1.1):1:(0.95-1.1).

[0018] As a preferred embodiment of the present application, the mass ratio of the composite iron source to the carbon source is 1:(0.1-0.4), preferably 1:(0.1-0.3), and more preferably 1:0.25.

[0019] As a preferred embodiment of the present application, the composite iron source comprises ferrous oxalate, iron powder and iron acetate, and the mass ratio of the ferrous oxalate, the iron powder and the iron acetate is 1:(0.2-0.8):(0.1-0.6), preferably 1:(0.4-0.8):(0.1-0.5), and more preferably 1:0.6:0.4. The use of the above specific composite iron source is conducive to the addition of metal powder and can also reduce the cost.

[0020] As a preferred embodiment of the present application, at least one of the following (a)-(c) is satisfied:

[0021] (a) the phosphorus source is one of ammonium dihydrogen phosphate, ammonium phosphate, phosphoric acid, diammonium hydrogen phosphate;

[0022] (b) the lithium source is one of lithium carbonate, lithium oxalate, lithium hydroxide, lithium dihydrogen phosphate, lithium acetate, lithium sulfate, lithium chloride, lithium nitrate;

[0023] (c) the carbon source is one of sucrose, glucose, starch, cellulose, magnesium stearate, stearic acid, polyethylene glycol, citric acid, oxalic acid, maltose, polyvinyl alcohol.

[0024] It should be noted that in the present application, the specific phosphorus source, lithium source or carbon source can be selected by those skilled in the art as needed, as long as the corresponding phosphorus supply, lithium supply and carbon supply effects are achieved.

[0025] As a preferred embodiment of the present application, the first part of the mixture accounts for 30-45% of the mass of the mixture, and the second part of the mixture accounts for 55-70% of the mass of the mixture.

[0026] As a preferred embodiment of the present application, the mass ratio of the first part of the mixture, the metal powder and the surfactant is 1:(0.01-0.05):(0.001-0.005); preferably 1:(0.02-0.05):(0.002-0.005), and more preferably 1:0.03:0.003. Further, under the specific preparation method of the present application, the incorporation amount of the metal powder and the surfactant is not the larger the better, and if the incorporation amount is too large, the agglomeration phenomenon becomes uncontrollable, leading to poor stability and thus causing a decrease in material performance.

[0027] As a preferred embodiment of the present application, the metal powder is copper powder, and the particle size D50 of the copper powder is 2-10 μm.

[0028] As a preferred embodiment of the present application, the surfactant is a silane coupling agent.

[0029] As a preferred embodiment of the present application, the mass ratio of the second part of the mixture and the carbon material is 1:(0.01-0.05), preferably 1:(0.01-0.04), and more preferably 1:0.03.

[0030] As a preferred embodiment of the present application, the carbon material is carbon nanotubes or graphene, preferably carbon nanotubes, and the particle size D50 of the carbon nanotubes is 2-8 μm.

[0031] In a preferred embodiment of the present invention, the sintering temperature in step (5) is 600-750°C and the sintering time is 4-10h.

[0032] In a preferred embodiment of the present invention, the material is pulverized to 4-20 μm in step (5).

[0033] A lithium iron phosphate cathode material is prepared using the method described above for preparing lithium iron phosphate cathode materials.

[0034] The present invention also proposes an application of the above-mentioned lithium iron phosphate cathode material in the preparation of lithium-ion batteries.

[0035] The beneficial effects of this invention are as follows: (1) By uniformly mixing composite iron source, phosphorus source, lithium source and carbon source to obtain a mixture, dividing the mixture into two parts, mixing them with metal powder, surfactant and carbon material respectively, and then blending and heat treating them, the conductivity and density of lithium iron phosphate cathode material are significantly improved, so that the internal resistance increases less during use, thereby significantly improving the electrochemical performance and cycle performance of lithium iron phosphate battery; (2) By incorporating metal powder into the first part of the mixture, this invention can effectively increase the lattice volume, avoid lattice distortion of lithium iron phosphate, reduce the bond energy of Li-O bond, and improve the Li-O bond energy. + The diffusion capacity is improved, thereby effectively enhancing the electrochemical performance and cycle performance; by incorporating carbon materials into the second mixture, the internal resistance can be reduced, the electron transport between lithium iron phosphates can be improved, and the electrical performance of the material can be enhanced; (3) In the process of preparing the lithium iron phosphate material, the present invention adopts a separate preparation method, mixing part of the mixture with metal powder and surfactant, and mixing part with carbon materials, which can effectively improve the dispersion performance of the materials, prevent the metal powder and carbon materials from agglomerating, and improve the stability of the material; (4) The present invention incorporates metal powder and carbon materials together, and the interaction between metal powder and carbon materials Under the mutual influence, the electrochemical performance and cycle performance are significantly improved. The inventors further explored the principle and found that it may be that both copper powder and carbon materials have good conductivity. The two work together to construct a conductive network, which significantly improves conductivity and reduces internal resistance, thereby significantly improving the electrochemical performance and cycle performance of lithium iron phosphate batteries. (5) Under the specific preparation method of this invention, the amount of metal powder and surfactant added is not as large as possible. If the amount added is too large, the agglomeration phenomenon becomes uncontrollable, resulting in poor stability and causing a decline in material performance. Therefore, it is necessary to control the amount of metal powder and surfactant added. Detailed Implementation

[0036] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be clearly and completely described below. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of the present application.

[0037] In the present application, the technical features described in an open way include both the closed technical solutions consisting of the listed features and the open technical solutions containing the listed features.

[0038] In the present application, if no special description is made, the numerical range is regarded as continuous and includes the minimum value and the maximum value of the range and each value between the minimum value and the maximum value. Further, when the range refers to an integer, each integer between the minimum value and the maximum value of the range is included. In addition, when multiple ranges are provided to describe a feature or a property, the ranges can be combined. In other words, unless otherwise specified, all the ranges disclosed herein should be understood to include any and all sub-ranges.

[0039] In the present application, the specific dispersion and stirring treatment method is not particularly limited.

[0040] Unless otherwise specified, the components and raw materials used in the embodiments and comparative examples of the present application are commercially available raw materials and instruments, and the components and raw materials used in each parallel experiment are the same.

[0041] Embodiment 1

[0042] A preparation method of a lithium iron phosphate positive electrode material, comprising the following steps:

[0043] (1) uniformly mixing a composite iron source, ammonium dihydrogen phosphate, lithium carbonate and glucose to obtain a mixture;

[0044] The composite iron source includes ferrous oxalate, iron powder and iron acetate, and the mass ratio of the ferrous oxalate, the iron powder and the iron acetate is 1:0.6:0.4.

[0045] The molar ratio of Li:Fe:P of the composite iron source, the ammonium dihydrogen phosphate and the lithium carbonate is 0.98:1:1.05.

[0046] The mass ratio of the composite iron source to the glucose is 1:0.25.

[0047] (2) dividing the mixture into two parts, i.e., a first part of the mixture and a second part of the mixture;

[0048] The first part of the mixture accounts for 40% of the mass of the mixture, and the second part of the mixture accounts for 60% of the mass of the mixture.

[0049] (3) The first part of the mixture, copper powder and gamma-aminopropyl triethoxysilane are uniformly mixed to obtain a first precursor;

[0050] The mass ratio of the first part of the mixture, copper powder and gamma-aminopropyl triethoxysilane is 1:0.03:0.003.

[0051] The D50 of the copper powder is 5 μm.

[0052] (4) The second part of the mixture and carbon nanotubes are uniformly mixed to obtain a second precursor;

[0053] The mass ratio of the second part of the mixture and carbon nanotubes is 1:0.03.

[0054] The particle size D50 of the carbon nanotubes is 5 μm.

[0055] (5) The first precursor and the second precursor are uniformly mixed, dried, sintered at 700°C for 6h, and crushed to 10 μm to obtain a lithium iron phosphate positive electrode material.

[0056] Example 2

[0057] A preparation method of a lithium iron phosphate positive electrode material comprises the following steps:

[0058] (1) A composite iron source, ammonium dihydrogen phosphate, lithium carbonate and glucose are uniformly mixed to obtain a mixture;

[0059] The composite iron source comprises ferrous oxalate, iron powder and iron acetate, and the mass ratio of the ferrous oxalate, iron powder and iron acetate is 1:0.6:0.4.

[0060] The molar ratio of Li:Fe:P of the composite iron source, ammonium dihydrogen phosphate and lithium carbonate is 0.98:1:1.05.

[0061] The mass ratio of the composite iron source and glucose is 1:0.25.

[0062] (2) The mixture is divided into two parts, which are a first part of the mixture and a second part of the mixture;

[0063] The first part of the mixture accounts for 30% of the mass of the mixture, and the second part of the mixture accounts for 70% of the mass of the mixture.

[0064] (3) The first part of the mixture, copper powder and gamma-aminopropyl triethoxysilane are uniformly mixed to obtain a first precursor;

[0065] The mass ratio of the first part of the mixture, copper powder and gamma-aminopropyl triethoxysilane is 1:0.03:0.003;

[0066] The D50 of the copper powder is 5 mu m.

[0067] (4) The second part of the mixture and the carbon nanotube are uniformly mixed to obtain a second precursor.

[0068] The mass ratio of the second part of the mixture and the carbon nanotube is 1:0.03.

[0069] The particle size D50 of the carbon nanotube is 5 mu m.

[0070] (5) The first precursor and the second precursor are uniformly mixed, dried, sintered at 700 DEG C for 6 hours, crushed to 10 mu m, and a lithium iron phosphate positive electrode material is obtained.

[0071] Example 3

[0072] A preparation method of a lithium iron phosphate positive electrode material comprises the following steps:

[0073] (1) A composite iron source, ammonium dihydrogen phosphate, lithium carbonate and glucose are uniformly mixed to obtain a mixture.

[0074] The composite iron source comprises ferrous oxalate, iron powder and iron acetate, and the mass ratio of the ferrous oxalate, iron powder and iron acetate is 1:0.6:0.4.

[0075] The molar ratio of Li:Fe:P of the composite iron source, ammonium dihydrogen phosphate and lithium carbonate is 0.98:1:1.05.

[0076] The mass ratio of the composite iron source and glucose is 1:0.25.

[0077] (2) The mixture is divided into two parts, which are a first part of the mixture and a second part of the mixture.

[0078] The first part of the mixture accounts for 45% of the mass of the mixture, and the second part of the mixture accounts for 55% of the mass of the mixture.

[0079] (3) The first part of the mixture, copper powder and gamma-aminopropyl triethoxysilane are uniformly mixed to obtain a first precursor.

[0080] The mass ratio of the first part of the mixture, copper powder and gamma-aminopropyl triethoxysilane is 1:0.03:0.003.

[0081] The D50 of the copper powder is 5 mu m.

[0082] (4) mixing the second part of the mixture and carbon nanotubes uniformly to obtain a second precursor;

[0083] The mass ratio of the second part of the mixture to the carbon nanotubes is 1:0.03.

[0084] The particle size D50 of the carbon nanotubes is 5 μm.

[0085] (5) mixing the first precursor and the second precursor uniformly, drying, sintering at 700 °C for 6 h, and crushing to 10 μm to obtain a lithium iron phosphate positive electrode material.

[0086] Example 4

[0087] A preparation method of a lithium iron phosphate positive electrode material comprises the following steps:

[0088] (1) mixing a composite iron source, ammonium dihydrogen phosphate, lithium carbonate and glucose uniformly to obtain a mixture;

[0089] The composite iron source comprises ferrous oxalate, iron powder and iron acetate, and the mass ratio of the ferrous oxalate, the iron powder and the iron acetate is 1:0.6:0.4.

[0090] The molar ratio of Li:Fe:P of the composite iron source, the ammonium dihydrogen phosphate and the lithium carbonate is 0.98:1:1.05.

[0091] The mass ratio of the composite iron source to glucose is 1:0.25.

[0092] (2) dividing the mixture into two parts, which are a first part of the mixture and a second part of the mixture;

[0093] The first part of the mixture accounts for 40% of the mass of the mixture, and the second part of the mixture accounts for 60% of the mass of the mixture.

[0094] (3) mixing the first part of the mixture, copper powder and γ-aminopropyltriethoxysilane uniformly to obtain a first precursor;

[0095] The mass ratio of the first part of the mixture, the copper powder and the γ-aminopropyltriethoxysilane is 1:0.01:0.001.

[0096] The D50 of the copper powder is 5 μm.

[0097] (4) mixing the second part of the mixture and carbon nanotubes uniformly to obtain a second precursor;

[0098] The mass ratio of the second part of the mixture to the carbon nanotubes is 1:0.05.

[0099] The particle size D50 of the carbon nanotubes is 5 μm.

[0100] (5)mixing the first precursor and the second precursor uniformly, drying, sintering at 700 DEG C for 6h, crushing to 10 mu m, to obtain the lithium iron phosphate positive electrode material.

[0101] Example 5

[0102] A preparation method of a lithium iron phosphate positive electrode material comprises the following steps:

[0103] (1)mixing a composite iron source, ammonium dihydrogen phosphate, lithium carbonate and glucose uniformly to obtain a mixture;

[0104] The composite iron source comprises ferrous oxalate, iron powder and iron acetate, and the mass ratio of the ferrous oxalate, the iron powder and the iron acetate is 1:0.6:0.4.

[0105] The molar ratio of Li:Fe:P of the composite iron source, the ammonium dihydrogen phosphate and the lithium carbonate is 0.98:1:1.05.

[0106] The mass ratio of the composite iron source to glucose is 1:0.25.

[0107] (2)dividing the mixture into two parts, a first part of the mixture and a second part of the mixture;

[0108] The first part of the mixture accounts for 45% of the mass of the mixture, and the second part of the mixture accounts for 55% of the mass of the mixture.

[0109] (3)mixing the first part of the mixture, copper powder and gamma-aminopropyl triethoxysilane uniformly to obtain a first precursor;

[0110] The mass ratio of the first part of the mixture, the copper powder and the gamma-aminopropyl triethoxysilane is 1:0.05:0.005.

[0111] The D50 of the copper powder is 5 mu m.

[0112] (4)mixing the second part of the mixture and carbon nanotubes uniformly to obtain a second precursor;

[0113] The mass ratio of the second part of the mixture to the carbon nanotubes is 1:0.01.

[0114] The particle size D50 of the carbon nanotubes is 5 mu m.

[0115] (5)mixing the first precursor and the second precursor uniformly, drying, sintering at 700 DEG C for 6h, crushing to 10 mu m, to obtain the lithium iron phosphate positive electrode material.

[0116] Example 6

[0117] A preparation method of a lithium iron phosphate positive electrode material, comprising the following steps:

[0118] (1) mixing a composite iron source, ammonium dihydrogen phosphate, lithium carbonate and glucose uniformly to obtain a mixture;

[0119] The composite iron source comprises ferrous oxalate, iron powder and iron acetate, and the mass ratio of the ferrous oxalate, the iron powder and the iron acetate is 1:0.6:0.4.

[0120] The molar ratio of Li:Fe:P of the composite iron source, the ammonium dihydrogen phosphate and the lithium carbonate is 0.9:1:1.1.

[0121] The mass ratio of the composite iron source to the glucose is 1:0.1.

[0122] (2) dividing the mixture into two parts, namely a first part of the mixture and a second part of the mixture;

[0123] The first part of the mixture accounts for 40% of the mass of the mixture, and the second part of the mixture accounts for 60% of the mass of the mixture.

[0124] (3) mixing the first part of the mixture, copper powder and gamma-aminopropyl triethoxysilane uniformly to obtain a first precursor;

[0125] The mass ratio of the first part of the mixture, the copper powder and the gamma-aminopropyl triethoxysilane is 1:0.03:0.003.

[0126] The D50 of the copper powder is 5 μm.

[0127] (4) mixing the second part of the mixture and carbon nanotubes uniformly to obtain a second precursor;

[0128] The mass ratio of the second part of the mixture to the carbon nanotubes is 1:0.03.

[0129] The particle size D50 of the carbon nanotubes is 5 μm.

[0130] (5) mixing the first precursor and the second precursor uniformly, drying, sintering at 700 DEG C for 6 h, crushing to 10 μm, and obtaining a lithium iron phosphate positive electrode material.

[0131] Example 7

[0132] A preparation method of a lithium iron phosphate positive electrode material, comprising the following steps:

[0133] (1) mixing a composite iron source, ammonium dihydrogen phosphate, lithium carbonate and glucose uniformly to obtain a mixture;

[0134] The composite iron source comprises ferrous oxalate, iron powder and iron acetate, and the mass ratio of the ferrous oxalate, the iron powder and the iron acetate is 1:0.6:0.4.

[0135] The molar ratio of Li:Fe:P of the composite iron source, the ammonium dihydrogen phosphate and the lithium carbonate is 1.1:1:0.95.

[0136] The mass ratio of the composite iron source to the glucose is 1:0.4.

[0137] (2) The mixture is divided into two parts, namely a first part of the mixture and a second part of the mixture;

[0138] The first part of the mixture accounts for 40% of the mass of the mixture, and the second part of the mixture accounts for 60% of the mass of the mixture.

[0139] (3) The first part of the mixture, copper powder and gamma-aminopropyl triethoxysilane are uniformly mixed to obtain a first precursor;

[0140] The mass ratio of the first part of the mixture, the copper powder and the gamma-aminopropyl triethoxysilane is 1:0.03:0.003.

[0141] The D50 of the copper powder is 5 μm.

[0142] (4) The second part of the mixture and carbon nanotubes are uniformly mixed to obtain a second precursor;

[0143] The mass ratio of the second part of the mixture to the carbon nanotubes is 1:0.03.

[0144] The particle size D50 of the carbon nanotubes is 5 μm.

[0145] (5) The first precursor and the second precursor are uniformly mixed, dried, sintered at 700 ℃ for 6 h, and crushed to 10 μm to obtain a lithium iron phosphate positive electrode material.

[0146] Comparative Example 1

[0147] Comparative Example 1 is different from Example 1 in that the iron source of Comparative Example 1 is single, and other aspects are the same.

[0148] The iron source of Comparative Example 1 is single ferrous oxalate.

[0149] Comparative Example 2

[0150] Comparative Example 2 is different from Example 1 in that the mass ratio of the first part of the mixture, the copper powder, the gamma-aminopropyl triethoxysilane and the second part of the mixture, the carbon nanotubes of Comparative Example 2 is not within the range of the present application, and other aspects are the same.

[0151] The mass ratio of the first part of mixture, copper powder, and surfactant in the present comparative example is 1:0.005:0.0005.

[0152] The mass ratio of the second part of mixture, carbon nanotube in the present comparative example is 1:0.005.

[0153] Comparative Example 3

[0154] Comparative Example 3 is different from Example 1 in that the mass ratio of the first part of mixture, copper powder, gamma-aminopropyl triethoxysilane, and the second part of mixture, carbon nanotube in Comparative Example 3 is not within the scope of the present application, and other aspects are the same.

[0155] The mass ratio of the first part of mixture, copper powder, and surfactant in the present comparative example is 1:0.08:0.008.

[0156] The mass ratio of the second part of mixture, carbon nanotube in the present comparative example is 1:0.08.

[0157] Comparative Example 4

[0158] Comparative Example 4 is different from Example 1 in that Comparative Example 4 does not add copper powder, and other aspects are the same.

[0159] A preparation method of a lithium iron phosphate positive electrode material, comprising the following steps:

[0160] (1) uniformly mixing a composite iron source, ammonium dihydrogen phosphate, lithium carbonate, and glucose to obtain a mixture;

[0161] The composite iron source comprises ferrous oxalate, iron powder, and iron acetate, and the mass ratio of the ferrous oxalate, iron powder, and iron acetate is 1:0.6:0.4.

[0162] The molar ratio of Li:Fe:P of the composite iron source, ammonium dihydrogen phosphate, and lithium carbonate is 0.98:1:1.05.

[0163] The mass ratio of the composite iron source and glucose is 1:0.25.

[0164] (2) uniformly mixing the mixture and carbon nanotubes to obtain a precursor;

[0165] The mass ratio of the mixture and carbon nanotubes is 1:0.03.

[0166] The particle size D50 of the carbon nanotubes is 5 μm.

[0167] (3) drying the precursor, sintering at 700°C for 6 h, crushing to 10 μm, to obtain a lithium iron phosphate positive electrode material.

[0168] Comparative Example 5

[0169] Comparative Example 5 differs from Example 1 in that no carbon nanotubes are added in Comparative Example 5.

[0170] A preparation method of a lithium iron phosphate positive electrode material comprises the following steps:

[0171] (1) mixing a composite iron source, ammonium dihydrogen phosphate, lithium carbonate and glucose uniformly to obtain a mixture;

[0172] The composite iron source comprises ferrous oxalate, iron powder and iron acetate, and the mass ratio of the ferrous oxalate, the iron powder and the iron acetate is 1:0.6:0.4.

[0173] The molar ratio of Li:Fe:P of the composite iron source, the ammonium dihydrogen phosphate and the lithium carbonate is 0.98:1:1.05.

[0174] The mass ratio of the composite iron source to the glucose is 1:0.25.

[0175] (2) mixing the mixture, copper powder and gamma-aminopropyl triethoxysilane uniformly to obtain a first precursor;

[0176] The mass ratio of the first part of the mixture, the copper powder and the gamma-aminopropyl triethoxysilane is 1:0.03:0.003.

[0177] The D50 of the copper powder is 5 μm.

[0178] (3) drying the precursor, sintering at 700 ℃ for 6 h, crushing to 10 μm to obtain the lithium iron phosphate positive electrode material.

[0179] Test Example

[0180] Preparation of a lithium ion battery:

[0181] (1) compounding the lithium iron phosphate positive electrode material, PVD and SP carbon in a mass ratio of 95:3:2, using NMP (N-methyl pyrrolidone) as a dispersant to perform sizing, coating on a flat aluminum foil, baking to dryness in an oven, and then roll pressing and punching into a positive electrode sheet with a diameter of 15 mm for standby;

[0182] (2) using graphite as a negative electrode, stirring and dispersing into a uniform slurry according to the weight ratio of the negative electrode material graphite: conductive agent SP: adhesive CMC = 96:2:2, using water as a solvent, then coating on a copper foil, drying, roll pressing and cutting into a negative electrode sheet for standby;

[0183] (3) separating the cut positive and negative electrode sheets with a polypropylene separator, winding together, then placing in an aluminum plastic film, drying the positive electrode sheet to remove moisture, then performing liquid injection in a drying room, and then heat sealing to form a battery.

[0184] Test: measure the initial volume V0 of the battery by drainage method, then transfer the battery to an oven at 60℃, after 1C / 1C charge-discharge cycle for 100 times, calculate the capacity retention rate, and measure the initial volume V1 of the battery by drainage method, and the volume expansion rate of the battery is obtained according to the following formula.

[0185] Volume expansion rate = (V1-V0) / V0*100%.

[0186] DCR test: charge the battery to 4.2V at 1C at 25, then discharge at 1C capacity for 30min, adjust to 50% SOC, then discharge at 1C for 10s, calculate the discharge DCR at 25℃, the calculation formula is △U / I, wherein △U is the change value of voltage before and after discharge, I is the discharge current, and the recorded results are shown in Table 2.

[0187] Table 1

[0188] DCR / mΩ volume expansion rate / % capacity retention rate / % Example 1 21.7 1.9％ 98.9％ Example 2 22.4 2.3％ 97.6％ Example 3 22.5 2.4％ 97.4％ Example 4 24.5 4.2％ 91.2％ Example 5 25.9 5.1％ 90.1％ Example 6 22.9 2.7％ 96.8％ Example 7 23.3 2.9％ 96.3％ Comparative Example 1 27.3 6.1％ 89.7％ Comparative Example 2 28.9 7.9％ 84.6％ Comparative Example 3 29.2 8.7％ 83.1％ Comparative Example 4 30.8 9.5％ 81.9％ Comparative Example 5 31.4 9.9％ 80.7％

[0189] As can be seen from Table 1, the lithium iron phosphate positive electrode material prepared by the application has excellent electrochemical performance and cycle performance.

[0190] As can be seen from Comparative Example 1 and Comparative Example 1, the application uses a composite iron source, which further improves the electrochemical performance and cycle performance.

[0191] As can be seen from Comparative Example 1 and Comparative Examples 2-3, by controlling the amount of carbon nanotubes and copper powder, the application further improves the electrochemical performance and cycle performance.

[0192] As can be seen from Comparative Example 1 and Comparative Examples 4-5, by adding carbon nanotubes and copper powder, the application significantly improves the electrochemical performance and cycle performance.

[0193] Finally, it should be noted that the above examples are only used to illustrate the technical solutions of the application and not to limit the protection scope of the application. Although the application has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the application can be modified or replaced by equivalents without departing from the essence and scope of the technical solutions of the application.

Claims

1. A method for preparing a lithium iron phosphate cathode material, characterized in that, Includes the following steps: (1) Mix the composite iron source, phosphorus source, lithium source and carbon source evenly to obtain a mixture; (2) Divide the mixture into two parts, namely the first part of the mixture and the second part of the mixture; (3) Mix the first part of the mixture, the metal powder, and the surfactant evenly to obtain the first precursor; (4) Mix the second part of the mixture and the carbon material evenly to obtain the second precursor; (5) Mix the first precursor and the second precursor evenly, dry, sinter, and pulverize to obtain lithium iron phosphate cathode material; The mass ratio of the first mixture, metal powder, and surfactant is 1:(0.01~0.05):(0.001~0.005).

2. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, In step (1), the molar ratio of Li:Fe:P is (0.9~1.1):1:(0.95~1.1).

3. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The mass ratio of the composite iron source to the carbon source is 1:(0.1~0.4).

4. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The composite iron source includes ferrous oxalate, iron powder, and ferric acetate, and the mass ratio of ferrous oxalate, iron powder, and ferric acetate is 1:(0.2~0.8):(0.1~0.6).

5. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, Satisfy at least one of the following (a) to (c): (a) The phosphorus source is one of ammonium dihydrogen phosphate, ammonium phosphate, phosphoric acid, and diammonium hydrogen phosphate; (b) The lithium source is one of lithium carbonate, lithium oxalate, lithium hydroxide, lithium dihydrogen phosphate, lithium acetate, lithium sulfate, lithium chloride, and lithium nitrate; (c) The carbon source is one of sucrose, glucose, starch, cellulose, magnesium stearate, stearic acid, polyethylene glycol, citric acid, oxalic acid, maltose, and polyvinyl alcohol.

6. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The first portion of the mixture accounts for 30-45% of the mass of the mixture, and the second portion of the mixture accounts for 55-70% of the mass of the mixture; and / or The mass ratio of the second mixture to the carbon material is 1:(0.01~0.05).

7. The method for preparing the lithium iron phosphate cathode material according to claim 1, characterized in that, The metal powder is copper powder, and the particle size D50 of the copper powder is 2~10μm; and / or The surfactant is a silane coupling agent; and / or The carbon material is carbon nanotubes or graphene.

8. The method for preparing lithium iron phosphate cathode material according to claim 1, characterized in that, In step (5), the sintering temperature is 600~750℃, and the sintering time is 4~10h; and / or In step (5), the material is pulverized to 4~20μm.

9. A lithium iron phosphate cathode material, characterized in that, It is prepared by the preparation method of lithium iron phosphate cathode material according to any one of claims 1 to 8.

10. The application of the lithium iron phosphate cathode material according to claim 9 in the preparation of lithium-ion batteries.