Cathode active material and preparation method thereof, secondary battery and electric device

Abstract

The invention provides a positive electrode active material and preparation thereof, a secondary battery and an electric device. The positive electrode active material comprises a lithium manganese iron phosphate matrix material and a carbon layer at least coating the surface of the lithium manganese iron phosphate matrix material, the powder compaction density of the positive electrode active material under 400MPa is greater than or equal to 2.35 g / cm < 3 >, and the positive electrode active material can improve the energy density of the battery.

Description

TECHNICAL FIELD

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

[0002] Secondary batteries have the advantages of high energy density, high working voltage, low self-discharge rate, small size and light weight, and are widely used in the field of consumer electronics.

[0003] At present, with the rapid development of electric vehicles and mobile electronic devices, people have higher and higher requirements for the energy density of secondary batteries. How to improve the energy density of batteries is a scientific and technical problem to be solved in the current application field of secondary batteries. SUMMARY

[0004] The purpose of the present application is to provide a positive electrode active material, a preparation method thereof, a secondary battery and an electric device, which can improve the energy density of the battery.

[0005] The first aspect of the present application provides a positive electrode active material, which comprises a lithium iron manganese phosphate base material and a carbon layer coated on at least the surface of the lithium iron manganese phosphate base material,

[0006] The positive electrode active material has a powder compaction density of 400MPa greater than or equal to 2.35g / cm 3 .

[0007] The positive electrode active material of the present application has a high powder compaction density, which is beneficial to improve the compaction density of the positive electrode film layer and improve the energy density of the battery.

[0008] In any embodiment, the positive electrode active material has a powder compaction density of 400MPa of 2.35g / cm 3 -2.50g / cm 3 .

[0009] The positive electrode active material has a high powder compaction density, which is beneficial to high energy density secondary batteries.

[0010] In any embodiment, the number percentage content of the positive electrode active material with a particle size of 4-6μm is greater than or equal to 90% based on the total amount of the positive electrode active material.

[0011] The number percentage content of the positive electrode active material with a particle size of 4-6 μm is greater than or equal to 90%, that is, the positive electrode active material has excellent particle size consistency, so that the discharge behavior of each particle tends to be consistent during battery cycling, and the possibility of overcharging and over-discharging of each particle during charging and discharging is reduced, which is beneficial to ensure the structural stability of the positive electrode active material. In addition, the particle size of the particles with a number percentage of greater than or equal to 90% is 4-6 μm, and the particle size of the material is relatively small, which is beneficial to improve the powder compaction density of the material. In addition, it can also reduce the influence of large particles on the mechanical integrity of the positive electrode active material particles, and can also slow down the possibility of side reactions between small particles and electrolyte, thereby comprehensively improving the low-temperature cycle performance of the battery and prolonging the service life of the battery.

[0012] In any embodiment, the tap density of the positive electrode active material is 1.0 g / cm 3 -1.3 g / cm 3 .

[0013] The tap density of the positive electrode active material is within a suitable range, which is beneficial to obtain a secondary battery with high energy density.

[0014] In any embodiment, the general formula of the lithium iron manganese phosphate-based material is:

[0015] Li z Fe x Mn y M 1-x-y PO4

[0016] wherein z is 0.9-1.2, x is 0.001-0.999, y is 0.001-0.999, and 1-x-y is 0-0.1; M includes at least one of Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb, and Ge.

[0017] In any embodiment, the mass content of the carbon layer is 1.5%-2.5% based on the mass of the lithium iron manganese phosphate-based material.

[0018] The second aspect of the present application provides a preparation method of a positive electrode active material, comprising the following steps:

[0019] Spray drying the mixed slurry containing lithium iron manganese phosphate and a carbon source to obtain an intermediate product;

[0020] Heat treating the intermediate product to obtain a positive electrode active material;

[0021] The heat source for spray drying includes microwaves,

[0022] The positive electrode active material comprises a lithium iron manganese phosphate base material and a carbon layer at least coated on the surface of the lithium iron manganese phosphate base material,

[0023] The powder compaction density of the positive electrode active material at 400 MPa is greater than or equal to 2.35 g / cm 3 .

[0024] The present application uses microwave spray drying slurry, under the action of microwave energy, the internal high-speed rotation of slurry droplets produces a thermal effect, so that the material instantaneously obtains heat source and loses moisture, the droplets are uniformly heated from the inside out, the internal particles are first dried, and then the outside is coated with small spherical particles, so that the lithium iron manganese phosphate intermediate product with solid spherical structure, low hollow rate and high powder compaction density can be obtained. At the same time, microwave is used as heat source to treat the slurry, the liquid drying time is short and uniform, which can ensure the uniformity of particle growth rate to the greatest extent, and the lithium iron manganese phosphate with uniform particle size distribution can be obtained. The lithium iron manganese phosphate intermediate product with solid spherical structure, low hollow rate, high powder compaction density and uniform particle size distribution is heat treated with carbon, so that the carbon-coated lithium iron manganese phosphate with solid spherical structure, low hollow rate, high powder compaction density and uniform particle size distribution can be obtained, which provides material basis for preparing batteries with high energy density and long cycle life.

[0025] In any embodiment, the microwave frequency of the microwave is 50 GHz-100 GHz.

[0026] The microwave frequency of the microwave is in a suitable range, so that the outlet air temperature of the spray drying is in a suitable range, which can uniformly and quickly dry the slurry, and the carbon-coated lithium iron manganese phosphate with solid spherical structure, low hollow rate, high powder compaction density and uniform particle size distribution can be obtained.

[0027] In any embodiment, the outlet air temperature of the spray drying is 100℃-120℃.

[0028] The outlet air temperature of the spray drying is in a suitable range, which can uniformly and quickly dry the slurry, and the carbon-coated lithium iron manganese phosphate with solid spherical structure, low hollow rate, high powder compaction density and uniform particle size distribution can be obtained.

[0029] In any embodiment, the solid content of the mixed slurry is 10%-30%.

[0030] The solid content of the mixed slurry is in a suitable range, the more compact the internal slurry droplets are, the more conducive to obtaining the lithium iron manganese phosphate intermediate product with low hollow rate and high compaction density. At the same time, the solid content of the mixed slurry is in a suitable range, and the viscosity of the mixed slurry is in a suitable range, so that the mixed slurry has good processing performance.

[0031] In any embodiment, the preparation method further comprises:

[0032] The initial slurry containing lithium manganese iron phosphate is subjected to stirring and ultrasonic dispersion treatment to obtain a first slurry;

[0033] The first slurry is mixed with a carbon source to obtain the mixed slurry containing lithium manganese iron phosphate and the carbon source.

[0034] In any embodiment, the volume average particle size Dv50 of the lithium manganese iron phosphate in the first slurry is 100 nm-300 nm.

[0035] The particle size of the lithium manganese iron phosphate in the first slurry is small, and the particle size of the lithium manganese iron phosphate raw material in the mixed slurry is small, which is conducive to obtaining a small-particle-size lithium manganese iron phosphate intermediate product, obtaining a positive electrode active material with high tap density, and also speeding up the drying speed of microwave spray drying and improving the preparation efficiency.

[0036] In any embodiment, the heat treatment specifically comprises:

[0037] The intermediate product is subjected to heat treatment under an inert atmosphere to obtain a positive electrode active material,

[0038] The treatment temperature of the heat treatment is 400°C-800°C; and / or the treatment time of the heat treatment is 4h-10h.

[0039] A third aspect of the present application provides a secondary battery comprising a positive electrode sheet, wherein the positive electrode sheet comprises the positive electrode active material of the first aspect or the positive electrode active material prepared by the preparation method of the second aspect.

[0040] A fourth aspect of the present application provides an electric device comprising the secondary battery of the third aspect. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 is a schematic diagram of a secondary battery of an embodiment of the present application;

[0042] Figure 2 is Figure 1 is an exploded view of the secondary battery of an embodiment of the present application shown in FIG. 8;

[0043] Figure 3 is a schematic diagram of a battery module of an embodiment of the present application;

[0044] Figure 4 is a schematic diagram of a battery pack of an embodiment of the present application;

[0045] Figure 5 is Figure 4 is an exploded view of the battery pack of an embodiment of the present application shown in FIG. 14;

[0046] Figure 6is a schematic view of a power consuming device using a secondary battery as a power source according to an embodiment of the present application.

[0047] Reference Signs:

[0048] 1: battery pack; 2: upper case; 3: lower case; 4: battery module; 5: secondary battery; 51: case; 52: electrode assembly; 53: cover plate. DETAILED DESCRIPTION

[0049] Hereinafter, embodiments of the positive electrode active material and the method for manufacturing the same, the secondary battery, and the power consuming device according to the present application will be specifically described with appropriate reference to the accompanying drawings. However, there will be cases where unnecessary detailed description is omitted. For example, there will be cases where detailed description of matters known well and repeated description of substantially the same structure are omitted. This is to avoid the following description from becoming unnecessarily lengthy and to facilitate understanding by those skilled in the art. In addition, the accompanying drawings and the following description are provided so that those skilled in the art can fully understand the present application, and are not intended to limit the subject matter recited in the claims.

[0050] The "ranges" disclosed in the present application are defined in the form of lower and upper limits, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The ranges defined in this way can be inclusive or exclusive of the end values, and can be arbitrarily combined, i.e., any lower limit can be combined with any upper limit to form a range. For example, if ranges of 60-120 and 80-110 are listed for a particular parameter, it is understood that ranges of 60-110 and 80-120 are also contemplated. In addition, if a minimum range value of 1 and 2 is listed, and if a maximum range value of 3, 4, and 5 is listed, then the following ranges are all contemplated: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise stated, a numerical range "a-b" represents a shorthand manner of describing each and every numerical value that is contained in the range between "a" and "b," wherein "a" and "b" are both real numbers. For example, the numerical range "0-5" indicates that all real numbers between "0" and "5" have been listed herein, and "0-5" is merely a shorthand manner of describing each and every numerical value that is contained in the range between "0" and "5." In addition, when it is stated that a parameter is an integer ≥ 2, it is equivalent to disclose that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, and the like.

[0051] If not specifically stated, all embodiments and optional embodiments of the present application can be combined with each other to form new technical solutions.

[0052] If not specifically stated, all technical features and optional technical features of the present application can be combined with each other to form new technical solutions.

[0053] If not specified otherwise, all steps of the present application can be carried out in sequence or randomly, preferably in sequence. For example, the method comprising steps (a) and (b) means that the method can comprise steps (a) and (b) in sequence, or steps (b) and (a) in sequence. For example, the method further comprising step (c) means that step (c) can be added to the method in any sequence, for example, the method can comprise steps (a), (b) and (c), or steps (a), (c) and (b), or steps (c), (a) and (b), etc.

[0054] If not specified otherwise, the terms "comprising" and "including" as used in the present application are open-ended. For example, the terms "comprising" and "including" can mean that other components not listed can also be included or can mean that only the listed components are included.

[0055] If not specified otherwise, the term "or" in the present application is inclusive. For example, the phrase "A or B" means "A, B, or both A and B." More specifically, any of the following satisfy the condition "A or B": A is true (or present) and B is false (or not present); A is false (or not present) and B is true (or present); or both A and B are true (or present).

[0056] Lithium manganese iron phosphate positive electrode active material has become one of the most concerned positive electrode active materials due to its advantages of high capacity, good safety performance, and abundant raw material sources. However, the lithium manganese iron phosphate particles currently have a hollow structure, which makes the lithium manganese iron phosphate powder have a low compaction density, and a battery with high energy density cannot be obtained.

[0057] [Positive electrode active material]

[0058] The present application provides a positive electrode active material, which comprises a lithium manganese iron phosphate base material and a carbon layer at least coated on the surface of the lithium manganese iron phosphate base material,

[0059] The powder compaction density of the positive electrode active material under 400 MPa is greater than or equal to 2.35 g / cm 3 .

[0060] In some embodiments, the powder compaction density of the positive electrode active material under 400 MPa can be selected as any one of the following: greater than or equal to 2.35 g / cm 3 , greater than or equal to 2.40 g / cm 3 , greater than or equal to 2.45 g / cm 3 , greater than or equal to 2.50 g / cm 3 .

[0061] The powder compaction density of the positive electrode active material at 400 MPa can be measured by methods and equipment known in the art. For example, referring to GB / T 24533-2009, 1 g of the positive electrode active material powder is weighed, added into a mold with a bottom area of 1.327 cm 2 , and pressed to a specific pressure, for example, 400 MPa, for 30 s, then released, and kept for 10 s. The powder compaction density of the positive electrode active material at the selected pressure (or selected pressure) is determined by an electronic pressure testing machine (such as a UTM7305 electronic pressure testing machine).

[0062] The positive electrode active material of the present application has a high powder compaction density, which is beneficial to improve the compaction density of the positive electrode film layer and improve the energy density of the battery.

[0063] In some embodiments, the powder compaction density of the positive electrode active material at 400 MPa is 2.35 g / cm 3 -2.50 g / cm 3 . In some embodiments, the powder compaction density of the positive electrode active material at 400 MPa can be selected from 2.35 g / cm 3 , 2.37 g / cm 3 , 2.39 g / cm 3 , 2.40 g / cm 3 , 2.42 g / cm 3 , 2.44 g / cm 3 , 2.46 g / cm 3 , 2.48 g / cm 3 , 2.50 g / cm 3 , or any numerical range between any two of them.

[0064] The positive electrode active material has a high powder compaction density, which is beneficial to high-energy-density secondary batteries.

[0065] In some embodiments, the number percentage content of the positive electrode active material with a particle size of 4-6 μm based on the total amount of the positive electrode active material is greater than or equal to 90%.

[0066] In some embodiments, the number percentage content of the positive electrode active material with a particle size of 4-6 μm based on the total amount of the positive electrode active material can be selected from any one of greater than or equal to 90%, greater than or equal to 92%, greater than or equal to 94%, greater than or equal to 96%, and greater than or equal to 98%.

[0067] In some embodiments, the number percentage content of the positive electrode active material with a particle size of 4-6 μm based on the total amount of the positive electrode active material can be selected from 90%, 92%, 94%, 96%, 98%, 99%, or any numerical range between any two of them.

[0068] The number percentage of the positive electrode active material having a particle size of 4 μm-6 μm can be measured by methods and devices known in the art. For example, an appropriate amount of the positive electrode active material is taken, 20 ml of deionized water is added (the sample concentration is ensured to be 8-12% obscuration), and ultrasonic dispersion is performed for 5 min (53 KHz / 120 W) to ensure that the sample is completely dispersed, and then the positive electrode active material sample is determined according to the GB / T 19077-2016 / ISO 13320:2009 standard, and the test device is a Malvern 2000 (MasterSizer 2000) laser particle size analyzer. The particle size number distribution graph is drawn according to the test data. From the particle size volume distribution graph, the number percentage of the active material having a particle size of 4 μm-6 μm based on the total number of the positive electrode active material is obtained.

[0069] The number percentage of the positive electrode active material having a particle size of 4 μm-6 μm is controlled to be greater than or equal to 90%, that is, the positive electrode active material has excellent particle size consistency, so that the discharge behavior of each particle tends to be consistent during the battery cycle process, and the possibility of overcharging and overdischarging of each particle during the charging and discharging process is reduced, which is beneficial to ensure the structural stability of the positive electrode active material. In addition, the particle size of the particle with a number percentage greater than or equal to 90% is 4 μm-6 μm, and the particle size of the material is relatively small, which is beneficial to improve the powder compaction density of the material. In addition, it can also reduce the influence of large particles on the mechanical integrity of the positive electrode active material particles, and can also slow down the possibility of side reactions between small particles and electrolyte, thereby comprehensively improving the low-temperature cycle performance of the battery and prolonging the service life of the battery.

[0070] In some embodiments, the tap density of the positive electrode active material is 1.0 g / cm 3 -1.3 g / cm 3 In some embodiments, the tap density of the positive electrode active material can be selected as 1.0 g / cm 3 , 1.1 g / cm 3 , 1.2 g / cm 3 , 1.3 g / cm 3 or any numerical range between any two of them.

[0071] The tap density of the positive electrode active material can be measured by methods and devices known in the art. For example, according to GB / T 5162-2006, a powder tap density tester is used for determination. The test instrument can use Dandong Bitai BT-301.

[0072] The tap density of the positive electrode active material is in a suitable range, which can improve the compaction density of the positive electrode film layer, and is beneficial to obtain a secondary battery with high energy density.

[0073] In some embodiments, the general formula of the lithium iron manganese phosphate-based material is:

[0074] Li z Fe x Mn y M 1-x-y PO4

[0075] wherein z is 0.9 to 1.2, x is 0.001 to 0.999, y is 0.001 to 0.999, 1-x-y is 0 to 0.1; M comprises at least one of Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb and Ge.

[0076] In some embodiments, z can be selected from 0.9, 1.0, 1.1, 1.2, or any numerical range between any two of them.

[0077] In some embodiments, x can be selected from 0.001, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.999, or any numerical range between any two of them.

[0078] In some embodiments, y can be selected from 0.001, 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 0.999, or any numerical range between any two of them.

[0079] In some embodiments, 1-x-y can be selected from 0, 0.01, 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, 0.09, 0.1, or any numerical range between any two of them.

[0080] In some embodiments, the mass content of the carbon layer is 1.5%-2.5% based on the mass of the lithium iron manganese phosphate base material.

[0081] In some embodiments, the mass content of the carbon layer can be selected from 1.5%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.5%, or any numerical range between any two of them, based on the mass of the lithium iron manganese phosphate base material.

[0082] The mass content of the carbon layer can be measured by methods and devices known in the art. For example, the mass content of the carbon layer can be tested by infrared absorption method, specifically, the sample to be tested is burned in oxygen flow to generate CO2. Since the infrared energy absorbed by CO2 is proportional to its concentration under a certain pressure, the mass content of the carbon layer can be calculated according to the energy change before and after the measured CO2 gas flows through the infrared absorber.

[0083] The mass content of the carbon layer is within an appropriate range, which can take into account the conductivity and gram capacity of the material, and comprehensively improve the rate performance and energy density of the battery.

[0084] A second aspect of the present application provides a method for preparing a positive electrode active material, comprising the following steps:

[0085] spray drying a mixed slurry comprising lithium manganese iron phosphate and a carbon source to obtain an intermediate product;

[0086] heat-treating the intermediate product to obtain a positive electrode active material;

[0087] Wherein, the heat source of the spray drying includes microwave,

[0088] The positive electrode active material includes a lithium iron manganese phosphate matrix material and a carbon layer at least coated on the surface of the lithium iron manganese phosphate matrix material.

[0089] The powder compaction density of the positive electrode active material at 400 MPa is greater than or equal to 2.35 g / cm 3 .

[0090] Lithium iron manganese phosphate (LiMnFePO4) is widely used in the field of secondary batteries due to its abundant raw materials, low cost, and high energy density. However, LiMnFePO4 suffers from low electrical conductivity. To improve the conductivity of LiMnFePO4, it is often coated with a carbon coating. Existing techniques often employ electric heating spray drying and high-temperature sintering for carbon coating. However, the carbon-coated LiMnFePO4 cathode active materials prepared in this manner have a high hollow ratio, resulting in a low powder compaction density, which affects their electrochemical performance.

[0091] The inventors of the present application found in the research process that using microwave as a heat source to spray dry the slurry can obtain carbon-coated lithium manganese iron phosphate with low hollow rate, high powder compaction density and uniform particle size distribution. Using microwave to spray dry the slurry, under the action of microwave energy, the internal high-speed rotation of the slurry droplet produces a thermal effect, so that the material instantaneously obtains heat source and loses moisture. The droplet is uniformly heated from the inside out, the internal particle loses moisture and dries first, and then the outside is coated with small spherical particles that have dried. The intermediate product of lithium manganese iron phosphate with solid spherical structure, low hollow rate and high powder compaction density can be obtained. At the same time, using microwave as a heat source to treat the slurry, the liquid drying time is short and uniform, which maximizes the uniformity of the particle growth rate, and the lithium manganese iron phosphate with uniform particle size distribution can be obtained. The solid spherical structure, low hollow rate, high powder compaction density and uniform particle size distribution of the lithium manganese iron phosphate can be obtained by heat treatment with carbon. This provides a material basis for the preparation of high-energy density and long cycle life batteries. In addition, compared with traditional electric heating spray drying, the drying speed is fast, the energy consumption is low, the efficiency is high, and it is beneficial to industrial production.

[0092] In some embodiments, the microwave frequency of the microwave is 50GHz-100GHz. In some embodiments, the microwave frequency of the microwave can be selected as 50GHz, 60GHz, 70GHz, 80GHz, 90GHz, 100GHz or any numerical range between any two of them.

[0093] The microwave frequency of the microwave is within a suitable range, so that the outlet air temperature of the spray drying is within a suitable range, which can uniformly and quickly dry the slurry, and carbon-coated lithium manganese iron phosphate with solid spherical structure, low hollow rate, high powder compaction density and uniform particle size distribution can be obtained.

[0094] In some embodiments, the outlet air temperature of the spray drying is 100℃-120℃. In some embodiments, the outlet air temperature of the spray drying can be selected as 100℃, 110℃, 120℃ or any numerical range between any two of them.

[0095] The outlet air temperature of the spray drying is within a suitable range, which can uniformly and quickly dry the slurry, and carbon-coated lithium manganese iron phosphate with solid spherical structure, low hollow rate, high powder compaction density and uniform particle size distribution can be obtained.

[0096] In some embodiments, the solid content of the mixed slurry is 10%-30%. In some embodiments, the solid content of the mixed slurry can be selected as 10%, 15%, 20%, 25%, 30% or any numerical range between any two of them.

[0097] The solid content of the mixed slurry can be measured by methods and equipment known in the art. For example, take aluminum foil into the loss on drying tester and weigh, record as M0, and zero. Take a small amount of mixed slurry and coat it on the aluminum foil, then put it into the moisture tester and weigh, record as M1. Close the equipment and start drying. After the end, record the weighing data as M2, and calculate the solid content, which is (M2-M0) / (M1-M0).

[0098] The solid content of the mixed slurry is within a suitable range, and the more compact the inside of the slurry droplets, the more conducive to obtaining a low hollow rate and high green density of the lithium manganese iron phosphate intermediate product. At the same time, the solid content of the mixed slurry is within a suitable range, and the viscosity of the mixed slurry is within a suitable range, and the mixed slurry has good processing performance.

[0099] In some embodiments, the preparation method further comprises:

[0100] The initial slurry containing lithium manganese iron phosphate is subjected to stirring and ultrasonic dispersion treatment to obtain a first slurry.

[0101] The first slurry is mixed with a carbon source to obtain the mixed slurry containing lithium manganese iron phosphate and a carbon source.

[0102] In some embodiments, the volume average particle size Dv50 of the lithium manganese iron phosphate in the first slurry is 100 nm-300 nm. In some embodiments, the volume average particle size Dv50 of the lithium manganese iron phosphate in the first slurry can be selected as 100 nm, 150 nm, 200 nm, 250 nm, 300 nm, or any numerical range between any two of them.

[0103] The volume average particle size Dv50 of the lithium manganese iron phosphate in the first slurry can be measured by methods and equipment known in the art. For example, take an appropriate amount of the first slurry, add 10 ml of deionized water (the sample concentration is guaranteed to have an 8-12% obscuration), and at the same time ultrasonically disperse for 5 min (53 KHz / 120 W) to ensure complete dispersion of the sample. Then, the first slurry is measured according to the GB / T 19077-2016 / ISO13320:2009 standard, and the test equipment is a Malvern 2000 (MasterSizer 2000) laser particle size analyzer. According to the test data, a particle size volume distribution graph is drawn. From the particle size volume distribution graph, the particle size Dv50 corresponding to the cumulative volume distribution percentage of the lithium manganese iron phosphate material in the first slurry reaching 50% is obtained, and the volume average particle size Dv50 of the lithium manganese iron phosphate in the first slurry is calculated.

[0104] The particle size of the lithium manganese iron phosphate in the first slurry is small, and the particle size of the lithium manganese iron phosphate raw material in the mixed slurry is small, which is beneficial to obtain a small-particle-size lithium manganese iron phosphate intermediate product, obtain a positive electrode active material with high tap density, and also accelerate the drying speed of microwave spray drying and improve the preparation efficiency.

[0105] In any embodiment, the heat treatment specifically comprises:

[0106] The intermediate product is subjected to heat treatment under an inert atmosphere to obtain a positive electrode active material,

[0107] The heat treatment has a treatment temperature of 400-800 DEG C, and / or a treatment time of 4-10 h.

[0108] In some embodiments, the treatment temperature of the heat treatment can be selected as 400 DEG C, 500 DEG C, 600 DEG C, 800 DEG C, or a numerical range between any two of them.

[0109] In some embodiments, the treatment time of the heat treatment can be selected as 4 h, 6 h, 8 h, 10 h, or a numerical range between any two of them.

[0110] Controlling the reaction time or reaction temperature of the heat treatment within a suitable range is beneficial to form a dense carbon coating layer on the surface of the lithium manganese iron phosphate intermediate product.

[0111] In some embodiments, the carbon source can be an organic carbon source or an inorganic carbon source.

[0112] In some embodiments, the carbon source includes at least one of glucose, sucrose, fructose, polyethylene glycol, starch, polydopamine, polyvinylpyrrolidone, or tannic acid.

[0113] [Positive electrode sheet]

[0114] The present application provides a positive electrode sheet, which comprises a positive electrode current collector and a positive electrode film layer arranged on at least one surface of the positive electrode current collector, the positive electrode film layer comprising a positive electrode active material, a binder and a conductive agent, the positive electrode active material being the positive electrode active material of the embodiments of the present application or being prepared by the preparation method of the embodiments of the present application.

[0115] For example, the positive electrode current collector has two opposite surfaces in the thickness direction of itself, and the positive electrode film layer is arranged on any one or both of the two opposite surfaces of the positive electrode current collector.

[0116] In some embodiments, the positive current collector can employ a metal foil or a composite current collector. For example, as a metal foil, an aluminum foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (aluminum, aluminum alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base material (e.g., a base material of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0117] In some embodiments, the positive electrode film layer can further optionally include a binder. As an example, the binder can include at least one of polyvinylidene fluoride (PVDF), polytetrafluoroethylene (PTFE), a vinylidene fluoride-tetrafluoroethylene-propylene terpolymer, a vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene terpolymer, a tetrafluoroethylene-hexafluoropropylene copolymer, and a fluorine-containing acrylic ester resin.

[0118] In some embodiments, the positive electrode film layer can further optionally include a conductive agent. As an example, the conductive agent can include at least one of super P, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0119] In some embodiments, the positive electrode tab can be prepared by dispersing the above-described components for preparing the positive electrode tab, such as the positive electrode active material, the conductive agent, the binder, and any other components, in a solvent (e.g., N-methylpyrrolidone) to form a positive electrode slurry, coating the positive electrode slurry on the positive electrode current collector, and subjecting the same to a drying, cold-pressing, or the like process to obtain the positive electrode tab.

[0120] [Negative electrode tab]

[0121] The negative electrode tab includes a negative current collector and a negative electrode film layer disposed on at least one surface of the negative current collector, the negative electrode film layer including a negative electrode active material.

[0122] As an example, the negative current collector has two surfaces opposite in the thickness direction thereof, and the negative electrode film layer is disposed on any one or both of the two opposite surfaces of the negative current collector.

[0123] In some embodiments, the negative current collector can employ a metal foil or a composite current collector. For example, as a metal foil, a copper foil can be employed. The composite current collector can include a polymer material base layer and a metal layer formed on at least one surface of the polymer material base layer. The composite current collector can be formed by forming a metal material (copper, copper alloy, nickel, nickel alloy, titanium, titanium alloy, silver, silver alloy, etc.) on a polymer material base layer (e.g., a base layer of polypropylene (PP), polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polystyrene (PS), polyethylene (PE), etc.).

[0124] In some embodiments, the negative active material can employ a negative active material for a battery known in the art. As an example, the negative active material can include at least one of artificial graphite, natural graphite, soft carbon, hard carbon, a silicon-based material, a tin-based material, and lithium titanate, etc. The silicon-based material can be selected from at least one of elemental silicon, a silicon oxide compound, a silicon-carbon composite, a silicon-nitrogen composite, and a silicon alloy. The tin-based material can be selected from at least one of elemental tin, a tin oxide compound, and a tin alloy. However, the present application is not limited to these materials, and other conventional materials that can be used as a negative active material for a battery can also be used. These negative active materials can be used alone or in combination of two or more.

[0125] In some embodiments, the negative film layer can further optionally include a binder. The binder can be selected from at least one of styrene butadiene rubber (SBR), polyacrylic acid (PAA), sodium polyacrylate (PAAS), polyacrylamide (PAM), polyvinyl alcohol (PVA), sodium alginate (SA), polymethacrylic acid (PMAA), and carboxymethyl chitosan (CMCS).

[0126] In some embodiments, the negative film layer can further optionally include a conductive agent. The conductive agent can be selected from at least one of super conductive carbon, acetylene black, carbon black, ketjen black, carbon dots, carbon nanotubes, graphene, and carbon nanofibers.

[0127] In some embodiments, the negative film layer can further optionally include other auxiliary agents, such as a thickening agent (e.g., sodium carboxymethyl cellulose (CMC-Na)) and the like.

[0128] In some embodiments, the negative electrode sheet can be prepared by dispersing the above-described components for preparing the negative electrode sheet, such as the negative active material, the conductive agent, the binder, and any other components, in a solvent (e.g., deionized water) to form a negative electrode slurry; coating the negative electrode slurry on a negative current collector; and drying, cold-pressing, and the like to obtain the negative electrode sheet.

[0129] [Electrolyte]

[0130] The electrolyte functions to conduct ions between the positive electrode sheet and the negative electrode sheet. The type of electrolyte is not particularly limited in the present application and can be selected as desired. For example, the electrolyte can be liquid, gel, or all-solid.

[0131] In some embodiments, the electrolyte employs an electrolyte solution. The electrolyte solution includes an electrolyte salt and a solvent.

[0132] In some embodiments, the electrolyte salt can be selected from at least one of lithium hexafluorophosphate, lithium tetrafluoroborate, lithium perchlorate, lithium hexafluoroarsenate, lithium bisfluorosulfonylimide, lithium bis-trifluoromethanesulfonylimide, lithium trifluoromethanesulfonate, lithium difluorophosphate, lithium difluoro oxalato borate, lithium di oxalato borate, lithium difluoro di oxalato phosphate, and lithium tetrafluoro oxalato phosphate.

[0133] In some embodiments, the solvent can be selected from at least one of ethylene carbonate, propylene carbonate, methyl ethyl carbonate, diethyl carbonate, dimethyl carbonate, dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, butylene carbonate, fluoroethylene carbonate, methyl formate, methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, methyl butyrate, ethyl butyrate, 1,4-butyrolactone, cyclobutane sulfone, dimethyl sulfone, methyl ethyl sulfone, and diethyl sulfone.

[0134] In some embodiments, the electrolyte solution can also optionally include an additive. For example, the additive can include a negative electrode film-forming additive, a positive electrode film-forming additive, and can also include an additive that can improve certain properties of the battery, such as an additive that improves overcharge performance of the battery, an additive that improves high or low temperature performance of the battery, etc.

[0135] [Separator]

[0136] In some embodiments, the secondary battery further includes a separator. The type of separator is not particularly limited in the present application and any known porous structure separator having good chemical stability and mechanical stability can be used.

[0137] In some embodiments, the material of the separator can be selected from at least one of glass fiber, non-woven fabric, polyethylene, polypropylene, and polyvinylidene fluoride. The separator can be a single layer film or a multi-layer composite film and is not particularly limited. When the separator is a multi-layer composite film, the materials of the layers can be the same or different and are not particularly limited.

[0138] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be used to make an electrode assembly through a winding process or a stacking process.

[0139] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0140] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, etc. can be listed.

[0141] [Secondary battery]

[0142] In one embodiment of the present application, a secondary battery is provided, which includes a positive electrode sheet, a separator, a negative electrode sheet, and an electrolyte, and the binder in the active material layer of the positive electrode sheet includes the polymer of any embodiment of the present application.

[0143] In some embodiments, the secondary battery is a lithium ion battery or a sodium ion battery. During the charging and discharging of the battery, active ions are inserted and extracted between the positive electrode sheet and the negative electrode sheet. The electrolyte plays a role in conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is arranged between the positive electrode sheet and the negative electrode sheet, mainly to prevent short circuiting of the positive and negative electrodes, while allowing ions to pass through.

[0144] In some embodiments, the positive electrode sheet, the negative electrode sheet, and the separator can be made into an electrode assembly through a rolling process or a stacking process.

[0145] In some embodiments, the secondary battery can include an outer package. The outer package can be used to package the electrode assembly and the electrolyte described above.

[0146] In some embodiments, the outer package of the secondary battery can be a hard shell, such as a hard plastic shell, an aluminum shell, a steel shell, etc. The outer package of the secondary battery can also be a soft package, such as a pouch. The material of the soft package can be plastic, and as plastic, polypropylene, polybutylene terephthalate, polybutylene succinate, etc. can be listed.

[0147] The shape of the secondary battery is not particularly limited in the present application, and it can be cylindrical, square, or any other shape. For example, Figure 1 is a square structure of a secondary battery 5 as an example.

[0148] In some embodiments, referring to Figure 2 , the outer package can include a shell 51 and a cover plate 53. The shell 51 can include a bottom plate and a side plate connected to the bottom plate, and the bottom plate and the side plate form an accommodation cavity. The shell 51 has an opening communicating with the accommodation cavity, and the cover plate 53 can be arranged on the opening to close the accommodation cavity. The positive electrode sheet, the negative electrode sheet, and the separator can be formed into an electrode assembly 52 through a rolling process or a stacking process. The electrode assembly 52 is packaged in the accommodation cavity. The electrolyte is impregnated in the electrode assembly 52. The number of electrode assemblies 52 contained in the secondary battery 5 can be one or more, and the skilled person can select according to the specific actual needs.

[0149] In some embodiments, the secondary battery can be assembled into a battery module, and the number of secondary batteries contained in the battery module can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery module.

[0150] Figure 3 The battery module 4 is an example. Referring to FIG. 1, the battery module 4 includes a plurality of secondary batteries 5 and a housing 6 having a receiving space for receiving the plurality of secondary batteries 5. Figure 3 In the battery module 4, the plurality of secondary batteries 5 can be arranged in sequence along the length direction of the battery module 4. Of course, other arbitrary arrangement can also be adopted. Further, the plurality of secondary batteries 5 can be fixed by fasteners.

[0151] Optionally, the battery module 4 can further include a housing having a receiving space for receiving the plurality of secondary batteries 5.

[0152] In some embodiments, the above-mentioned battery module can also be assembled into a battery pack, and the number of battery modules contained in the battery pack can be one or more, and the specific number can be selected by those skilled in the art according to the application and capacity of the battery pack.

[0153] Figure 4 And Figure 5 The battery pack 1 is an example. Referring to FIG. 2, the battery pack 1 includes a plurality of battery modules 4 and a battery box 7 for receiving the plurality of battery modules 4. Figure 4 And Figure 5 In the battery pack 1, the battery box and the plurality of battery modules 4 arranged in the battery box can be included. The battery box includes an upper box body 2 and a lower box body 3, and the upper box body 2 can be arranged on the lower box body 3 to form a closed space for receiving the battery modules 4. The plurality of battery modules 4 can be arranged in the battery box in any manner.

[0154] In addition, the application also provides a power utilization device, which includes at least one of the secondary battery, the battery module, or the battery pack provided by the application. The secondary battery, the battery module, or the battery pack can be used as a power supply of the power utilization device, or can be used as an energy storage unit of the power utilization device. The power utilization device can include a mobile device (such as a mobile phone, a notebook computer, etc.), an electric vehicle (such as a pure electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, an electric bicycle, an electric scooter, an electric golf cart, an electric truck, etc.), an electric train, a ship and a satellite, an energy storage system, etc., but is not limited thereto.

[0155] As the power utilization device, the secondary battery, the battery module, or the battery pack can be selected according to the use requirement thereof.

[0156] Figure 6The electric device is, for example, an electric vehicle, a hybrid electric vehicle, or a plug-in hybrid electric vehicle. In order to meet the demand for high power and high energy density of the secondary battery for the electric device, a battery pack or a battery module can be used.

[0157] The device is, for example, a mobile phone, a tablet computer, a notebook computer, or the like. The device is generally required to be thin and light, and a secondary battery can be used as a power source.

[0158] Example

[0159] Hereinafter, the embodiments of the present application will be described. The embodiments described below are exemplary and are for the purpose of explanation of the present application and should not be understood as a limitation of the present application. In the embodiments, the specific techniques or conditions not mentioned are performed according to the techniques or conditions described in the literature in the art or according to the product manual. The reagents or instruments not mentioned by the manufacturer are all conventional products that can be obtained on the market.

[0160] I. Preparation method

[0161] Example 1

[0162] 1) Preparation of positive active material

[0163] 10 kg of lithium manganese iron phosphate (LiFe 0.3 Mn 0.6 Co 0.1 PO4) slurry (solvent: deionized water) having a solid content of 20% was poured into a homogenizer, and was sufficiently stirred by a high-speed dispersion disc at a revolution speed of 2000 r / min and a rotation speed of 50 r / min. Then, the stirred slurry was transferred to an ultrasonic disperser, and an ultrasonic frequency of 60 Hz and an ultrasonic time of 0.5 h were set to obtain a first slurry. The volume average particle size Dv50 of the lithium manganese iron phosphate in the first slurry was 300 nm.

[0164] The first slurry was mixed with 0.2 kg of glucose to obtain a mixed slurry having a solid content of 20%. The mixed slurry was placed in a spray drying storage tank, and was stirred at a rotation speed of 15 r / min. A "outlet temperature - magnetron on" interlock mode was selected, and the microwave frequency was directly controlled to 100 GHz by a magnetron to control the outlet temperature of the powder to 120°C. A peristaltic pump was turned on, and the slurry in the storage tank was rapidly introduced into a drying main tower through a nozzle. The intermediate product was obtained by drying for 28 minutes.

[0165] The intermediate product was calcined at a high temperature under nitrogen protection. The calcination temperature was 500°C, and the calcination time was 6 hours. The carbon-coated lithium manganese iron phosphate material was obtained by cooling. The mass content of the carbon layer was 2% based on the lithium manganese iron phosphate material.

[0166] 2) Preparation of positive electrode sheet

[0167] The positive electrode active material, conductive carbon black, and binder polyvinylidene fluoride in each of the examples or comparative examples were dispersed in N-methylpyrrolidone solvent at a weight ratio of 92%:2.5%:5.5%, mixed uniformly with sufficient stirring, and a positive electrode slurry was obtained. The positive electrode slurry was coated on both sides of an aluminum foil having a thickness of 12 μm using a doctor blade, and after the coating was completed, the positive electrode sheet was prepared by drying, cold pressing, slitting, and the like.

[0168] 3) Negative electrode sheet

[0169] The negative electrode material hard carbon, conductive agent acetylene black, binder styrene butadiene rubber, and thickening agent sodium carboxymethyl cellulose were dispersed in deionized water solvent at a weight ratio of 95:2:2:1, mixed uniformly with sufficient stirring, and a negative electrode slurry having a solid content of 15% was obtained. The negative electrode slurry was coated on both sides of an aluminum foil having a thickness of 12 μm using a doctor blade, and after the coating was completed, the negative electrode sheet was prepared by drying, cold pressing, slitting, and the like.

[0170] 4) Electrolyte

[0171] In an argon atmosphere glove box (H2O < 0.1 ppm, O2 < 0.1 ppm), organic solvents ethylene carbonate (EC) and methyl ethyl carbonate (EMC) were mixed uniformly at a volume ratio of 3:7, and LiPF6 lithium salt was dissolved in the organic solvents to prepare a solution having a weight content of 12.5%, and an electrolyte was obtained.

[0172] 5) Separator

[0173] A polypropylene film was used as the separator film.

[0174] 6) Preparation of battery

[0175] The positive electrode sheet, separator, and negative electrode sheet were stacked in order with the separator between the positive electrode and the negative electrode to function as a separator, and a bare cell was obtained by winding. The bare cell was placed in an outer package, and an electrolyte prepared as described above was injected to obtain a cell. The cell was subjected to processes such as packaging, liquid injection, formation, and degassing, and a lithium ion battery was obtained.

[0176] Examples 2-5

[0177] Examples 2-5 were compared with Example 1, and the microwave frequency or the solid content of the mixed slurry was adjusted. The specific parameters are shown in Table 1.

[0178] Comparative Example 1

[0179] The preparation method of the positive electrode active material was adjusted compared with Example 1, and the details are as follows:

[0180] 10Kg of manganese iron lithium phosphate slurry with a solid content of 20% was poured into a homogenizer, and fully stirred by a high-speed dispersion disc for 0.5h, with a revolution speed of 2000r / min and a rotation speed of 50r / min. Then the stirred slurry was transferred to an ultrasonic disperser, with a set ultrasonic frequency (60Hz) and ultrasonic time of 0.5h, to obtain a first slurry.

[0181] The first slurry was mixed with 0.2kg of glucose to obtain a mixed slurry, with a volume average particle size Dv50 of manganese iron lithium phosphate in the mixed slurry of 300nm. The mixed slurry was placed in a spray drying storage tank, stirred at a rotation speed of 15r / min, and the "inlet air temperature-electric heating" interlocking mode was selected, with the powder outlet air temperature directly controlled to 120℃ by adjusting the electric heating power. The peristaltic pump was started, and the slurry in the storage tank was quickly introduced into the drying main tower through the nozzle, and the intermediate was obtained after drying for 105 minutes.

[0182] The intermediate was calcined at a high temperature under nitrogen protection, with a calcination temperature of 500℃ and a calcination time of 6 hours, to obtain a carbon-coated manganese iron lithium phosphate material, with a mass content of the carbon layer of 2% based on the manganese iron lithium phosphate material.

[0183] II. Test method

[0184] 1. Low temperature cycle performance of the secondary battery

[0185] At a temperature of 25℃, the battery was rested for 2 hours, then charged at a constant current and constant voltage to 4.3V at a rate of 1C, rested for 2 hours at 4.3V, and then discharged at a constant current to 2.0V at a rate of 1C at a temperature of 25℃, which was a normal temperature charging and normal temperature discharging process. The discharge capacity at this time was the discharge capacity of the battery at 25℃, denoted as C1. The battery was rested for 2 hours, then charged at a constant current and constant voltage to 4.3V at a rate of 1C, rested for 2 hours at 4.3V, and then discharged at a constant current to 2.0V at a rate of 1C at a temperature of -20℃, which was a normal temperature charging and low temperature discharging process. The discharge capacity at this time was the discharge capacity of the battery at -20℃, denoted as C2. The low temperature capacity retention rate was calculated according to the following formula:

[0186] Low temperature capacity retention rate (%) = (C2 / C1) x 100%.

[0187] 2. Energy density of the secondary battery

[0188] The battery cells of the embodiment and comparative example were left at 25°C for 2 hours to ensure that the temperature of the battery cells was 25°C. The battery cells were charged to 4.3V at 0.33C at 25°C, and constant voltage charging was continued at 4.3V until the current reached 0.05C, at which point charging was terminated (where C0 represents the rated capacity of the battery). After the battery cells were left at 25°C for 1 hour, they were discharged to 2.0V at 0.33C at 25°C, and the total discharge energy of the battery cells was recorded as E0.

[0189] The measured cell weight is M0;

[0190] Cell weight energy density = total discharge energy E0 of the cell / cell weight M0.

[0191] 3. Analysis of test results of various embodiments and comparative examples

[0192] The positive electrode active materials and secondary batteries of the embodiments and comparative examples were prepared according to the above methods, and various parameters were measured. The results are shown in the table below.

[0193] Table 1

[0194]

[0195] The positive electrode active material in Examples 1-5 comprises a lithium manganese iron phosphate matrix material and a carbon layer at least coated on the surface of the lithium manganese iron phosphate matrix material, wherein the powder compaction density of the positive electrode active material at 400 MPa is greater than or equal to 2.35 g / cm 3 .

[0196] From the comparison of Examples 1-5 and Comparative Example 1, it can be seen that the use of the positive electrode active material of the present application can improve the energy density and low-temperature cycle performance of the battery.

[0197] It should be noted that the present application is not limited to the above-mentioned embodiments. The above-mentioned embodiments are merely examples, and any embodiments having substantially the same structure and effect as the technical concept within the scope of the present application are all included in the technical scope of the present application. In addition, without departing from the scope of the present application, any other embodiments that can be conceived by those skilled in the art and that combine some of the constituent elements in the embodiments are also included in the scope of the present application.

Claims

1. A positive electrode active material, characterized in that The positive electrode active material includes a lithium iron manganese phosphate matrix material and a carbon layer at least coated on the surface of the lithium iron manganese phosphate matrix material. The powder compaction density of the positive electrode active material at 400 MPa is greater than or equal to 2.35 g / cm 3 .

2. The positive electrode active material according to claim 1, characterized in that The powder compaction density of the positive electrode active material at 400 MPa is 2.35 g / cm 3 -2.50g / cm 3 .

3. The positive electrode active material according to claim 1 or 2, characterized in that Based on the total amount of the positive electrode active material, the positive electrode active material having a particle size of 4 μm to 6 μm accounts for 90% or more.

4. The positive electrode active material according to any one of claims 1 to 3, characterized in that The tap density of the positive electrode active material is 1.0 g / cm 3 -1.3g / cm 3 .

5. The positive electrode active material according to any one of claims 1 to 4, characterized in that The general formula of the lithium manganese iron phosphate matrix material is: Li z Fe x Mn y M 1-x-y PO4 Wherein z is 0.9 to 1.2, x is 0.001 to 0.999, y is 0.001 to 0.999, and 1-xy is 0 to 0.1; M includes at least one of Al, Cu, Mg, Zn, Ni, Ti, V, Zr, Co, Ga, Sn, Sb, Nb, and Ge.

6. The positive electrode active material according to any one of claims 1 to 5, characterized in that Based on the mass of the lithium manganese iron phosphate matrix material, the mass content of the carbon layer is 1.5%-2.5%.

7. A method for preparing a positive electrode active material, characterized in that: The steps include: spray drying a mixed slurry comprising lithium manganese iron phosphate and a carbon source to obtain an intermediate product; heat-treating the intermediate product to obtain a positive electrode active material; Wherein, the heat source of the spray drying includes microwave, The positive electrode active material includes a lithium iron manganese phosphate matrix material and a carbon layer at least coated on the surface of the lithium iron manganese phosphate matrix material. The powder compaction density of the positive electrode active material at 400 MPa is greater than or equal to 2.35 g / cm 3 .

8. The preparation method according to claim 7, characterized in that The microwave frequency of the microwave is 50 GHz to 100 GHz.

9. The preparation method according to claim 7 or 8, characterized in that The outlet air temperature of the spray drying is 100°C-120°C.

10. The preparation method according to any one of claims 7 to 9, characterized in that The solid content of the mixed slurry is 10%-30%.

11. The preparation method according to any one of claims 7 to 10, characterized in that The preparation method further comprises: Stirring and ultrasonically dispersing an initial slurry containing lithium manganese iron phosphate to obtain a first slurry; The first slurry is mixed with a carbon source to obtain the mixed slurry containing lithium manganese iron phosphate and the carbon source.

12. The preparation method according to any one of claims 7 to 11, characterized in that The volume average particle size Dv50 of the lithium manganese iron phosphate in the first slurry is 100 nm to 300 nm.

13. The preparation method according to any one of claims 7 to 12, characterized in that The heat treatment specifically includes: The intermediate product is heat-treated under an inert atmosphere to obtain a positive electrode active material. Wherein, the treatment temperature of the heat treatment is 400° C.-800° C.; and / or the treatment time of the heat treatment is 4 h-10 h.

14. A secondary battery comprising a positive electrode plate, characterized in that: The positive electrode sheet includes the positive electrode active material according to any one of claims 1 to 6 or the positive electrode active material prepared by the preparation method according to any one of claims 7 to 13.

15. An electrical device, characterized in that: The secondary battery according to claim 14 is included.

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