Positive electrode active material, positive electrode, battery, and power-driven device

Abstract

The application provides a positive electrode active material, which comprises a carbon core, a lithium iron manganese phosphate layer covering the carbon core, and a carbon layer covering the lithium iron manganese phosphate layer. The positive electrode active material has excellent rate capability and low-temperature performance, and is beneficial to use. The application further provides a positive electrode, a battery and an electric device.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of batteries, in particular to a positive electrode active material, a positive electrode, a battery and an electric device. BACKGROUND

[0002] Lithium iron phosphate is widely used in battery positive electrodes due to its high cycle stability and safety, however, the energy density of lithium iron phosphate is low, which cannot meet the use requirements of batteries. Therefore, manganese-doped lithium iron manganese phosphate material with high energy density is obtained by doping manganese on lithium iron phosphate. However, the low electrical conductivity of lithium manganese iron phosphate also affects the improvement of battery performance, and thus further research and improvement of lithium manganese iron phosphate material are needed. SUMMARY

[0003] Therefore, the present application provides a positive electrode active material, a positive electrode, a battery and an electric device, the conductive performance of the lithium iron phosphate layer is improved by setting a carbon core and a carbon layer, so that the positive electrode active material has excellent electrical conductivity, specific capacity and energy density, and the positive electrode and the battery with the positive electrode active material have excellent low-temperature performance and high-rate performance.

[0004] In a first aspect, the present application provides a positive electrode active material, comprising a carbon core, a lithium manganese iron phosphate layer covering the carbon core, and a carbon layer covering the lithium manganese iron phosphate layer.

[0005] Optionally, the lithium manganese iron phosphate layer comprises lithium manganese iron phosphate particles, and the particle size of the lithium manganese iron phosphate particles is 100 nm-400 nm.

[0006] Optionally, the mass content of the lithium manganese iron phosphate layer in the positive electrode active material is 92%-98.9%.

[0007] Optionally, the particle size of the carbon core is 10 nm-50 nm.

[0008] Optionally, the mass content of the carbon core in the positive electrode active material is 0.1%-5%.

[0009] Optionally, the thickness of the carbon layer is 1 nm-20 nm.

[0010] Optionally, the mass content of the carbon layer in the positive electrode active material is 0.1%-3%.

[0011] Optionally, the coating rate of the lithium manganese iron phosphate layer is 20%-80%.

[0012] Optionally, the coating rate of the carbon layer is 20%-80%.

[0013] Optionally, the lithium manganese iron phosphate layer comprises LiFe 1-α Mn αPO4, and a is 0.6-0.8.

[0014] In a second aspect, the present application provides a positive electrode, comprising a positive electrode current collector and a positive electrode active material layer arranged on the surface of the positive electrode current collector, wherein the positive electrode active material layer comprises the positive electrode active material according to the first aspect.

[0015] In a third aspect, the present application provides a battery, comprising a negative electrode and the positive electrode according to the second aspect.

[0016] In a fourth aspect, the present application provides an electric device, comprising the battery according to the third aspect.

[0017] The positive electrode active material provided by the present application has high conductivity due to the carbon core and the carbon layer, thereby obtaining a positive electrode active material with excellent conductivity, which helps to improve the specific capacity, low-temperature performance and rate performance of the positive electrode active material, and is conducive to the use of the positive electrode active material in the positive electrode and the battery, improves the overall performance of the battery, and helps to use the battery in the electric device and improve the use performance of the electric device. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments or the prior art description will be briefly introduced. The specific embodiments described herein are only used to explain the present application and do not limit the present application.

[0019] Figure 1 A cross-sectional schematic diagram of the positive electrode active material provided by an embodiment of the present application.

[0020] Figure 2 A cross-sectional schematic diagram of the positive electrode provided by an embodiment of the present application. DETAILED DESCRIPTION

[0021] The technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative labor fall within the scope of protection of the present application.

[0022] Please refer to Figure 1A cross-sectional schematic view of a positive electrode active material according to an embodiment of the present application is provided, wherein the positive electrode active material 10 comprises a carbon core 11, a lithium manganese iron phosphate layer 12 covering the carbon core 11, and a carbon layer 13 covering the lithium manganese iron phosphate layer 12. Compared with lithium iron phosphate, the electronic conductivity of lithium manganese iron phosphate is low, and the large resistance in the crystal lattice leads to a low ion diffusion coefficient, affecting the rate performance, and the activity of lithium manganese iron phosphate decreases at low temperature conditions (for example, subzero temperature, such as -10℃, -15℃, -20℃, -30℃, etc.), exacerbating the decrease in conductivity, limiting the use of lithium manganese iron phosphate. In the present application, a carbon core is arranged inside the lithium manganese iron phosphate layer, and a carbon layer is arranged outside the lithium manganese iron phosphate layer, so that a conductive network is formed inside and outside the lithium manganese iron phosphate, which is conducive to the transmission of electrons, improves the electronic conductivity of the positive electrode active material, and also provides an effective lithium ion diffusion channel to improve the rate performance of the positive electrode active material; at the same time, the positive electrode active material can still maintain a high level of conductivity at low temperature conditions, has excellent low temperature performance, and is conducive to the application of the positive electrode active material.

[0023] The inventors have found that when a carbon layer is coated on the surface of lithium manganese iron phosphate particles, a conductive network can be formed on the surface of lithium manganese iron phosphate, which improves the conductive performance of lithium manganese iron phosphate close to the carbon layer, but the carbon layer has little effect on lithium manganese iron phosphate far from the carbon layer, that is, the carbon coating only improves part of the conductive performance of lithium manganese iron phosphate, and the conductive performance inside lithium manganese iron phosphate is still very low, which is not conducive to the use of lithium manganese iron phosphate. Therefore, in the present application, a carbon core is arranged inside the lithium manganese iron phosphate layer to improve the conductivity inside the lithium manganese iron phosphate layer, thereby improving the rate performance and low temperature performance of the positive electrode active material.

[0024] In an embodiment of the present application, the mass content of the carbon core in the positive electrode active material is 0.1%-5%, which is conducive to further improving the conductivity of the positive electrode active material, and also ensures the mass proportion of the lithium manganese iron phosphate layer in the positive electrode active material, thereby helping to further improve the rate performance and low temperature performance of the positive electrode active material. Specifically, the mass content of the carbon core in the positive electrode active material can be, but is not limited to, 0.1%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, or 5%, etc. In an embodiment, the mass content of the carbon core in the positive electrode active material can be 0.3%-5%. In another embodiment, the mass content of the carbon core in the positive electrode active material can be 0.3%-3%. In yet another embodiment, the mass content of the carbon core in the positive electrode active material can be 0.3%-2%, which is conducive to further improving the low temperature capacity retention rate and discharge capacity retention rate of the positive electrode active material.

[0025] In an embodiment of the present application, the particle size of the carbon core is 10 nm-50 nm. The above particle size range not only improves the electrical conductivity of the positive electrode active material, but also ensures the mass proportion of the lithium manganese iron phosphate layer in the positive electrode active material, thereby helping to further improve the rate performance and low-temperature performance of the positive electrode active material, and the carbon core is not difficult to prepare, which is conducive to the reduction of the preparation cost of the positive electrode active material. Specifically, the particle size of the carbon core can be, but is not limited to, 10 nm, 15 nm, 20 nm, 25 nm, 30 nm, 35 nm, 40 nm, 45 nm or 50 nm, etc. In an embodiment, the particle size of the carbon core can be 20 nm-50 nm.

[0026] In the present application, the material of the carbon core is not limited and can be selected as needed. Specifically, the material of the carbon core can be, but is not limited to, graphite, soft carbon, hard carbon, etc.

[0027] Lithium manganese iron phosphate has a wide source, high theoretical specific capacity, and high platform voltage, and can obtain a high-energy-density positive electrode and battery. In the present application, the lithium manganese iron phosphate layer in the positive electrode active material can be uniformly coated on the surface of the carbon core. Compared with the lithium manganese iron phosphate particles directly coated with a carbon layer, the use of the lithium manganese iron phosphate layer in the present application is also conducive to improving the wetting effect of the electrolyte, shortening the ion migration path, and improving the use performance of the positive electrode active material. In an embodiment of the present application, the mass content of the lithium manganese iron phosphate layer in the positive electrode active material is 92%-98.9%, which is conducive to improving the specific capacity and energy density of the positive electrode active material, while also ensuring the electrical conductivity of the positive electrode active material, which is conducive to the use of the positive electrode active material. Specifically, the mass content of the lithium manganese iron phosphate layer in the positive electrode active material can be, but is not limited to, 92%, 92.5%, 93%, 93.2%, 94%, 95%, 95.5%, 96%, 96.2%, 97%, 97.2%, 97.9%, 98%, 98.7% or 98.9%, etc. In an embodiment, the mass content of the lithium manganese iron phosphate layer in the positive electrode active material can be 92%-98%. In another embodiment, the mass content of the lithium manganese iron phosphate layer in the positive electrode active material can be 93%-97.5%, which is conducive to further improving the performance of the positive electrode active material. In yet another embodiment, the mass content of the lithium manganese iron phosphate layer in the positive electrode active material can be 96.2%-97.9%.

[0028] In the related art, the transmission path of ions is reduced by reducing the particle size of lithium manganese iron phosphate. In the present application, a carbon core is arranged inside the lithium manganese iron phosphate layer and a carbon layer is arranged outside, so as to shorten the transmission path of ions and improve the performance of the positive electrode active material. The particle size of lithium manganese iron phosphate does not need to be specially controlled, which is more conducive to the preparation and use of the positive electrode active material. In an embodiment of the present application, the lithium manganese iron phosphate layer includes lithium manganese iron phosphate particles, and the particle size of the lithium manganese iron phosphate particles is 100 nm-400 nm, which is conducive to the conduction and movement of electrons and ions, further improves the rate performance and low-temperature performance of the positive electrode active material, and the preparation difficulty of lithium manganese iron phosphate is low, which reduces the preparation cost of the positive electrode active material. Specifically, the particle size of the lithium manganese iron phosphate particles can be, but is not limited to, 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, 350 nm or 400 nm, etc. In an embodiment, the particle size of the lithium manganese iron phosphate particles can be 200 nm-600 nm. In another embodiment, the particle size of the lithium manganese iron phosphate particles can be 300 nm-400 nm. In yet another embodiment, the particle size of the lithium manganese iron phosphate particles can be 100 nm-350 nm.

[0029] In an embodiment of the present application, the lithium manganese iron phosphate layer includes LiFe 1-α Mn α PO4, and α is 0.6-0.8, which is conducive to improving the energy density and reducing the influence of Jahn-Teller effect as much as possible, and improving the comprehensive performance of the positive electrode active material.

[0030] In an embodiment of the present application, the mass content of manganese element in the lithium manganese iron phosphate layer is 0.2%-0.8%, which can improve the platform voltage of lithium manganese iron phosphate, thereby improving the energy density of the battery with the positive electrode active material, and can also ensure the discharge specific capacity of the positive electrode active material, so that the positive electrode active material has excellent comprehensive performance. Specifically, the mass content of manganese element in the lithium manganese iron phosphate layer can be, but is not limited to, 0.2%, 0.3%, 0.4%, 0.5%, 0.6%, 0.7% or 0.8%, etc. In an embodiment, the mass content of manganese element in the lithium manganese iron phosphate layer can be 0.2%-0.5%. In another embodiment, the mass content of manganese element in the lithium manganese iron phosphate layer can be 0.3%-0.7%. In yet another embodiment, the mass content of manganese element in the lithium manganese iron phosphate layer can be 0.5%-0.8%.

[0031] In an embodiment of the present application, the mass content of manganese in the lithium manganese iron phosphate layer gradually decreases in the direction from the carbon core to the carbon layer, so that the positive electrode active material has a high voltage platform and a high capacity, and the performance of the lithium manganese iron phosphate layer close to the outside is closer to that of lithium iron phosphate, and has better cycle performance, thereby improving the use performance of the positive electrode active material. Specifically, the mass content of manganese in the lithium manganese iron phosphate layer can be linearly reduced, gradiently reduced, or irregularly reduced.

[0032] In an embodiment of the present application, the material of the lithium manganese iron phosphate layer includes lithium manganese iron phosphate, and the lithium manganese iron phosphate has a doping element. Specifically, the doping element includes at least one of magnesium, titanium, cobalt, nickel, and vanadium. The addition of the doping element is beneficial to further improving the rate performance, low-temperature performance, specific capacity, etc. of the lithium manganese iron phosphate. In an embodiment of the present application, the mass content of the doping element in the lithium manganese iron phosphate can be 500 ppm-6000 ppm. Specifically, the mass content of the doping element in the lithium manganese iron phosphate can be, but is not limited to, 600 ppm, 900 ppm, 1000 ppm, 1500 ppm, 2000 ppm, 2500 ppm, 3000 ppm, 4000 ppm, 5000 ppm, or 5500 ppm, etc.

[0033] The coating rate of the lithium manganese iron phosphate layer refers to the ratio of the surface area of the carbon core covered by the lithium manganese iron phosphate layer to the total surface area of the carbon core. In an embodiment of the present application, the coating rate of the lithium manganese iron phosphate layer is greater than or equal to 20%, which is beneficial to the improvement of the conductivity of the lithium manganese iron phosphate and the performance of the positive electrode active material. Specifically, the coating rate of the lithium manganese iron phosphate layer can be, but is not limited to, greater than or equal to 20%, greater than or equal to 30%, greater than or equal to 40%, greater than or equal to 50%, greater than or equal to 60%, greater than or equal to 70%, greater than or equal to 80%, greater than or equal to 90%, or 100%, etc. In an embodiment of the present application, the coating rate of the lithium manganese iron phosphate layer is 20%-80%, which is beneficial to the improvement of the performance of the positive electrode active material, and can also reduce the preparation difficulty and the preparation cost of the positive electrode active material. In an embodiment, the coating rate of the lithium manganese iron phosphate layer can be 30%-80%. In another embodiment, the coating rate of the lithium manganese iron phosphate layer can be 40%-75%. In still another embodiment, the coating rate of the lithium manganese iron phosphate layer can be 50%-80%.

[0034] In the present application, the carbon layer forms a conductive network on the surface of the lithium iron manganese phosphate, further improving the conductive performance of the positive electrode active material, and can also prevent the agglomeration between the positive electrode active materials, so that the positive electrode active material has better dispersion performance; the carbon layer can be uniformly coated on the surface of the lithium iron manganese phosphate layer. In an embodiment of the present application, the thickness of the carbon layer is 1 nm-20 nm, so that a conductive network can be formed on the surface of the positive electrode active material to improve the electron migration rate on the surface of the positive electrode active material, while not affecting the ion migration to ensure the use performance of the positive electrode active material. Specifically, the thickness of the carbon layer can be, but is not limited to, 1 nm, 5 nm, 6 nm, 8 nm, 9 nm, 10 nm, 11 nm, 12 nm, 13 nm, 14 nm, 15 nm, 16 nm, 17 nm, 18 nm, 19 nm or 20 nm, etc. In an embodiment, the thickness of the carbon layer is 5 nm-15 nm. In another embodiment, the thickness of the carbon layer is 10 nm-15 nm.

[0035] In an embodiment of the present application, the mass content of the carbon layer in the positive electrode active material is 0.1%-3%, so that the conductive performance of the positive electrode active material can be improved, and the ion migration rate can be ensured, so that the positive electrode active material has excellent comprehensive performance. Specifically, the mass content of the carbon layer in the positive electrode active material can be, but is not limited to, 0.3%, 0.5%, 0.8%, 1%, 1.1%, 1.2%, 1.3%, 1.4%, 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2%, 2.3%, 2.5%, 2.8% or 3%, etc. In an embodiment, the mass content of the carbon layer in the positive electrode active material can be 0.1%-1.8%, which is beneficial to further improve the low-temperature capacity retention rate and discharge capacity retention rate of the positive electrode active material. In another embodiment, the mass content of the carbon layer in the positive electrode active material can be 1%-1.8%.

[0036] In an embodiment of the present application, the material of the carbon layer can include at least one of hard carbon, soft carbon, artificial graphite, natural graphite, acetylene black, carbon black, mesocarbon microbeads, carbon nanotubes, carbon nanofibers, graphene and carbon fibers, so as to improve the conductive performance of the positive electrode active material.

[0037] The coating rate of the carbon layer refers to the ratio of the surface area of the lithium iron manganese phosphate layer covered by the carbon layer to the total surface area of the lithium iron manganese phosphate layer. In an embodiment of the present application, the coating rate of the carbon layer is greater than or equal to 20%, which is beneficial to the improvement of the conductivity of the lithium iron manganese phosphate and the performance of the positive active material. Specifically, the coating rate of the carbon layer can be, but is not limited to, greater than or equal to 20%, greater than or equal to 30%, greater than or equal to 40%, greater than or equal to 50%, greater than or equal to 60%, greater than or equal to 70%, greater than or equal to 80%, greater than or equal to 90%, or 100%, etc. In an embodiment of the present application, the coating rate of the carbon layer is 20%-80%, which is beneficial to the improvement of the performance of the positive active material, reduces the preparation difficulty and the preparation cost of the positive active material, and is beneficial to the improvement of the infiltration effect of the electrolyte and the shortening of the ion diffusion path. In an embodiment, the coating rate of the carbon layer can be 30%-75%. In another embodiment, the coating rate of the carbon layer can be 40%-70%. In yet another embodiment, the coating rate of the carbon layer can be 50%-80%.

[0038] In an embodiment of the present application, the tap density of the positive active material is greater than or equal to 0.5 g / cm 3 . Specifically, the tap density of the positive active material can be, but is not limited to, 0.6 g / cm 3 , 0.8 g / cm 3 , 1 g / cm 3 , 1.1 g / cm 3 , 1.3 g / cm 3 , 1.5 g / cm 3 , etc.

[0039] In an embodiment of the present application, the carbon core can be mixed with a manganese source, an iron source, and a phosphorus source, dried after reaction, mixed with a carbon source, and sintered to obtain the positive active material. It can be understood that the sintering temperature can be selected according to the material performance. The carbon core can be modified before being mixed with the manganese source, the iron source, and the phosphorus source to improve the bonding force between the lithium iron manganese phosphate layer and the carbon core. The carbon core can be, but is not limited to, a carbon microsphere, and the modification can be achieved by mixing with a modifier (such as polyacrylic acid, sodium polyacrylate, etc.).

[0040] The present application also provides a positive electrode comprising the positive active material in any of the above embodiments. The positive electrode with the positive active material has excellent specific capacity, energy density, rate performance, and low-temperature performance, which is beneficial to its use in batteries.

[0041] Please refer to Figure 2A cross-sectional schematic view of a positive electrode according to an embodiment of the present application is provided, wherein the positive electrode 20 includes a positive electrode current collector 21 and a positive electrode active material layer 22 disposed on the surface of the positive electrode current collector 21, and the positive electrode active material layer 22 includes the positive electrode active material according to any of the embodiments described above. The material of the positive electrode current collector can be, but is not limited to, aluminum.

[0042] In an embodiment of the present application, the mass content of the positive electrode active material in the positive electrode active material layer is greater than or equal to 85%, thereby ensuring the electrochemical performance of the positive electrode. Specifically, the mass content of the positive electrode active material in the positive electrode active material layer can be, but is not limited to, 85%, 86%, 87%, 88%, 89%, 90%, 91%, 92%, 93%, 94%, 95%, 96%, 97%, 98%, or 99%, etc.

[0043] In an embodiment of the present application, the positive electrode active material layer can further include a positive electrode conductive agent. Specifically, the positive electrode conductive agent includes at least one of conductive carbon black, acetylene black, carbon nanowires, graphite, and graphene. In an embodiment of the present application, the mass content of the positive electrode conductive agent in the positive electrode active material layer can be 0.1%-10%. Specifically, the mass content of the positive electrode conductive agent in the positive electrode active material layer can be, but is not limited to, 0.1%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, or 10%, etc.

[0044] In an embodiment of the present application, the positive electrode active material layer can further include a positive electrode binder. Specifically, the positive electrode binder can include, but is not limited to, at least one of polythiophene, polypyrrole, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polystyrene, polyacrylamide, ethylene-propylene-diene copolymer resin, styrene butadiene rubber, polybutadiene, fluoroelastomer, polyvinylpyrrolidone, polyester resin, acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, carboxypropyl cellulose, ethyl cellulose, polyethylene oxide, sodium carboxymethyl cellulose, and styrene butadiene rubber. In an embodiment of the present application, the mass content of the positive electrode binder in the positive electrode active material layer can be 0.5%-5%. Specifically, the mass content of the positive electrode binder in the positive electrode active material layer can be, but is not limited to, 0.5%, 1%, 2%, 3%, 4%, or 5%, etc.

[0045] In the present application, the positive electrode active material can be dispersed in an organic solvent to form a positive electrode active slurry, the positive electrode active slurry can be coated on the surface of the positive electrode current collector, and the positive electrode can be prepared after drying. Specifically, at least one of a positive electrode conductive agent and a positive electrode binder can be further added to the positive electrode active slurry; the organic solvent can include, but is not limited to, at least one of ethanol, toluene, xylene, anisole, acetonitrile, heptane, decane, ethyl acetate, ethyl propionate, butyl butyrate, N-methyl pyrrolidone, acetone, etc.

[0046] The application further provides a battery comprising the positive electrode and the negative electrode.

[0047] In an embodiment of the application, the battery further comprises a separator arranged between the positive electrode and the negative electrode. In an embodiment of the application, the battery further comprises an electrolyte. At least part of the positive electrode and at least part of the negative electrode are soaked in the electrolyte. The negative electrode, the separator and the electrolyte are not particularly limited in the application, and can be, but are not limited to, substances capable of being used as a battery negative electrode, a separator and an electrolyte in the art.

[0048] The application further provides a power-consuming device comprising the battery of any of the above embodiments. Specifically, the power-consuming device can be a vehicle, an electronic device, an energy storage system, etc., and the battery can be arranged in the power-consuming device in the form of a single battery, a battery module, a battery pack, etc.

[0049] The effects of the technical solutions of the application are further described below through specific examples.

[0050] Example 1

[0051] A positive electrode active material comprises a carbon core (particle size: 20 nm), a lithium manganese iron phosphate layer (lithium manganese iron phosphate particle size: 350 nm) covering the carbon core, and a carbon layer (thickness: 15 nm) covering the lithium manganese iron phosphate layer. The mass content of the carbon core in the positive electrode active material is 0.3%, the mass content of the lithium manganese iron phosphate layer is 97.9%, and the mass content of the carbon layer is 1.8%.

[0052] Example 2

[0053] The example is substantially the same as example 1, except that the mass content of the carbon core in the positive electrode active material is 0.3%, the particle size of the carbon core is 25 nm, the mass content of the lithium manganese iron phosphate layer is 98.7%, the mass content of the carbon layer is 1%, and the thickness of the carbon layer is 8 nm.

[0054] Example 3

[0055] The example is substantially the same as example 1, except that the mass content of the carbon core in the positive electrode active material is 1%, the particle size of the carbon core is 40 nm, the mass content of the lithium manganese iron phosphate layer is 97.2%, the mass content of the carbon layer is 1.8%, and the thickness of the carbon layer is 12 nm.

[0056] Example 4

[0057] The same as in Example 1 except that the mass content of the carbon core in the positive electrode active material is 2%, the particle diameter of the carbon core is 50 nm, the mass content of the lithium manganese iron phosphate layer is 96.2%, the mass content of the carbon layer is 1.8%, and the thickness of the carbon layer is 10 nm.

[0058] Example 5

[0059] The same as in Example 1 except that the mass content of the carbon core in the positive electrode active material is 0.3%, the particle diameter of the carbon core is 20 nm, the mass content of the lithium manganese iron phosphate layer is 97.9%, the particle diameter of the lithium manganese iron phosphate particle is 100 nm, the mass content of the carbon layer is 1.8%, and the thickness of the carbon layer is 15 nm.

[0060] Example 6

[0061] The same as in Example 1 except that the mass content of the carbon core in the positive electrode active material is 0.3%, the particle diameter of the carbon core is 20 nm, the mass content of the lithium manganese iron phosphate layer is 97.9%, the particle diameter of the lithium manganese iron phosphate particle is 400 nm, the mass content of the carbon layer is 1.8%, and the thickness of the carbon layer is 15 nm.

[0062] Example 7

[0063] The same as in Example 6 except that the particle diameter of the lithium manganese iron phosphate particle is 200 nm.

[0064] Example 8

[0065] The same as in Example 1 except that the mass content of the carbon core in the positive electrode active material is 8%, the particle diameter of the carbon core is 110 nm, the mass content of the lithium manganese iron phosphate layer is 88%, the mass content of the carbon layer is 4%, and the thickness of the carbon layer is 30 nm.

[0066] Example 9

[0067] The same as in Example 6 except that the particle diameter of the lithium manganese iron phosphate particle is 20 nm.

[0068] Example 10

[0069] The same as in Example 6 except that the particle diameter of the lithium manganese iron phosphate particle is 700 nm.

[0070] Comparative Example 1

[0071] The same as in Example 1 except that the positive electrode active material includes lithium manganese iron phosphate and a carbon layer coating the lithium manganese iron phosphate layer, the mass content of the lithium manganese iron phosphate in the positive electrode active material is 98.2%, and the mass content of the carbon layer is 1.8%.

[0072] Comparative Example 2

[0073] The same as Example 1, except that the positive active material comprises a carbon core and a lithium manganese iron phosphate layer covering the carbon core, the mass content of the carbon core in the positive active material is 0.3%, and the mass content of the lithium manganese iron phosphate layer is 99.7%.

[0074] Performance detection

[0075] The positive active material prepared in the above examples and comparative examples is mixed with conductive carbon black, polyvinylidene fluoride and N-methyl pyrrolidone to form a positive active paste, the mass content of the positive active material in the positive active paste is 95%, the mass content of the conductive carbon black is 2%, and the mass content of the polyvinylidene fluoride is 3%. The positive active material is coated on the surface of an aluminum foil, and after coating and drying, a positive active material layer with a thickness of 40-60 μm is obtained, and a positive electrode is prepared.

[0076] The positive electrode and a separator, an electrolyte and a lithium sheet are assembled to form a battery, and performance detection is carried out, the capacity test is 25℃, 0.1C constant voltage charging to 4.3V, recording the capacity C1, 0.1C discharging to 2V, recording the capacity C2; the low temperature test is to place the full battery at-20℃ for 2h, and then discharge to 2V at a rate of 0.25C, and record the discharge capacity C3, and the results are shown in Table 1; wherein the specific capacity (unit mAh·g -1 ) is the ratio of C2 to the mass of the positive active material, the low temperature capacity retention at-20℃ is the ratio of the low temperature capacity to the normal temperature capacity, and the 2C discharge capacity retention is the ratio of the 2C discharge capacity (0.1C charging, 2C discharging) to the 0.1C discharge capacity.

[0077] Table 1 Performance detection results

[0078]

[0079] It can be seen that in Comparative Example 1, no carbon core is provided, and the low temperature capacity retention and the 2C discharge capacity retention of the battery prepared are low, in Comparative Example 2, no carbon layer is provided, and the overall performance of the battery prepared is poor; the positive active material prepared in Examples 1-10 of the present application has a high specific capacity, and excellent low temperature capacity retention and 2C discharge capacity retention, and has good low temperature performance and rate performance. Therefore, the positive active material provided by the present application has excellent electrical conductivity, specific capacity and energy density, which is beneficial to its use in batteries.

[0080] The above is the preferred embodiment of the present application, but it cannot be interpreted as limiting the scope of the present application. It should be noted that for ordinary skilled persons in the art, without departing from the principles of the present application, a number of improvements and refinements can be made, which are also considered within the protection scope of the present application.

Claims

1. A positive electrode active material, characterized in that, The material includes a carbon core, a lithium manganese iron phosphate layer covering the carbon core, and a carbon layer covering the lithium manganese iron phosphate layer. Along the direction from the carbon core to the carbon layer, the mass content of manganese in the lithium manganese iron phosphate layer gradually decreases. The material of the lithium manganese iron phosphate layer includes lithium manganese iron phosphate. The lithium manganese iron phosphate has doping elements, including at least one of magnesium, titanium, cobalt, nickel, and vanadium.

2. The positive electrode active material as described in claim 1, characterized in that, The lithium manganese iron phosphate layer includes lithium manganese iron phosphate particles with a particle size of 100nm-400nm. The mass content of the lithium manganese iron phosphate layer in the positive electrode active material is 92%-98.9%.

3. The positive electrode active material as described in claim 1, characterized in that, The particle size of the carbon core is 10nm-50nm; The carbon core in the positive electrode active material has a mass content of 0.1%-5%.

4. The positive electrode active material as described in claim 1, characterized in that, The thickness of the carbon layer is 1nm-20nm; The carbon layer in the positive electrode active material has a mass content of 0.1%-3%.

5. The positive electrode active material as described in claim 1, characterized in that, The coating rate of the lithium iron phosphate manganese phosphate layer is 20%-80%.

6. The positive electrode active material as described in claim 1, characterized in that, The carbon layer has a coating rate of 20%-80%.

7. The positive electrode active material as described in claim 1, characterized in that, The lithium manganese iron phosphate layer includes LiFe 1-α Mn α PO4, α is 0.6-0.

8.

8. A positive electrode, characterized in that, It includes a positive current collector and a positive active material layer disposed on the surface of the positive current collector, wherein the positive active material layer includes the positive active material according to any one of claims 1-7.

9. A battery, characterized in that, It includes the negative electrode and the positive electrode as described in claim 8.

10. An electrical appliance, characterized in that, Includes the battery as described in claim 9.

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