Spherical lithium iron phosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a spherical lithium iron phosphate positive electrode material and a preparation method thereof, and belongs to the technical field of lithium batteries, and the spherical lithium iron phosphate positive electrode material is of a multi-layer coating structure; the spherical lithium iron phosphate positive electrode material sequentially comprises spherical lithium iron phosphate particles, a nanoscale nickel-cobalt-manganese coating layer and a carbon layer from inside to outside. The spherical lithium iron phosphate positive electrode material has relatively high tap density and relatively large specific surface area, so that solid-phase diffusion paths of lithium ions among lithium iron phosphate particles can be reduced, and the contact resistance of the particles is reduced; the nanoscale nickel-cobalt-manganese coating layer can improve the overall electronic conductivity and lithium ion diffusion rate and reduce lithium ion migration barriers; and the outermost carbon layer can form a high-conductivity network and can also be used as a physical barrier to maintain the structural stability of the material, so that the influence of a low-temperature environment on the transmission efficiency of lithium ions and electrons is further reduced, and the low-temperature performance of the spherical lithium iron phosphate positive electrode material is improved.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium batteries, and in particular relates to a spherical lithium iron phosphate positive electrode material and a preparation method thereof. Background Art

[0002] With the rapid development of electric vehicles and portable electronic devices, lithium-ion batteries, as the core of modern energy storage technology, have higher and higher performance requirements, and the performance of the positive electrode material of lithium-ion batteries directly determines the energy density, service life and applicable temperature range of the battery. Lithium iron phosphate has the advantages of high safety, long cycle life and high cost-effectiveness, and has become one of the mainstream positive electrode materials in the field of power batteries and energy storage.

[0003] However, the olivine crystal structure of lithium iron phosphate results in low electronic conductivity and slow diffusion kinetics of lithium ions along one-dimensional channels, especially under low temperature conditions. The low temperature environment will aggravate the lithium ion migration barrier in the lattice of lithium iron phosphate positive electrode materials, resulting in a significant slowdown in lithium ion deintercalation kinetics, increased interface polarization, and increased activation energy of charge transfer reactions. The electrochemical impedance increases, which severely limits the power output and rate performance of lithium iron phosphate positive electrode materials, and has low potential for low-temperature applications.

[0004] From the perspective of positive electrode materials, existing technologies attempt to improve the low-temperature performance of lithium iron phosphate positive electrode materials through various means, but the effect achieved is single, and it is difficult to enhance the rate performance of the material while improving the low-temperature performance, and it may also lead to the problem of reduced stability. Therefore, it is urgent to develop a technology that uses a combination of multiple modification methods to effectively improve the low-temperature rate performance of lithium iron phosphate positive electrode materials, while maintaining the stability and economy of the material. This is of great significance for breaking through the performance bottleneck of lithium iron phosphate materials in low-temperature and high-rate scenarios, as well as for the application of electric vehicles and energy storage systems in high-cold areas. Summary of the invention

[0005] The object of the present invention is to provide a spherical lithium iron phosphate positive electrode material and a preparation method thereof, so as to solve the problem that the lithium iron phosphate positive electrode material has limited rate performance and power output at low temperature.

[0006] The purpose of the present invention can be achieved by the following technical solutions: In a first aspect, the present invention provides a spherical lithium iron phosphate positive electrode material, which has a multi-layer coating structure; the spherical lithium iron phosphate positive electrode material comprises, from the inside to the outside, spherical lithium iron phosphate particles, a nano-scale nickel-cobalt-manganese coating layer and a carbon layer.

[0007] Preferably, the thickness of the nano-scale nickel-cobalt-manganese coating layer is 5 to 30 nm.

[0008] Preferably, the raw materials of the nano-scale nickel-cobalt-manganese coating layer include nickel carbonate, manganese carbonate, cobalt carbonate and lithium salt in a molar ratio of (0.1-0.15):(0.5-0.6):(0.1-0.15):(1.1-1.3).

[0009] Preferably, the lithium salt includes a combination of one or more of lithium carbonate, lithium phosphate and lithium hydroxide.

[0010] By adopting the above technical solution, the spherical lithium iron phosphate particles have a relatively high tap density and a large specific surface area. The high tap density will form a continuous electron transmission network inside the obtained positive electrode material, and the contact resistance between the particles will be reduced, so that electrons can be efficiently transmitted even in a low temperature environment; and the high tap density reduces the solid-phase diffusion path of lithium ions between lithium iron phosphate particles, compensating for the problem of decreased lithium ion migration rate at low temperatures.

[0011] The large specific surface area can provide more surface active sites and diffusion channels for lithium ions. Compared with irregular particles, the migration path of lithium ions in spherical particles is also shorter, which can reduce the problem of increased lithium ion migration barriers due to low temperature. In addition, the positive electrode material is coated with a carbon layer, which can form a conductive network. The synergistic effect of the high specific surface area and the conductive network can accelerate the transmission of electrons on the surface and interface of the particles, inhibit the polarization phenomenon during charging and discharging, and provide electrical contact between the positive electrode material particles and the electrolyte and conductive agent, reducing the impedance between particles, especially at low temperatures, which will significantly reduce the impact of low temperature on charge impedance.

[0012] The spherical lithium iron phosphate particles are coated with a nano-scale nickel-cobalt-manganese coating layer. The nano-scale nickel-cobalt-manganese coating layer has good conductivity. As another conductive network, it can accelerate the transfer speed of charge on the surface of the spherical lithium iron phosphate particles more quickly and reduce the impact of low temperature. In addition, since the electronic conductivity of the nano-scale nickel-cobalt-manganese coating layer is also much higher than that of the lithium iron phosphate particles, it can help reduce the contact resistance between the positive electrode material particles and greatly alleviate the problem of increased polarization and decreased rate performance due to electron transfer at low temperatures.

[0013] At the same time, the nano-scale nickel-cobalt-manganese coating has a high lithium ion diffusion coefficient and can act as an "ion bridge" to promote the cross-interface transmission of lithium ions and reduce the barriers to lithium ion migration. At the same time, under the protection of the nano-scale nickel-cobalt-manganese coating, the direct contact between the spherical lithium iron phosphate positive electrode material and the electrolyte can be reduced, thereby reducing the interface passivation problem caused by the increased viscosity of the electrolyte at low temperatures. It can also help buffer the lattice strain of the lithium iron phosphate positive electrode material during the lithium ion deintercalation process, thereby reducing the phenomenon of lithium iron phosphate capacity attenuation under low-temperature cycles.

[0014] However, when general nickel-cobalt-manganese ternary materials are combined with lithium iron phosphate positive electrode materials, the uniformity of the coating will be affected due to process mismatch, which will lead to local blockage of electron and ion transmission paths, which will in turn cause local polarization to increase and reduce the rate performance of the obtained lithium iron phosphate positive electrode material.

[0015] In order to further improve the interfacial bonding force between the nano-scale nickel-cobalt-manganese coating layer and the spherical lithium iron phosphate particles, the present invention adopts an in-situ coating method. Specifically, the raw materials of the nano-scale nickel-cobalt-manganese coating layer are all carbonate compounds. By utilizing a precipitation conversion reaction, phosphate ions on the surface of the lithium iron phosphate positive electrode material react with the carbonate compounds of the nickel-cobalt-manganese alloy, and the nickel-cobalt-manganese alloy ions react and connect with the phosphates on the surface of the lithium iron phosphate positive electrode material. Finally, lithium salts are mixed and calcined to form a nano-scale nickel-cobalt-manganese coating layer. Compared with general solid-solid phase mechanical mixing coating, the reaction between the solid surface and the solution in the solid-liquid chemical reaction is more sufficient, which is beneficial to the uniformity and consistency of the nano-scale nickel-cobalt-manganese coating layer, can amplify the effect of the nano-scale nickel-cobalt-manganese coating layer, and improve the low-temperature performance of the obtained spherical lithium iron phosphate positive electrode material.

[0016] In a second aspect, the present invention provides a method for preparing a spherical lithium iron phosphate positive electrode material, comprising the following process steps: S1. Add an iron source, a phosphorus source and a lithium source to a solvent, mix well, add a pore-forming agent and a dispersant, stir and dissolve for 2 to 3 hours under a nitrogen atmosphere, then react at 180 to 200 ° C for 12 to 24 hours, and finally filter, wash and spray dry to obtain spherical lithium iron phosphate particles; S2. Add nickel carbonate, cobalt carbonate and manganese carbonate to an acidic aqueous solution, stir and dissolve to form a carbonate mixed solution, then add spherical lithium iron phosphate particles, raise the temperature to 80-85°C, stir and react for 40-60 minutes, then add lithium salt, stir and dissolve, calcine at 450-500°C for 5-6 hours to form a nano-scale nickel-cobalt-manganese coating layer, and finally grind to obtain a primary coated spherical lithium iron phosphate; S3. Add the once coated spherical lithium iron phosphate to the organic carbon source aqueous solution, immerse for 24 to 48 hours, and finally obtain the spherical lithium iron phosphate positive electrode material through suction filtration, drying and calcination.

[0017] Preferably, the concentration of the carbonate mixed solution is 0.15-0.25 g / mL; the mass volume ratio of the spherical lithium iron phosphate particles to the carbonate mixed solution is 1 g: (4-6) mL.

[0018] Preferably, the molar ratio of the phosphorus source, the iron source and the lithium source is 1:(0.9-1.05):(0.95-1.1).

[0019] Preferably, the phosphorus source includes a combination of one or more of phosphoric acid, ammonium dihydrogen phosphate, diammonium hydrogen phosphate, lithium hydrogen phosphate, lithium dihydrogen phosphate, sodium phosphate and potassium phosphate.

[0020] Preferably, the iron source comprises a combination of one or more of ferric nitrate, ferric sulfate, ferric carbonate, ferric chloride, ferrous nitrate and ferric oxide.

[0021] Preferably, the lithium source includes one or a combination of lithium hydroxide, lithium acetate, lithium carbonate, lithium oxalate, lithium chloride and lithium acetate.

[0022] Preferably, the acidic aqueous solution includes any one of a hydrochloric acid aqueous solution, a nitric acid aqueous solution and a sulfuric acid aqueous solution with a mass fraction of 5 to 10%.

[0023] Preferably, the calcination temperature in step S3 is 500-550° C. and the calcination time is 5-6 hours.

[0024] By adopting the above technical scheme, the phosphorus source, iron source and lithium source first form spherical lithium iron phosphate particles with porous surface and uniform particle size distribution with the assistance of pore-forming agent and dispersant, and then disperse them in a mixed solution of carbonate, and the phosphate on the surface of the spherical lithium iron phosphate particles will react with the carbonate of nickel, cobalt and manganese, gradually combining the nickel, cobalt and manganese alloy ions on the surface of the spherical lithium iron phosphate particles, and then adding lithium salt to combine with the nickel, cobalt and manganese alloy ions, and finally calcining to coat a nano-scale nickel, cobalt and manganese coating layer on the surface of the spherical lithium iron phosphate particles. Finally, the spherical lithium iron phosphate particles that have been coated once are dispersed in an organic carbon source aqueous solution, and a carbon layer is formed by calcining to obtain a spherical lithium iron phosphate positive electrode material with a multi-layer coating structure.

[0025] Preferably, the pore-forming agent includes a combination of one or more of urea, ammonium bicarbonate, ammonium oxalate and ammonium carbonate; the molar ratio of the pore-forming agent to the phosphorus source is (0.4-0.5):1.

[0026] Preferably, the dispersant includes a combination of one or more of citric acid, oxalic acid and polyvinyl pyrrolidone; the molar ratio of the dispersant to the phosphorus source is (0.2-0.3):1.

[0027] By adopting the above technical solution, pore-forming agent and dispersant are added in the process of forming spherical lithium iron phosphate particles. The dispersant can prevent the spherical lithium iron phosphate particles from agglomerating during the synthesis process, ensuring that the obtained spherical particles are uniform in size and evenly distributed.

[0028] The pore-forming agent can decompose during the high-temperature reaction, release gas and then further form mesopores on the surface of the spherical lithium iron phosphate particles, which can increase the specific surface area. The porous structure provides a surface diffusion channel for lithium ions and improves the solid-phase diffusion coefficient. In addition, the surface porous spherical lithium iron phosphate particles can also enhance the mechanical interlocking effect with the nano-scale nickel-cobalt-manganese coating, thereby enhancing the bonding force with the nano-scale nickel-cobalt-manganese coating and optimizing the transmission path of lithium ions and charges.

[0029] By changing the structural characteristics of spherical lithium iron phosphate particles with the assistance of dispersants and pore-forming agents, the active sites of spherical lithium iron phosphate particles can be more effectively improved, and the contact resistance between particles can be reduced. The multi-level pores will greatly shorten the solid-phase diffusion distance of lithium ions, reduce the charge transfer impedance, and improve the capacity retention rate at low temperatures. It can effectively maintain the effective penetration of the electrolyte at low temperatures, thereby improving the low-temperature performance of the spherical lithium iron phosphate material positive electrode material.

[0030] Preferably, the organic carbon source includes a combination of one or more of glucose, sucrose and starch; the amount of the organic carbon source added is 5 to 8% of the mass of the primary coated spherical lithium iron phosphate.

[0031] By adopting the above technical solution, the present invention uses an organic carbon source to coat lithium iron phosphate particles and forms a conductive carbon layer after calcination. The continuous conductive network formed by the carbon layer can constitute an electron transmission path throughout the electrode, thereby improving the overall electronic conductivity.

[0032] More importantly, under low-temperature cycles, the structural stability of the material and the side reactions with the electrolyte will intensify, and the manganese ions and cobalt ions in the nano-scale nickel-cobalt-manganese coating are easily dissolved in this process, resulting in a decrease in the overall structural stability of the material and a decrease in rate performance. After carbon coating, the carbon layer can act as a physical barrier to effectively reduce the dissolution of manganese and cobalt ions, avoiding the problem of decreased rate performance of spherical lithium iron phosphate cathode materials caused by loss of active substances at low temperatures, thereby greatly improving the stability of the surface structure and improving the low-temperature performance of spherical lithium iron phosphate cathode materials.

[0033] Beneficial effects of the present invention: The spherical lithium iron phosphate positive electrode material of the present invention has a relatively high tap density and a large specific surface area, which can reduce the solid phase diffusion path of lithium ions between lithium iron phosphate particles, reduce the particle contact resistance, and reduce the impact of low temperature environment on the transmission efficiency of lithium ions and electrons.

[0034] The spherical lithium iron phosphate positive electrode material of the present invention is coated with a nano-scale nickel-cobalt-manganese coating layer, which can improve the overall electronic conductivity and lithium ion diffusion rate, reduce the lithium ion migration barrier, and improve the low-temperature performance of the spherical lithium iron phosphate positive electrode material. In order to avoid the problem of reduced rate performance caused by uneven coating of the nickel-cobalt-manganese ternary material, the present invention adopts an in-situ coating method, using the reaction between the carbonate compound of the nickel-cobalt-manganese alloy and the phosphate radical to obtain a nano-scale nickel-cobalt-manganese coating layer with good interface bonding and uniformity.

[0035] The outermost layer of the spherical lithium iron phosphate positive electrode material of the present invention is also coated with a carbon layer. In addition to being able to form a highly conductive network, the carbon layer can also serve as a physical barrier to effectively prevent the dissolution of cobalt ions and manganese ions in the nano-scale nickel-cobalt-manganese coating layer, thereby effectively avoiding the problem of decreased rate performance of the spherical lithium iron phosphate positive electrode material due to loss of active substances at low temperatures. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] The present invention will be further described below in conjunction with the accompanying drawings.

[0037] Figure 1 It is a scanning electron microscope image of Example 1 of the present invention. DETAILED DESCRIPTION

[0038] The following will be combined with the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0039] Example Example 1: A spherical lithium iron phosphate positive electrode material is prepared according to the following method: S1. Add ferric nitrate, ammonium dihydrogen phosphate and lithium hydroxide to ethylene glycol, wherein the molar ratio of ammonium dihydrogen phosphate, ferric nitrate and lithium hydroxide is 1:0.95:1; after mixing evenly, add urea and citric acid, wherein the molar ratio of ammonium dihydrogen phosphate to urea is 1:0.4, and the molar ratio of ammonium dihydrogen phosphate to citric acid is 1:0.3; stir and dissolve under nitrogen atmosphere for 2h, then react at 200°C for 15h, and finally obtain spherical lithium iron phosphate particles by suction filtration, washing and spray drying; S2. Add nickel carbonate, cobalt carbonate and manganese carbonate to an 8% hydrochloric acid aqueous solution, stir and dissolve to form a carbonate mixed solution, wherein the concentration of the carbonate mixed solution is 0.2g / mL, then add spherical lithium iron phosphate particles, the mass volume ratio of the spherical lithium iron phosphate particles to the carbonate mixed solution is 1g:5mL; increase the temperature to 85°C, stir and react for 40 to 60 minutes, and then add lithium carbonate, wherein the molar ratio of nickel carbonate, manganese carbonate, cobalt carbonate and lithium carbonate is 0.13:0.54:0.13:1.2; stir and dissolve, calcine at 500°C for 6h to form a nano-scale nickel-cobalt-manganese coating layer, and finally grind to obtain a primary coated spherical lithium iron phosphate; S3. Add the once coated spherical lithium iron phosphate into the glucose aqueous solution, wherein the amount of glucose added is 6% of the mass of the once coated spherical lithium iron phosphate; immerse for 48 hours, and finally filter, dry and calcine to obtain the spherical lithium iron phosphate positive electrode material, wherein the calcination temperature is 550°C; and the calcination time is 5 hours.

[0040] Example 2, a spherical lithium iron phosphate positive electrode material, differs from Example 1 only in that the molar ratio of diammonium phosphate, ferric nitrate and lithium hydroxide is 1:0.9:0.95; the molar ratio of diammonium phosphate to urea is 1:0.4; and the molar ratio of diammonium phosphate to citric acid is 1:0.2.

[0041] Example 3, a spherical lithium iron phosphate positive electrode material, is different from Example 1 only in that the molar ratio of diammonium phosphate, ferric nitrate and lithium hydroxide is 1:1.05:1.1; the molar ratio of diammonium phosphate to urea is 1:0.5; and the molar ratio of diammonium phosphate to citric acid is 1:0.3.

[0042] Example 4, a spherical lithium iron phosphate positive electrode material, is different from Example 1 only in that the molar ratio of nickel carbonate, manganese carbonate, cobalt carbonate and lithium carbonate is 0.11:0.57:0.12:1.2.

[0043] Example 5, a spherical lithium iron phosphate positive electrode material, is different from Example 1 only in that the molar ratio of nickel carbonate, manganese carbonate, cobalt carbonate and lithium carbonate is 0.15:0.5:0.15:1.2.

[0044] Example 6, a spherical lithium iron phosphate positive electrode material, differs from Example 1 only in that the mass volume ratio of spherical lithium iron phosphate particles to carbonate mixed solution is 1g:4mL; the amount of glucose added is 8% of the mass of the once-coated spherical lithium iron phosphate.

[0045] Example 7, a spherical lithium iron phosphate positive electrode material, differs from Example 1 only in that the mass volume ratio of spherical lithium iron phosphate particles to carbonate mixed solution is 1g:6mL; the amount of glucose added is 5% of the mass of the spherical lithium iron phosphate coated once.

[0046] Example 8, a spherical lithium iron phosphate positive electrode material, is different from Example 1 only in that the mass volume ratio of the spherical lithium iron phosphate particles to the carbonate mixed solution is 1 g:2 mL.

[0047] Example 9, a spherical lithium iron phosphate positive electrode material, is different from Example 1 only in that the mass volume ratio of the spherical lithium iron phosphate particles to the carbonate mixed solution is 1 g:8 mL.

[0048] Comparative Example Comparative Example 1: A spherical lithium iron phosphate positive electrode material is prepared according to the following method: S1. Add ferric nitrate, ammonium dihydrogen phosphate and lithium hydroxide to ethylene glycol, wherein the molar ratio of ammonium dihydrogen phosphate, ferric nitrate and lithium hydroxide is 1:0.95:1; after mixing evenly, add urea and citric acid, wherein the molar ratio of ammonium dihydrogen phosphate to urea is 1:0.4, and the molar ratio of ammonium dihydrogen phosphate to citric acid is 1:0.3; stir and dissolve under nitrogen atmosphere for 2h, then react at 200°C for 15h, and finally obtain spherical lithium iron phosphate particles by suction filtration, washing and spray drying; S3. Add spherical lithium iron phosphate to a glucose aqueous solution, wherein the amount of glucose added is 6% of the mass of the spherical lithium iron phosphate; immerse for 48 hours, and finally obtain a spherical lithium iron phosphate positive electrode material by suction filtration, drying and calcination, wherein the calcination temperature is 550°C; and the calcination time is 5 hours.

[0049] Comparative Example 2, a spherical lithium iron phosphate positive electrode material is prepared according to the following method: S1. Add ferric nitrate, ammonium dihydrogen phosphate and lithium hydroxide to ethylene glycol, wherein the molar ratio of ammonium dihydrogen phosphate, ferric nitrate and lithium hydroxide is 1:0.95:1; after mixing evenly, add urea and citric acid, wherein the molar ratio of ammonium dihydrogen phosphate to urea is 1:0.4, and the molar ratio of ammonium dihydrogen phosphate to citric acid is 1:0.3; stir and dissolve under nitrogen atmosphere for 2h, then react at 200°C for 15h, and finally obtain spherical lithium iron phosphate particles by suction filtration, washing and spray drying; S2. Add nickel carbonate, cobalt carbonate and manganese carbonate to an 8% by mass aqueous solution of hydrochloric acid, stir and dissolve to form a carbonate mixed solution, wherein the concentration of the carbonate mixed solution is 0.2 g / mL, then add spherical lithium iron phosphate particles, the mass volume ratio of the spherical lithium iron phosphate particles to the carbonate mixed solution is 1 g: 5 mL; increase the temperature to 85°C, stir and react for 40 to 60 minutes, and then add lithium carbonate, wherein the molar ratio of nickel carbonate, manganese carbonate, cobalt carbonate and lithium carbonate is 0.13:0.54:0.13:1.2; stir and dissolve, calcine at 500°C for 6 hours to form a nano-scale nickel-cobalt-manganese coating layer, and finally grind to obtain a once-coated spherical lithium iron phosphate, i.e., a spherical lithium iron phosphate positive electrode material.

[0050] Comparative Example 3, a spherical lithium iron phosphate positive electrode material, is different from Example 1 only in that urea is not added in step S1.

[0051] Comparative Example 4, a spherical lithium iron phosphate positive electrode material, is different from Example 1 only in that citric acid is not added in step S1.

[0052] Comparative Example 5, a lithium iron phosphate positive electrode material is prepared according to the following method: S1. Add ferric nitrate, ammonium dihydrogen phosphate and lithium hydroxide to ethylene glycol, wherein the molar ratio of ammonium dihydrogen phosphate, ferric nitrate and lithium hydroxide is 1:0.95:1; after mixing evenly, add urea and citric acid, wherein the molar ratio of ammonium dihydrogen phosphate to urea is 1:0.4, and the molar ratio of ammonium dihydrogen phosphate to citric acid is 1:0.3; stir and dissolve under nitrogen atmosphere for 2h, then react at 200°C for 6h, and finally obtain lithium iron phosphate particles by suction filtration, washing and drying; S2. Add nickel carbonate, cobalt carbonate and manganese carbonate to an aqueous solution of hydrochloric acid with a mass fraction of 8%, stir and dissolve to form a carbonate mixed solution, wherein the concentration of the carbonate mixed solution is 0.2g / mL, then add lithium iron phosphate particles, and the mass volume ratio of lithium iron phosphate particles to the carbonate mixed solution is 1g:5mL; increase the temperature to 85°C, stir and react for 40 to 60 minutes, and then add lithium carbonate, wherein the molar ratio of nickel carbonate, manganese carbonate, cobalt carbonate and lithium carbonate is 0.13:0.54:0.13:1.2; stir and dissolve, calcine at 500°C for 6h to form a nano-scale nickel-cobalt-manganese coating layer, and finally grind to obtain a primary coated lithium iron phosphate; S3. Add the once coated lithium iron phosphate into the glucose aqueous solution, wherein the added amount of glucose is 6% of the mass of the once coated lithium iron phosphate; immerse for 48 hours, and finally obtain the lithium iron phosphate positive electrode material through suction filtration, drying and calcination, wherein the calcination temperature is 550°C; and the calcination time is 5 hours.

[0053] Performance testing Sample preparation: The positive electrode materials obtained in the examples and comparative examples were mixed with a conductive agent (Super P) and a PVDF solution dissolved in NMP in a ratio of 90:5:5, respectively, and then coated on a carbon-coated aluminum foil. After vacuum drying at 110°C, they were pressed into a 12 mm diameter disc as the positive electrode, a metal lithium sheet as the negative electrode, a Celgard 2300 microporous membrane as the separator, and a 1.0 mol / L LiPF6 solution of ethylene carbonate (EC): dimethyl carbonate (DMC) = 1:1 as the electrolyte. The cells were assembled into R2025 button cells in a glove box as samples for electrical performance testing.

[0054] Performance Testing: (1) After standing for 8 hours at room temperature (25°C), the gram capacity (mAh / g) at 0.2C and 1C discharge rates were tested respectively.

[0055] (2) Place the sample battery in a -20℃ constant temperature box for 1 hour and then take it out to test the retention rate (%) of gram capacity at a discharge rate of 1C.

[0056] The above test results are shown in Table 1:

[0057] According to Table 1, in combination with Example 1, Example 8, Example 9 and Comparative Example 1, it can be seen that the retention rate of the capacitance at low temperature in Example 8, Example 9 and Comparative Example 1 is reduced compared with Example 1, and the reduction in Comparative Example 1 is more obvious, indicating that the low-temperature performance of Example 8, Example 9 and Comparative Example 1 is reduced compared with Example 1. The reason is that the difference between Example 8, Example 9 and Comparative Example 1 compared with Example 1 is only that the content of the nano-scale nickel-cobalt-manganese coating layer coated on the surface of the spherical lithium iron phosphate particles is adjusted, wherein the nano-scale nickel-cobalt-manganese coating layer coated in Example 8 is less, which will lead to an increase in charge transfer impedance, affect the diffusion of lithium ions at low temperatures, and extend the migration path of lithium ions in low temperature environments; and the coating layer is less and the coverage is low, which will lead to an increase in side reactions at the interface during the low-temperature cycle, and the structural stability is reduced, and the phenomenon of local structural collapse may occur during low-temperature charging and discharging. In Comparative Example 1, there is no nano-scale nickel-cobalt-manganese coating layer, and the low-temperature performance of the obtained positive electrode material is more significantly reduced. In Example 9, the number of nano-scale nickel-cobalt-manganese coating layers increased, resulting in an increase in the thickness of the coating layer, which will also increase and extend the diffusion path of lithium ions, and an overly thick coating layer will further increase the diffusion resistance, resulting in an increase in the polarization voltage. Moreover, due to the mismatch between the thermal expansion coefficient of the nickel-cobalt-manganese ternary material and lithium iron phosphate, the effect is more obvious when blended. In the present invention, too many coating layers will lead to greater interfacial stress in low-temperature cycles, causing the coating layer to crack or peel off, thereby affecting the low-temperature performance of the material.

[0058] Combining Example 1 and Comparative Example 2, it can be seen that the low-temperature performance of Comparative Example 2 is significantly reduced compared with that of Example 1. The reason is that the spherical lithium iron phosphate positive electrode material in Comparative Example 2 is not coated with a carbon layer, which causes a significant decrease in the conductivity of the positive electrode material and a decrease in the electron transfer efficiency; and lacks the protective effect of the carbon layer, and the manganese ions and cobalt ions in the nano-scale nickel-cobalt-manganese coating layer are easily dissolved during low-temperature cycles, resulting in a decrease in the overall structural stability of the material, thereby affecting the low-temperature performance.

[0059] In combination with Example 1, Comparative Example 3 and Comparative Example 4, it can be seen that the low-temperature performance of Comparative Example 3 and Comparative Example 4 is lower than that of Example 1. The reason is that in Comparative Example 3, no pore-forming agent is added during the preparation of the spheroidized lithium iron phosphate particles, and the surface mesoporous structure of the obtained spheroidized lithium iron phosphate particles is greatly reduced, the specific surface area is reduced, the solid phase diffusion coefficient is reduced, and the diffusion efficiency of lithium ions is reduced; in Comparative Example 4, no dispersant is added during the preparation of the spheroidized lithium iron phosphate particles. The reduction of dispersant will make the spheroidized lithium iron phosphate particles easy to agglomerate during the synthesis process, resulting in the obtained spheroidized lithium iron phosphate particles being uneven in size, seriously affecting the subsequent coating treatment and the capacitance of the spherical lithium iron phosphate positive electrode material.

[0060] Combining Example 1 and Comparative Example 5, it can be seen that the low-temperature performance of Comparative Example 5 is lower than that of Example 1. The reason is that the lithium iron phosphate particles synthesized in Comparative Example 5 are not uniformly distributed non-spherical particles. The lithium iron phosphate particles of different shapes and sizes will affect the uniformity of the coating layer on the one hand, and on the other hand, the tap density and specific surface area of ​​the obtained lithium iron phosphate positive electrode material are reduced, thereby increasing the contact resistance between the particles, and the transmission efficiency of electrons and lithium ions is reduced in a low temperature environment. The problem of increased barriers to lithium ion migration will also increase, and the impedance between the particles will increase, resulting in a decrease in the low-temperature performance of the obtained lithium iron phosphate positive electrode material.

[0061] It should be noted that, in this article, relational terms such as first and second, etc. are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "include", "comprise" or any other variants thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device.

[0062] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A spherical lithium iron phosphate positive electrode material, characterized in that: The spherical lithium iron phosphate positive electrode material is a multi-layer coating structure; the spherical lithium iron phosphate positive electrode material is composed of spherical lithium iron phosphate particles, a nano-scale nickel-cobalt-manganese coating layer and a carbon layer from the inside to the outside.

2. The spherical lithium iron phosphate positive electrode material according to claim 1, characterized in that: The thickness of the nano-scale nickel-cobalt-manganese coating layer is 5 to 30 nm.

3. The spherical lithium iron phosphate positive electrode material according to claim 1, characterized in that: The raw materials of the nano-scale nickel-cobalt-manganese coating layer include nickel carbonate, manganese carbonate, cobalt carbonate and lithium salt in a molar ratio of (0.1-0.15): (0.5-0.6): (0.1-0.15): (1.1-1.3).

4. The spherical lithium iron phosphate positive electrode material according to claim 3, characterized in that: The lithium salt includes one or more of lithium carbonate, lithium phosphate and lithium hydroxide.

5. The method for preparing a spherical lithium iron phosphate positive electrode material according to any one of claims 1 to 4, characterized in that: The process steps include: S1. Add an iron source, a phosphorus source and a lithium source to a solvent, mix well, add a pore-forming agent and a dispersant, stir and dissolve for 2 to 3 hours under a nitrogen atmosphere, then react at 180 to 200 ° C for 12 to 24 hours, and finally filter, wash and spray dry to obtain spherical lithium iron phosphate particles; S2. Add nickel carbonate, cobalt carbonate and manganese carbonate to an acidic aqueous solution, stir and dissolve to form a carbonate mixed solution, then add spherical lithium iron phosphate particles, raise the temperature to 80-85°C, stir and react for 40-60 minutes, then add lithium salt, stir and dissolve, calcine at 450-500°C for 5-6 hours to form a nano-scale nickel-cobalt-manganese coating layer, and finally grind to obtain a primary coated spherical lithium iron phosphate; S3. Add the once coated spherical lithium iron phosphate to the organic carbon source aqueous solution, immerse for 24 to 48 hours, and finally obtain the spherical lithium iron phosphate positive electrode material through suction filtration, drying and calcination.

6. The method for preparing a spherical lithium iron phosphate positive electrode material according to claim 5, characterized in that: The concentration of the carbonate mixed solution is 0.15-0.25 g / mL; the mass volume ratio of the spherical lithium iron phosphate particles to the carbonate mixed solution is 1 g: (4-6) mL.

7. The method for preparing a spherical lithium iron phosphate positive electrode material according to claim 5, characterized in that: The molar ratio of the phosphorus source, the iron source and the lithium source is 1:(0.9-1.05):(0.95-1.1).

8. The method for preparing a spherical lithium iron phosphate positive electrode material according to claim 5, characterized in that: The pore-forming agent comprises a combination of one or more of urea, ammonium bicarbonate, ammonium oxalate and ammonium carbonate; the molar ratio of the pore-forming agent to the phosphorus source is (0.4-0.5):

1.

9. The method for preparing a spherical lithium iron phosphate positive electrode material according to claim 5, characterized in that: The dispersant includes a combination of one or more of citric acid, oxalic acid and polyvinyl pyrrolidone; the molar ratio of the dispersant to the phosphorus source is (0.2-0.3):

1.

10. The method for preparing a spherical lithium iron phosphate positive electrode material according to claim 5, characterized in that: The organic carbon source comprises a combination of one or more of glucose, sucrose and starch; the added amount of the organic carbon source is 5-8% of the mass of the primary coated spherical lithium iron phosphate.

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