Coated lithium iron phosphate positive electrode material, preparation method thereof, secondary battery and electric device

Abstract

The application provides a co-coated lithium iron phosphate positive electrode material and a preparation method thereof, a secondary battery and an electric device, and relates to the field of secondary batteries. The co-coated lithium iron phosphate positive electrode material comprises a lithium iron phosphate base body and a coating layer on the surface of the lithium iron phosphate base body, wherein the coating layer comprises carbon and silicon dioxide; the weight of the silicon dioxide accounts for 0.1-5% of the total weight of the positive electrode material; the weight of the carbon accounts for 0.1-6% of the total weight of the positive electrode material; the particle size of the lithium iron phosphate base body is 0.1-20 mu m; and the thickness of the coating layer is 1-20 nm. The carbon in the coating layer can improve the electronic conductivity, the silicon dioxide in the coating layer can enhance the structural stability, and the carbon and the silicon dioxide in the coating layer can cooperate with each other to effectively improve the cycle life and the rate performance of the battery.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of secondary batteries, in particular to a co-coated lithium iron phosphate positive electrode material, a preparation method thereof, a secondary battery and an electric device. BACKGROUND

[0002] Among lithium ion battery positive electrode materials, lithium iron phosphate has become one of the most promising materials due to its high safety, low cost, long cycle and other advantages. However, the electronic transition resistance of Fe 3+ / Fe 2+ in the olivine structure of lithium iron phosphate is large, and its intrinsic electronic conductivity is only 10 -9 S cm -1 , which restricts the high rate performance of the battery.

[0003] The current mainstream method is to improve the conductivity of lithium iron phosphate by carbon coating. However, this method has problems such as uneven coating and loose carbon layer, which leads to poor rate performance and cycle stability. SUMMARY

[0004] The purpose of the present application is to provide a co-coated lithium iron phosphate positive electrode material, a preparation method thereof, a secondary battery and an electric device to solve the above problems.

[0005] To achieve the above purpose, the following technical solutions are adopted in the present application:

[0006] A co-coated lithium iron phosphate positive electrode material, comprising a lithium iron phosphate matrix and a coating layer located on the surface of the lithium iron phosphate matrix, wherein the coating layer comprises carbon and silicon dioxide;

[0007] The weight of the silicon dioxide accounts for 0.1% to 5% of the total weight of the positive electrode material;

[0008] The weight of the carbon accounts for 0.1% to 6% of the total weight of the positive electrode material;

[0009] The Dv50 particle size of the lithium iron phosphate matrix is 0.1 to 20 μm;

[0010] The thickness of the coating layer is 1 to 20 nm.

[0011] According to the embodiments of the present application, the weight of the silicon dioxide accounts for 0.3 to 1.2% of the total weight of the positive electrode material;

[0012] The weight of the carbon accounts for 0.5 to 2% of the total weight of the positive electrode material;

[0013] The Dv50 particle size of the lithium iron phosphate matrix is 0.9 to 1.2 μm;

[0014] The thickness of the coating layer is 2 to 18 nm.

[0015] The application also provides a preparation method of the co-coated lithium iron phosphate positive electrode material as described above, comprising:

[0016] mixing the silane coupling agent with water to perform a hydrolysis reaction to obtain a silane coupling agent dispersion liquid;

[0017] dispersing an iron source in water to obtain an iron source dispersion liquid, and mixing the silane coupling agent dispersion liquid with the iron source dispersion liquid to obtain a mixed liquid;

[0018] mixing the mixed liquid with a lithium source, a carbon source and an optional phosphorus source to obtain a slurry, and grinding and drying the slurry to obtain a precursor;

[0019] sintering the precursor in an inert atmosphere to obtain the co-coated lithium iron phosphate positive electrode material.

[0020] According to the embodiments of the application, the structural formula of the silane coupling agent is Y 4-n -Si-(OR) n wherein n is 1-3, Y includes at least one of alkenyl, amino, epoxy, methacryloyloxy, mercapto, phenyl, alkyl, aminoalkyl and fluoroalkyl, and R includes alkyl;

[0021] and / or the iron source includes at least one of an iron salt, a ferrous salt and an oxide of iron, and the iron salt includes iron phosphate;

[0022] and / or the lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium acetate, lithium dihydrogen phosphate and lithium nitrate;

[0023] and / or the phosphorus source includes at least one of ammonium dihydrogen phosphate, ammonium phosphate, iron phosphate, lithium dihydrogen phosphate and phosphoric acid;

[0024] and / or in the step of preparing the silane coupling agent dispersion liquid, the mass ratio of the silane coupling agent to water is 1: (1-10);

[0025] and / or the method further comprises adding an alcohol solvent to the hydrolysis reaction system;

[0026] and / or the alcohol solvent includes at least one of methanol, ethanol, ethylene glycol and isopropyl alcohol;

[0027] and / or the volume ratio of the water to the alcohol solvent is 1: (1-3);

[0028] and / or the method further comprises adding a pH adjuster to the hydrolysis reaction system to make the pH of the hydrolysis reaction system 3-7;

[0029] and / or the pH adjuster includes at least one of formic acid and acetic acid.

[0030] and / or, the temperature of the hydrolysis reaction is 20-60℃;

[0031] and / or, the time of the hydrolysis reaction is 0.5-6 hours;

[0032] and / or, the solid content of the slurry is 30%-70%;

[0033] and / or, the carbon source comprises at least one of a sugar compound, an acid compound and an alcohol compound, the sugar compound comprises at least one of glucose, sucrose and starch, the acid compound comprises ascorbic acid, and the alcohol compound comprises polyethylene glycol.

[0034] According to the embodiments of the present application, the silane coupling agent comprises at least one of hexadecyltrimethoxysilane, octyltrimethoxysilane, vinyltrimethoxysilane, γ-aminopropyltriethoxysilane, phenyltriethoxysilane, methyltrimethoxysilane and fluoroalkyltrimethoxysilane.

[0035] According to the embodiments of the present application, the mass ratio of the silane coupling agent to the iron source is 0.001-0.08:1;

[0036] and / or, the molar ratio of Li in the lithium source, Fe in the iron source and P in the raw material is (0.95-1.05):(0.95-1.05):1, wherein the P in the raw material is from the iron source and / or the phosphorus source;

[0037] and / or, the mass of the carbon source accounts for 5-14% of the mass of the iron source.

[0038] According to the embodiments of the present application, the Dv50 particle size of the ground slurry is 0.1-10μm;

[0039] and / or, the particle size of the precursor is 1.0-30μm.

[0040] According to the embodiments of the present application, the heating rate of the sintering is 1-8℃ / min;

[0041] and / or, the temperature of the sintering is 700-830℃;

[0042] and / or, the time of the sintering is 4-10h;

[0043] and / or, the inert atmosphere comprises at least one of nitrogen and argon, and the oxygen content of the inert atmosphere is <30ppm.

[0044] The application also provides a secondary battery comprising the co-coated lithium iron phosphate positive electrode material described above or the co-coated lithium iron phosphate positive electrode material prepared by the preparation method described above.

[0045] The application also provides an electric device comprising the secondary battery described above.

[0046] Compared with the prior art, the application has the following beneficial effects:

[0047] The application provides a carbon and silicon dioxide co-coated lithium iron phosphate material. The carbon in the coating layer can improve the electronic conductivity, the silicon dioxide in the coating layer can enhance the structural stability, and the carbon and silicon dioxide in the coating layer can cooperate with each other to effectively improve the cycle life and rate performance of the battery.

[0048] Specifically, the intrinsic electronic conductivity of lithium iron phosphate is low, and the electronic conductivity of the material can be effectively improved after carbon coating, so that the electron is more easily conducted in the electrode material, thereby improving the charge and discharge performance of the battery, improving the rate performance of the battery, and enabling the battery to have better performance under the condition of large-current charge and discharge. However, the carbon layer is often loose and porous, which is not conducive to the stability and long-life cycle of the battery. Silicon dioxide has good chemical stability and mechanical properties, and after forming a co-coated layer with carbon, the structural stability of the coated material can be enhanced, the lattice distortion caused by the insertion and extraction of lithium ions during the charge and discharge process can be reduced, thereby improving the cycle performance of the material and prolonging the service life of the battery. In addition, silicon dioxide also has high thermal stability, which can play a certain role in heat insulation and heat dissipation during the operation of the battery, which helps to improve the thermal stability of lithium iron phosphate and enhance the safety of the battery in a high-temperature environment, thereby reducing the risk of thermal runaway.

[0049] The preparation method of the application has the advantages of simple process and easy operation, is compatible with existing lithium iron phosphate production and preparation equipment, is easy to industrialize, and is convenient for industrialization and application. BRIEF DESCRIPTION OF DRAWINGS

[0050] In order to more clearly illustrate the technical solutions of the embodiments of the application, the following will briefly introduce the drawings needed to be used in the embodiments. It should be understood that the following drawings only show some embodiments of the application, and therefore should not be regarded as a limitation on the scope of the application.

[0051] Figure 1 A structural evolution diagram in the preparation process of the positive electrode material of the application;

[0052] Figure 2 XRD patterns of the positive electrode materials of Examples 1-3 and Comparative Examples 1-2;

[0053] Figure 3 This is a TEM image of the cathode material in Example 1;

[0054] Figure 4 This is a TEM image of the cathode material in Example 2;

[0055] Figure 5 This is a TEM image of the cathode material in Example 3;

[0056] Figure 6 This is a TEM image of the cathode material in Example 4;

[0057] Figure 7 This is a TEM image of the cathode material in Example 5;

[0058] Figure 8 This is a TEM image of the cathode material in Comparative Example 1;

[0059] Figure 9 This is a TEM image of the cathode material in Comparative Example 2;

[0060] Figure 10 This is a TEM image of the cathode material in Comparative Example 3;

[0061] Figure 11 This is a TEM image of the cathode material in Comparative Example 4. Detailed Implementation

[0062] As used in this article:

[0063] "Prepared from" is synonymous with "comprising". The terms "comprising", "including", "having", "containing", or any other variations thereof as used herein are intended to cover non-exclusive inclusion. For example, a composition, step, method, article, or apparatus that includes the listed elements is not necessarily limited to those elements, but may include other elements not expressly listed or elements inherent to such composition, step, method, article, or apparatus.

[0064] The conjunction "composed of..." excludes any unspecified elements, steps, or components. If used in a claim, this phrase makes the claim closed, excluding materials other than those described, except for associated conventional impurities. When the phrase "composed of..." appears in a clause of the body of a claim rather than immediately following it, it limits only the elements described in that clause; other elements are not excluded from the claim as a whole.

[0065] When expressing amounts, concentrations, or other values or parameters of a substance in a range, preferably a range, or a series of upper preferred values and lower preferred values, it is to be understood that the disclosure specifically envisions all ranges formed from any of the upper values or preferred values with any of the lower values or preferred values, even if that range is not expressly disclosed. For example, where a range "1-5" is disclosed, the disclosure is to be interpreted to include ranges such as "1-4," "1-3," "1-2," "1-3 and 5," etc. When numerical ranges are disclosed, unless otherwise stated, the range is intended to include both the upper and lower values and all intervening values, including fractions of the unit, unless the context clearly indicates otherwise.

[0066] In these embodiments, the parts and percentages described are by mass, unless otherwise indicated.

[0067] "Mass parts" refers to a basic unit of measurement that represents the mass ratio relationship of multiple components, 1 part can represent any unit mass, such as 1 g, 2.689 g, etc. If we say that the mass parts of component A is a parts, and the mass parts of component B is b parts, it means that the mass ratio of component A to component B is a:b. Alternatively, it means that the mass of component A is aK, and the mass of component B is bK (K is an arbitrary number, indicating a multiple factor). It should not be misunderstood that unlike mass parts, the sum of the mass parts of all components is not limited to 100 parts.

[0068] "And / or" is used to indicate that one or both of the described situations can occur, for example, A and / or B includes (A and B) and (A or B).

[0069] In order to better illustrate the technical solutions provided in the present application, before the embodiments, the technical solutions are stated as a whole, as follows:

[0070] A co-coated lithium iron phosphate positive electrode material, comprising a lithium iron phosphate base and a coating layer on the surface of the lithium iron phosphate base, wherein the coating layer comprises carbon and silicon dioxide;

[0071] The weight of the silicon dioxide accounts for 0.1% to 5% of the total weight of the positive electrode material;

[0072] The weight of the carbon accounts for 0.1% to 6% of the total weight of the positive electrode material;

[0073] The Dv50 particle size of the lithium iron phosphate base is 0.1 to 20 μm;

[0074] The thickness of the coating layer is 1-20 nm.

[0075] The weight of the silicon dioxide in the application accounts for 0.1%-5% of the total weight of the positive electrode material. When the weight of the silicon dioxide is within the above range, the silicon dioxide and the carbon can cooperate with each other to effectively enhance the compactness and stability of the coating layer. If the weight of the silicon dioxide is too small, the silicon dioxide cannot effectively fill the gaps in the carbon layer, and the improvement of the stability of the coating layer is small, and the problem of loose carbon layer in the conventional carbon coating cannot be effectively improved. If the weight of the silicon dioxide is too large, too much silicon dioxide will hinder the electron conduction, reduce the conductivity of the material, and thus affect the rate performance of the battery.

[0076] The weight of the carbon in the application accounts for 0.1%-6% of the total weight of the positive electrode material. When the weight of the carbon is within the above range, the carbon can form a continuous and effective conductive path, which can effectively improve the electronic conductivity of the lithium iron phosphate and meet the demand of the high rate performance of the battery. If the proportion of the carbon is too small, a complete conductive network cannot be formed, the conductivity of the material is not obviously improved, and the problem of low intrinsic electronic conductivity of the lithium iron phosphate cannot be solved. If the proportion of the carbon is too large, excessive carbon will form a thick layered structure on the surface of the lithium iron phosphate, hinder the diffusion of lithium ions, and cause the charge and discharge performance of the battery to decrease.

[0077] The Dv50 particle size of the lithium iron phosphate matrix in the application is 0.1-20 μm. When the Dv50 particle size of the lithium iron phosphate matrix is within the above range, the appropriate particle size can ensure that the material has good fluidity, which is convenient for subsequent electrode preparation and other processing processes. If the particle size of the lithium iron phosphate matrix is too small, the specific surface area of the particles is too large, which will increase the surface energy of the material, cause the particles to easily agglomerate, be not conducive to dispersion and processing, and also increase the side reaction with the electrolyte, and affect the cycle performance of the battery. If the particle size of the lithium iron phosphate matrix is too large, the migration path of lithium ions in the particles becomes long, which will reduce the diffusion speed of lithium ions, and cause the rate performance of the battery to be poor.

[0078] The thickness of the coating layer in the application is 1-20 nm. When the thickness of the coating layer is too small, the coating layer cannot completely cover the surface of the lithium iron phosphate matrix, and cannot effectively form an effective conductive and protective network, and the stability and conductivity of the material are limitedly improved. If the thickness of the coating layer is too large, the coating layer will become an obstacle for the migration of lithium ions, increase the diffusion resistance of lithium ions, and cause the charge and discharge efficiency and the high rate performance of the battery to decrease.

[0079] In some embodiments, the weight of the silicon dioxide accounts for 0.1%, 0.2%, 0.3%, 0.5%, 0.8%, 1%, 1.2%, 2%, 3%, 4%, 5% or any value between 0.1%-5% of the total weight of the positive electrode material.

[0080] In some embodiments, the weight of the carbon is 0.1%, 0.5%, 1%, 1.5%, 2%, 2.5%, 3%, 3.5%, 4%, 4.5%, 5%, 5.5%, 6%, or any value between 0.1% and 6% of the total weight of the positive electrode material.

[0081] In some embodiments, the Dv50 particle size of the lithium iron phosphate matrix can be 0.1 μm, 0.5 μm, 0.9 μm, 1 μm, 1.2 μm, 3 μm, 5 μm, 8 μm, 10 μm, 13 μm, 15 μm, 18 μm, 20 μm, or any value between 0.1 μm and 20 μm.

[0082] In some embodiments, the thickness of the coating layer can be 1 nm, 2 nm, 3 nm, 5 nm, 8 nm, 10 nm, 13 nm, 15 nm, 18 nm, 20 nm, or any value between 1 nm and 20 nm.

[0083] According to embodiments of the present application, the weight of the silicon dioxide is 0.3-1.2% of the total weight of the positive electrode material;

[0084] The weight of the carbon is 0.5-2% of the total weight of the positive electrode material;

[0085] The Dv50 particle size of the lithium iron phosphate matrix is 0.9-1.2 μm;

[0086] The thickness of the coating layer is 2-18 nm.

[0087] In some embodiments, the silicon of the silicon dioxide is connected to the lithium iron phosphate through a Si-O-Fe bond.

[0088] In some embodiments, the Dv50 particle size of the co-coated lithium iron phosphate positive electrode material is 0.8-1.5 μm.

[0089] The present application also provides a preparation method of the co-coated lithium iron phosphate positive electrode material as described above, comprising:

[0090] Mixing the silane coupling agent with water to perform a hydrolysis reaction to obtain a silane coupling agent dispersion;

[0091] Dispersing an iron source in water to obtain an iron source dispersion, mixing the silane coupling agent dispersion with the iron source dispersion to obtain a mixed solution;

[0092] Mixing the mixed solution with a lithium source, a carbon source, and an optional phosphorus source to obtain a slurry, grinding and drying the slurry to obtain a precursor;

[0093] Sintering the precursor in an inert atmosphere to obtain the co-coated lithium iron phosphate positive electrode material.

[0094] When the iron source of the present application is dispersed in water, Fe 3+ The ion as a Lewis acid site dissociates and adsorbs water molecules:

[0095] ≡Fe 3+ + H2O→≡Fe-OH + H +

[0096] The hydroxyl group (-Si-OH) generated after the hydrolysis of the silane coupling agent can react with the hydroxyl group on the surface of the iron source (for example, iron phosphate), to generate a Si-O-Fe bond. Moreover, due to the formation of the Si-O-Fe bond, the peeling of the coating material can be avoided.

[0097] When the iron source is iron phosphate, the phosphate group (PO4 3- ) is protonated under acidic conditions (≡P—O - + H + → ≡P—OH), which can further increase the density of the hydroxyl group on the surface of the iron source. The -Si-OH generated after the hydrolysis of the silane coupling agent can not only react with ≡Fe-OH to generate a Fe-O-Si bond, but also can undergo a condensation reaction with ≡P-OH to generate a P-O-Si bond. The Fe-O-Si bond and the P-O-Si bond jointly constitute a “double anchoring” of the coating layer and the substrate, which can more effectively inhibit the peeling of the coating layer in the charge and discharge cycle, and significantly improve the structural stability of the material.

[0098] It should be noted that “≡Fe 3+ ”, “≡Fe-OH”, “≡P—O - ”, and “≡P—OH” in “≡” represent the connection sites on the surface of the solid.

[0099] When the iron source is iron phosphate, since it can also provide phosphorus, it is not necessary to add a phosphorus source. If the iron source is other substances, a phosphorus source needs to be added.

[0100] After the silane coupling agent is coated on the surface of the iron phosphate, it can act as a physical barrier to inhibit the grain growth and agglomeration during the sintering process, so as to prepare iron phosphate lithium particles with uniform particle size distribution.

[0101] According to the embodiments of the present application, the structural formula of the silane coupling agent is Y 4-n -Si-(OR) n , wherein n is 1-3, Y includes at least one of alkenyl, amino, epoxy, methacryloyloxy, mercapto, phenyl, alkyl, aminoalkyl, and fluoroalkyl, and R includes at least one of alkyl, preferably methyl and ethyl;

[0102] The silane coupling agent is hydrolyzed to generate -Si-OH, which condenses with the hydroxyl group (-Fe-OH) on the surface of the iron source to form a Si-O-Fe covalent bond, and the alkyl chain in the silane coupling agent can be carbonized to generate carbon after high-temperature sintering.

[0103] And / or, the iron source includes at least one of an iron salt, a ferrous salt, and an oxide of iron, and the iron salt includes iron phosphate;

[0104] And / or, the lithium source includes at least one of lithium hydroxide, lithium carbonate, lithium acetate, lithium dihydrogen phosphate, and lithium nitrate;

[0105] And / or, the phosphorus source includes at least one of ammonium dihydrogen phosphate, ammonium phosphate, iron phosphate, lithium dihydrogen phosphate, and phosphoric acid;

[0106] And / or, in the step of preparing the silane coupling agent dispersion, the mass ratio of the silane coupling agent to water is 1:(1-10); the amount of water directly affects the degree of hydrolysis of the silane coupling agent. If the amount of water is too small, the hydrolysis of the silane coupling agent may not be complete, leaving residual unhydrolyzed alkoxy groups, which affects the subsequent combination with iron phosphate; if the amount of water is too large, it will dilute the system, affecting the efficiency of the subsequent process.

[0107] For example, the mass ratio of the silane coupling agent to water can be 1:1, 1:2, 1:3, 1:4, 1:5, 1:6, 1:7, 1:8, 1:9, 1:10, or any value between 1:(1-10).

[0108] And / or, the method further includes adding an alcohol solvent to the system of the hydrolysis reaction;

[0109] And / or, the alcohol solvent includes at least one of methanol, ethanol, ethylene glycol, and isopropyl alcohol; the silane coupling agent (such as hexadecyl trimethoxysilane) is prone to local polycondensation in pure water due to too fast hydrolysis speed, forming insoluble siloxane agglomerates. Alcohol, as a polar organic solvent, can slow down the hydrolysis rate of the Si-OR group in the silane molecule by reducing the dielectric constant of water, making the hydrolysis reaction proceed more uniformly.

[0110] And / or, the volume ratio of the water to the alcohol solvent is 1:(1-3); for example, the volume ratio of water to alcohol solvent can be 1:1, 1:2, 1:3, or any value between 1:(1-3).

[0111] A weakly acidic or weakly basic aqueous solution can promote the hydrolysis of the silane coupling agent. Silane coupling agents with acidic or basic groups are more prone to hydrolysis, such as silane coupling agents with amino groups, which can promote the hydrolysis reaction using their own basicity. Non-amino silanes can adjust the pH of the system by adding a pH adjuster to promote hydrolysis.

[0112] And / or, the method further comprises: adding a pH regulator to the hydrolysis reaction system to make the pH of the hydrolysis reaction system 3-7; by adding a pH regulator, the pH of the hydrolysis reaction system can be adjusted to promote the progress of the hydrolysis reaction.

[0113] And / or, the pH regulator comprises at least one of formic acid, acetic acid; further, 0.1%-0.5% formic acid or 0.1%-0.5% acetic acid can be used.

[0114] And / or, the temperature of the hydrolysis reaction is 20-60℃; for example, the temperature of the hydrolysis reaction can be 20℃, 30℃, 40℃, 50℃, 60℃ or any value between 20-60℃.

[0115] And / or, the time of the hydrolysis reaction is 0.5-6 hours; for example, the time of the hydrolysis reaction can be 0.5 hours, 1 hour, 2 hours, 3 hours, 4 hours, 5 hours, 6 hours or any value between 0.5-6 hours.

[0116] And / or, the solid content of the slurry is 30%-70%; for example, the solid content of the slurry can be 30%, 40%, 50%, 60%, 70% or any value between 30%-70%.

[0117] And / or, the carbon source comprises at least one of a sugar compound, an acid compound and an alcohol compound, the sugar compound comprises at least one of glucose, sucrose and starch, the acid compound comprises ascorbic acid, and the alcohol compound comprises polyethylene glycol.

[0118] According to the embodiments of the present application, the silane coupling agent comprises at least one of hexadecyl trimethoxysilane, octyl trimethoxysilane, vinyl trimethoxysilane, γ-aminopropyl triethoxysilane, phenyl triethoxysilane, methyl trimethoxysilane and fluoroalkyl trimethoxysilane.

[0119] According to the embodiments of the present application, the mass ratio of the silane coupling agent to the iron source is 0.001-0.08:1; if the amount of silane coupling agent is too much, agglomeration of the silane coupling agent is prone to occur. If the amount of silane coupling agent is too little, the coating effect cannot be achieved.

[0120] For example, the mass ratio of the silane coupling agent to the iron source can be 0.001:1, 0.01:1, 0.02:1, 0.03:1, 0.04:1, 0.05:1, 0.06:1, 0.07:1, 0.08:1 or any value between 0.001-0.08:1.

[0121] In some embodiments, the mass ratio of the silane coupling agent to the iron source is preferably 0.02-0.08:1.

[0122] and / or, the molar ratio of Li in the lithium source, Fe in the iron source and P in the raw material is (0.95-1.05):(0.95-1.05):1, wherein the P in the raw material is from the iron source and / or the phosphorus source; if the content of Li, Fe and P does not meet the above content range, the electrochemical performance of the positive electrode material will be poor, and specifically, if the iron-phosphorus ratio is too high, the electrochemical performance of the positive electrode material will be poor.

[0123] For example, the molar ratio of Li in the lithium source, Fe in the iron source and P in the raw material can be 0.95:0.95:1, 0.95:1:1, 0.95:1.05:1, 1:0.95:1, 1.05:0.95:1, 1:1:1, 1:1.05:1, 1.05:1:1, 1.05:1.05:1 or any value between (0.95-1.05):(0.95-1.05):1.

[0124] and / or, the mass of the carbon source accounts for 5-14% of the mass of the iron source. If the use amount ratio of the carbon source to the iron source is too large, the capacity density of the positive electrode material will be reduced. If the use amount ratio of the carbon source to the iron source is too small, the conductivity of the positive electrode material will be poor.

[0125] For example, the mass of the carbon source accounts for 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14% or any value between 5-14% of the mass of the iron source.

[0126] Figure 1 The above is a schematic diagram of the structure evolution in the preparation process of the positive electrode material of the application. After the silane coupling agent is hydrolyzed, it reacts with the hydroxyl groups on the surface of the iron phosphate to adhere to the surface of the iron phosphate. After high-temperature sintering, a co-coating layer containing silicon and silicon dioxide is formed on the surface of the lithium iron phosphate.

[0127] According to the embodiments of the application, the Dv50 particle size of the ground slurry is 0.1-10 μm.

[0128] For example, the Dv50 particle size of the ground slurry can be 0.1 μm, 0.2 μm, 0.3 μm, 0.5 μm, 0.8 μm, 1 μm, 3 μm, 5 μm, 8 μm, 10 μm or any value between 0.1-10 μm.

[0129] and / or, the particle size of the precursor is 1.0-30 μm.

[0130] For example, the particle size of the precursor can be 1.0 μm, 5 μm, 10 μm, 15 μm, 20 μm, 25 μm, 30 μm or any value between 1.0-30 μm.

[0131] According to an embodiment of this application, the heating rate of the sintering is 1-8℃ / min; for example, the heating rate of the sintering can be any value between 1℃ / min, 2℃ / min, 3℃ / min, 4℃ / min, 5℃ / min, 6℃ / min, 7℃ / min, 8℃ / min or 1-8℃ / min.

[0132] And / or, the sintering temperature is 700-830℃; if the sintering temperature is too high, the carbon content in the cathode material will decrease, the conductivity will deteriorate, and the particle size will increase. If the sintering temperature is too low, the sintering will be insufficient, resulting in poor electrochemical performance.

[0133] For example, the sintering temperature can be any value between 700℃, 730℃, 750℃, 780℃, 800℃, 830℃, or 700-830℃.

[0134] And / or, the sintering time is 4-10h; for example, the sintering time can be any value between 4h, 5h, 6h, 7h, 8h, 9h, 10h or 4-10h.

[0135] And / or, the inert atmosphere includes at least one of nitrogen and argon, and the oxygen content of the inert atmosphere is <30ppm.

[0136] This application also provides a secondary battery, which includes the co-coated lithium iron phosphate cathode material described above or the co-coated lithium iron phosphate cathode material prepared by the preparation method of the co-coated lithium iron phosphate cathode material described above.

[0137] This application also provides an electrical device, which includes the secondary battery described above.

[0138] The implementation schemes of this application will be described in detail below with reference to specific embodiments. However, those skilled in the art will understand that the following embodiments are only for illustrating this application and should not be regarded as limiting the scope of this application. Unless otherwise specified in the embodiments, conventional conditions or conditions recommended by the manufacturer shall apply. Reagents or instruments used without specified manufacturers are all conventional products that can be purchased commercially.

[0139] Example 1

[0140] In this embodiment, a co-coated lithium iron phosphate cathode material was prepared according to the following method:

[0141] (1) 160 g hexadecyltrimethoxysilane (HDTMS) was added to 1 kg deionized water, and an appropriate amount of acetic acid was added to adjust the pH to 6, and the hexadecyltrimethoxysilane was fully hydrolyzed by fully stirring at 30℃ for 1 hour to obtain a uniform silane coupling agent dispersion.

[0142] (2) 4 kg of iron phosphate was dispersed in 8 kg of water, and fully dispersed at room temperature for 40 minutes to obtain an iron phosphate dispersion, and the silane coupling agent dispersion obtained in step (1) was added to the iron phosphate dispersion, and fully dispersed by stirring to obtain a mixture, wherein the mass ratio of the silane coupling agent (hexadecyltrimethoxysilane) to the iron source (iron phosphate) was 0.04:1.

[0143] (3) 983 g of lithium carbonate, 180 g of glucose, and 200 g of PEG were added to the mixture obtained in step (2), and fully stirred at 30℃ for 1 hour. The molar ratio of the total iron element to the total phosphorus element was 1:1; the molar ratio of the total iron element to the lithium element was 1:1.003; the mass of the carbon source (glucose and PEG) accounted for 9.5% of the mass of the iron source (iron phosphate); and the solid content of the slurry was 38.03%. The dispersed slurry was ground, and the grinding was stopped when the slurry particle size Dv50 was 0.5 μm, and the ground slurry was spray dried to obtain a precursor;

[0144] (4) The precursor obtained in step (3) was sintered under a nitrogen atmosphere (oxygen content <5 ppm). The temperature was raised to 780℃ at a rate of 5℃ / min, and maintained at this temperature for 10 hours, and then naturally cooled to room temperature. After airflow crushing, a co-coated lithium iron phosphate positive electrode material was obtained.

[0145] Figure 2 The XRD pattern of the positive electrode material of Example 1-3 and Comparative Examples 1-2 is shown in FIG. 1. Figure 2 As can be seen from FIG. 1, the peak position of Example 1 is completely consistent with the standard card of lithium iron phosphate, indicating that the lithium iron phosphate positive electrode material is successfully prepared.

[0146] The positive electrode material of Example 1 includes a lithium iron phosphate matrix and a coating layer located on the surface of the lithium iron phosphate matrix, the coating layer contains carbon and silicon dioxide; the weight of the silicon dioxide accounts for 0.6% of the total weight of the positive electrode material, the weight of the carbon accounts for 1.3% of the total weight of the positive electrode material, and the Dv50 particle size of the lithium iron phosphate matrix is 1.05 μm, Figure 3 The TEM pattern of the positive electrode material of Example 1 is shown in FIG. 2. Figure 3 As can be seen from FIG. 2, the thickness of the coating layer in Example 1 is about 5 nm.

[0147] Example 2

[0148] The co-coated lithium iron phosphate cathode material of the present embodiment is prepared according to the following method:

[0149] (1) 80 g of hexadecyltrimethoxysilane (HDTMS) is added to 0.5 kg of deionized water, and an appropriate amount of acetic acid is added to adjust the pH to 6, and the mixture is stirred at 30°C for 1 hour to hydrolyze the hexadecyltrimethoxysilane and obtain a uniform dispersion.

[0150] (2) 4 kg of iron phosphate is dispersed in 8 kg of water at room temperature for 40 minutes to obtain an iron phosphate dispersion, and the dispersion obtained in step (1) is added to the iron phosphate dispersion and stirred to obtain a mixture, wherein the mass ratio of the silane coupling agent (hexadecyltrimethoxysilane) to the iron source (iron phosphate) is 0.02:1.

[0151] (3) 983 g of lithium carbonate, 180 g of glucose, and 200 g of PEG are added to the mixture obtained in step (2) and stirred at 30°C for 1 hour. The molar ratio of total iron elements to total phosphorus elements is 1:1, the molar ratio of total iron elements to lithium elements is 1:1.003, the mass of the carbon source (glucose and PEG) accounts for 9.5% of the mass of the iron source (iron phosphate), and the solid content of the slurry is 39.04%. The dispersed slurry is ground, and the grinding is stopped when the slurry particle size Dv50 is 0.5 μm. The ground slurry is then spray dried to obtain a precursor;

[0152] (4) The precursor obtained in step (3) is sintered under a nitrogen atmosphere (oxygen content <5 ppm). The temperature is raised to 780°C at a rate of 5°C / min, and the temperature is maintained at this temperature for 10 hours, and then naturally cooled to room temperature. After air flow crushing, a co-coated lithium iron phosphate cathode material is obtained.

[0153] Figure 2 The XRD pattern of the cathode material of Example 2 is shown in FIG. 2. Figure 2 As can be seen from FIG. 2, the peak position of Example 2 is completely consistent with the standard card of lithium iron phosphate, indicating that the lithium iron phosphate cathode material is successfully prepared.

[0154] The cathode material of Example 2 includes a lithium iron phosphate matrix and a coating layer on the surface of the lithium iron phosphate matrix, the coating layer contains carbon and silicon dioxide; the weight of silicon dioxide accounts for 0.3% of the total weight of the cathode material, the weight of carbon accounts for 1.25% of the total weight of the cathode material, and the Dv50 particle size of the lithium iron phosphate matrix is 1.15 μm, Figure 4 The TEM pattern of the cathode material of Example 2 is shown in FIG. 3. Figure 4 As can be seen from FIG. 3, the thickness of the coating layer in Example 2 is about 3 nm.

[0155] Example 3

[0156] The co-coated lithium iron phosphate cathode material of the present embodiment is prepared according to the following method:

[0157] (1) 320 g of hexadecyltrimethoxysilane (HDTMS) is added to 2 kg of deionized water, and an appropriate amount of acetic acid is added to adjust the pH to 6, and the mixture is stirred at 30°C for 1 hour to hydrolyze the hexadecyltrimethoxysilane and obtain a uniform dispersion.

[0158] (2) 4 kg of iron phosphate is dispersed in 7 kg of water at room temperature for 40 minutes to obtain an iron phosphate dispersion, and the dispersion obtained in step (1) is added to the iron phosphate dispersion and stirred to obtain a mixture, wherein the mass ratio of the silane coupling agent (hexadecyltrimethoxysilane) to the iron source (iron phosphate) is 0.08:1.

[0159] (3) 983 g of lithium carbonate, 180 g of glucose, and 200 g of PEG are added to the mixture obtained in step (2) and stirred at 30°C for 1 hour. The molar ratio of total iron elements to total phosphorus elements is 1:1; the molar ratio of total iron elements to lithium elements is 1:1.003; the mass of the carbon source (glucose and PEG) accounts for 9.5% of the mass of the iron source (iron phosphate); and the solid content of the slurry is 38.7%. The dispersed slurry is ground, and the grinding is stopped when the slurry particle size Dv50 is 0.5 μm. The ground slurry is then spray dried to obtain a precursor;

[0160] (4) The precursor obtained in step (3) is sintered under a nitrogen atmosphere (oxygen content <5 ppm). The temperature is raised to 780°C at a rate of 5°C / min, and the temperature is maintained at this temperature for 10 hours, and then naturally cooled to room temperature. After air flow crushing, a co-coated lithium iron phosphate cathode material is obtained.

[0161] Figure 2 The XRD pattern of the cathode material of Example 3 is shown in FIG. 3. Figure 2 As can be seen from FIG. 3, the peak position of Example 3 is completely consistent with the standard card of lithium iron phosphate, indicating that the lithium iron phosphate cathode material is successfully prepared.

[0162] The cathode material of Example 3 includes a lithium iron phosphate matrix and a coating layer on the surface of the lithium iron phosphate matrix, and the coating layer contains carbon and silicon dioxide; the weight of the silicon dioxide accounts for 1.2% of the total weight of the cathode material, the weight of the carbon accounts for 1.02% of the total weight of the cathode material, and the Dv50 particle size of the lithium iron phosphate matrix is 0.96 μm, Figure 5 The TEM pattern of the cathode material of Example 3 is shown in FIG. 4. Figure 5 As can be seen from FIG. 4, the thickness of the coating layer in Example 3 is about 12 nm.

[0163] Example 4

[0164] The co-coated lithium iron phosphate positive electrode material in this example was prepared according to the following method:

[0165] (1) 160 g of hexadecyltrimethoxysilane (HDTMS) was added to 1 kg of deionized water, and an appropriate amount of acetic acid was added to adjust the pH to 6. The mixture was stirred at 30°C for 1 hour to hydrolyze the hexadecyltrimethoxysilane and obtain a uniform dispersion.

[0166] (2) 4 kg of iron phosphate was dispersed in 8 kg of water at room temperature for 40 minutes to obtain an iron phosphate dispersion. The dispersion obtained in step (1) was added to the iron phosphate dispersion, and the mixture was stirred and dispersed to obtain a mixture. The mass ratio of the silane coupling agent (hexadecyltrimethoxysilane) to the iron source (iron phosphate) was 0.04:1.

[0167] (3) 983 g of lithium carbonate, 80 g of glucose, and 150 g of PEG were added to the mixture obtained in step (2) and stirred at 30°C for 1 hour. The molar ratio of total iron elements to total phosphorus elements was 1:1, the molar ratio of total iron elements to lithium elements was 1:1.003, the mass of the carbon source (glucose and PEG) accounted for 5.75% of the mass of the iron source (iron phosphate), and the solid content of the slurry was 37.38%. The dispersed slurry was ground, and the grinding was stopped when the slurry particle size Dv50 was 0.5 μm. The ground slurry was then spray dried to obtain a precursor;

[0168] (4) The precursor obtained in step (3) was sintered under a nitrogen atmosphere (oxygen content <5 ppm). The temperature was raised to 780°C at a rate of 5°C / min, and the temperature was maintained at 780°C for 10 hours. The temperature was then naturally cooled to room temperature, and the sintered product was pulverized by airflow to obtain a co-coated lithium iron phosphate positive electrode material.

[0169] The positive electrode material in Example 4 included a lithium iron phosphate base and a coating layer on the surface of the lithium iron phosphate base. The coating layer included carbon and silicon dioxide. The weight of the silicon dioxide accounted for 0.6% of the total weight of the positive electrode material, and the weight of the carbon accounted for 0.55% of the total weight of the positive electrode material. The Dv50 particle size of the lithium iron phosphate base was 1.02 μm. Figure 6 The TEM image of the positive electrode material in Example 4 is shown in FIG. 2. Figure 6 As can be seen from FIG. 2, the thickness of the coating layer in Example 4 was about 3 nm.

[0170] Example 5

[0171] The co-coated lithium iron phosphate positive electrode material in this example was prepared according to the following method:

[0172] (1) Put 160 g of hexadecyltrimethoxysilane (HDTMS) into 1 kg of deionized water, add an appropriate amount of acetic acid to adjust the pH to 6, and stir thoroughly at 30°C for 1 hour to hydrolyze the hexadecyltrimethoxysilane and obtain a uniform dispersion.

[0173] (2) Disperse 4 kg of iron phosphate in 8 kg of water at room temperature for 40 minutes to obtain an iron phosphate dispersion, and add the dispersion obtained in step (1) to the iron phosphate dispersion and stir thoroughly to obtain a mixture, wherein the mass ratio of the silane coupling agent (hexadecyltrimethoxysilane) to the iron source (iron phosphate) is 0.04:1.

[0174] (3) Add 983 g of lithium carbonate, 280 g of glucose, and 250 g of PEG to the mixture obtained in step (2) and stir thoroughly at 30°C for 1 hour. The molar ratio of total iron elements to total phosphorus elements is 1:1; the molar ratio of total iron elements to lithium elements is 1:1.003; the mass of the carbon source (glucose and PEG) accounts for 13.25% of the mass of the iron source (iron phosphate); and the solid content of the slurry is 38.66%. Grind the dispersed slurry, stop grinding when the slurry particle size Dv50 is 0.5 μm, and spray dry the ground slurry to obtain a precursor;

[0175] (4) Sinter the precursor obtained in step (3) under a nitrogen atmosphere (oxygen content <5 ppm). Increase the temperature to 780°C at a rate of 5°C / min and maintain the temperature for 10 hours, and then naturally cool to room temperature. After air flow crushing, a co-coated lithium iron phosphate positive electrode material is obtained.

[0176] The positive electrode material of Example 5 includes a lithium iron phosphate base and a coating layer on the surface of the lithium iron phosphate base, the coating layer containing carbon and silicon dioxide; the weight of the silicon dioxide accounts for 0.6% of the total weight of the positive electrode material, the weight of the carbon accounts for 1.89% of the total weight of the positive electrode material, and the Dv50 particle size of the lithium iron phosphate base is 1.19 μm, Figure 7 is a TEM image of the positive electrode material of Example 5, and Figure 7 It can be seen that the thickness of the coating layer in Example 5 is about 15 nm.

[0177] Example 6

[0178] In this example, a co-coated lithium iron phosphate positive electrode material is prepared according to the following method:

[0179] (1) 160 g hexadecyltrimethoxysilane (HDTMS) was added to 1 kg deionized water, and an appropriate amount of acetic acid was added to adjust the pH to 6, and the mixture was stirred at 30°C for 1 hour to hydrolyze the hexadecyltrimethoxysilane sufficiently to obtain a uniform dispersion.

[0180] (2) 4 kg of iron phosphate was dispersed in 8 kg of water at room temperature for 40 minutes to obtain an iron phosphate dispersion, and the dispersion obtained in step (1) was added to the iron phosphate dispersion, and the mixture was stirred and dispersed to obtain a mixture, wherein the mass ratio of the silane coupling agent (hexadecyltrimethoxysilane) to the iron source (iron phosphate) was 0.04:1.

[0181] (3) 983 g of lithium carbonate, 180 g of glucose, and 200 g of PEG were added to the mixture obtained in step (2), and the mixture was stirred at 30°C for 1 hour. The molar ratio of the total iron element to the total phosphorus element was 1:1, the molar ratio of the total iron element to the lithium element was 1:1.003, the mass of the carbon source (glucose and PEG) accounted for 9.5% of the mass of the iron source (iron phosphate), and the solid content of the slurry was 38.03%. The dispersed slurry was ground, and the grinding was stopped when the slurry particle size Dv50 was 0.5 μm. The ground slurry was spray dried to obtain a precursor;

[0182] (4) The precursor obtained in step (3) was sintered under a nitrogen atmosphere (oxygen content <5 ppm). The temperature was increased to 700°C at a rate of 5°C / min, and the temperature was maintained at 700°C for 10 hours, and then naturally cooled to room temperature. After air flow crushing, a co-coated lithium iron phosphate positive electrode material was obtained.

[0183] The positive electrode material of Example 6 comprises a lithium iron phosphate base and a coating layer on the surface of the lithium iron phosphate base, the coating layer comprising carbon and silicon dioxide; the weight of the silicon dioxide accounts for 0.6% of the total weight of the positive electrode material, the weight of the carbon accounts for 1.65% of the total weight of the positive electrode material, the Dv50 particle size of the lithium iron phosphate base is 1.2 μm, and the thickness of the coating layer is about 18 nm.

[0184] Example 7

[0185] In this example, a co-coated lithium iron phosphate positive electrode material was prepared according to the following method:

[0186] (1) 160 g hexadecyltrimethoxysilane (HDTMS) was added to 1 kg deionized water, and an appropriate amount of acetic acid was added to adjust the pH to 6, and the mixture was stirred at 30°C for 1 hour to hydrolyze the hexadecyltrimethoxysilane sufficiently to obtain a uniform dispersion.

[0187] (2) 4 kg of iron phosphate was dispersed in 8 kg of water for 40 minutes at room temperature to obtain an iron phosphate dispersion liquid, and the dispersion liquid obtained in step (1) was added to the iron phosphate dispersion liquid and dispersed by stirring to obtain a mixed liquid, wherein the mass ratio of the silane coupling agent (hexadecyltrimethoxysilane) to the iron source (iron phosphate) was 0.04:1.

[0188] (3) 983 g of lithium carbonate, 180 g of glucose, and 200 g of PEG were added to the mixed liquid obtained in step (2) and stirred at 30°C for 1 hour. The molar ratio of the total iron element to the total phosphorus element was 1:1, the molar ratio of the total iron element to the lithium element was 1:1.003, the mass of the carbon source (glucose and PEG) accounted for 9.5% of the mass of the iron source (iron phosphate), and the solid content of the slurry was 38.03%. The dispersed slurry was ground, and the grinding was stopped when the slurry particle size Dv50 was 0.5 μm. The ground slurry was spray dried to obtain a precursor;

[0189] (4) The precursor obtained in step (3) was sintered under a nitrogen atmosphere (oxygen content <5 ppm). The temperature was raised to 830°C at a rate of 5°C / min, and the temperature was maintained for 10 hours, and then naturally cooled to room temperature. After airflow crushing, a co-coated lithium iron phosphate positive electrode material was obtained.

[0190] The positive electrode material of Example 7 comprises a lithium iron phosphate base and a coating layer on the surface of the lithium iron phosphate base, the coating layer comprising carbon and silicon dioxide; the weight of the silicon dioxide accounts for 0.6% of the total weight of the positive electrode material, the weight of the carbon accounts for 0.5% of the total weight of the positive electrode material, the Dv50 particle size of the lithium iron phosphate base is 0.9 μm, and the thickness of the coating layer is about 2 nm.

[0191] Comparative Example 1

[0192] The lithium iron phosphate positive electrode material of this comparative example was prepared according to the following method:

[0193] (1) 983 g of lithium carbonate, 4 kg of iron phosphate, 180 g of glucose, and 200 g of PEG were added to 8 kg of deionized water and stirred at 30°C for 1 hour. The molar ratio of the total iron element to the total phosphorus element was 1:1, the molar ratio of the total iron element to the lithium element was 1:1.003, and the solid content of the slurry was 40.13%. The dispersed slurry was ground, and the grinding was stopped when the slurry particle size Dv50 was 0.5 μm. The ground slurry was spray dried to obtain a precursor;

[0194] (2) The precursor obtained in step 1 is sintered under a nitrogen atmosphere (oxygen content <5 ppm). The temperature is raised to 780°C at a heating rate of 5°C / min, and the temperature is maintained at 780°C for 10 hours, and then naturally cooled to room temperature. After air flow crushing, the lithium iron phosphate positive electrode material is obtained.

[0195] The positive electrode material of Comparative Example 1 includes a lithium iron phosphate base and a coating layer located on the surface of the lithium iron phosphate base, and the coating layer contains carbon; the weight of the carbon accounts for 1.27% of the total weight of the positive electrode material, the Dv50 particle size of the lithium iron phosphate base is 1.1 μm, Figure 8 The TEM image of the positive electrode material of Comparative Example 1 is shown in FIG. 1. Figure 8 It can be seen that the thickness of the coating layer in Comparative Example 1 is about 7-14 nm.

[0196] Comparative Example 2

[0197] The lithium iron phosphate positive electrode material of the present comparative example is prepared according to the following method:

[0198] (1) 160 g of hexadecyltrimethoxysilane (HDTMS) is added to 1 kg of deionized water, and an appropriate amount of acetic acid is added to adjust the pH to 6, and the hexadecyltrimethoxysilane is fully hydrolyzed at 30°C for 1 hour with sufficient stirring to obtain a uniform dispersion.

[0199] (2) 4 kg of iron phosphate is dispersed in 8 kg of water, and fully dispersed at room temperature for 40 minutes to obtain an iron phosphate dispersion, and the dispersion obtained in step (1) is added to the iron phosphate dispersion, and fully dispersed with stirring to obtain a mixture, wherein the mass ratio of the silane coupling agent (hexadecyltrimethoxysilane) to the iron source (iron phosphate) is 0.04:1.

[0200] (3) 983 g of lithium carbonate is added to the mixture obtained in step (2) and fully stirred at 30°C for 1 hour. The molar ratio of total iron elements to total phosphorus elements is 1:1; the molar ratio of total iron elements to lithium elements is 1:1.003; and the solid content of the slurry is 36.36%. The dispersed slurry is ground, and the grinding is stopped when the slurry particle size Dv50 is 0.5 μm. The ground slurry is spray dried to obtain a precursor;

[0201] (4) The precursor obtained in step (3) is sintered under a nitrogen atmosphere (oxygen content <5 ppm). The temperature is raised to 780°C at a heating rate of 5°C / min, and the temperature is maintained at 780°C for 10 hours, and then naturally cooled to room temperature. After air flow crushing, the lithium iron phosphate positive electrode material is obtained.

[0202] The positive electrode material of Comparative Example 2 includes a lithium iron phosphate substrate and a coating layer located on the surface of the lithium iron phosphate substrate, the coating layer comprising silicon dioxide; the weight of the silicon dioxide accounts for 0.6% of the total weight of the positive electrode material, the Dv50 particle size of the lithium iron phosphate substrate is 0.9 μm, Figure 9 The TEM image of the positive electrode material of Comparative Example 2 is shown in FIG. 2. Figure 9 It can be seen that the thickness of the coating layer in Comparative Example 2 is about 3 nm.

[0203] Comparative Example 3

[0204] The co-coated lithium iron phosphate positive electrode material of the present comparative example was prepared according to the following method:

[0205] (1) 40 g of hexadecyltrimethoxysilane (HDTMS) was added to 0.5 kg of deionized water, and an appropriate amount of acetic acid was added to adjust the pH to 6, and the mixture was stirred at 30°C for 1 hour to hydrolyze the hexadecyltrimethoxysilane and obtain a uniform dispersion.

[0206] (2) 4 kg of iron phosphate was dispersed in 8 kg of water and dispersed at room temperature for 40 minutes to obtain an iron phosphate dispersion, and the dispersion obtained in step (1) was added to the iron phosphate dispersion and dispersed by stirring to obtain a mixture, wherein the mass ratio of the silane coupling agent (hexadecyltrimethoxysilane) to the iron source (iron phosphate) was 0.01:1.

[0207] (3) 983 g of lithium carbonate, 60 g of glucose, and 120 g of PEG were added to the mixture obtained in step (2) and stirred at 30°C for 1 hour. The molar ratio of total iron element to total phosphorus element was 1:1; the molar ratio of total iron element to lithium element was 1:1.003; the mass of the carbon source (glucose and PEG) accounted for 4.5% of the mass of the iron source (iron phosphate); and the solid content of the slurry was 37.97%. The dispersed slurry was ground, and the grinding was stopped when the slurry particle size Dv50 was 0.5 μm. The ground slurry was then spray dried to obtain a precursor;

[0208] (4) The precursor obtained in step (3) was sintered under a nitrogen atmosphere (oxygen content <5 ppm). The temperature was raised to 780°C at a rate of 5°C / min, and the temperature was maintained at this temperature for 10 hours, and then naturally cooled to room temperature. After air flow crushing, a co-coated lithium iron phosphate positive electrode material was obtained.

[0209] The positive electrode material of Comparative Example 3 includes a lithium iron phosphate substrate and a coating layer located on the surface of the lithium iron phosphate substrate, the coating layer comprising carbon and silicon dioxide; the weight of the silicon dioxide accounts for 0.15% of the total weight of the positive electrode material, the weight of the carbon accounts for 0.3% of the total weight of the positive electrode material, the Dv50 particle size of the lithium iron phosphate substrate is 1.17 μm, Figure 10The TEM image of the positive electrode material of Comparative Example 3 is shown in Figure 2. Figure 10 It can be seen that the thickness of the coating layer in Comparative Example 3 is about 3 nm.

[0210] Comparative Example 4

[0211] The co-coated lithium iron phosphate positive electrode material was prepared according to the following method:

[0212] (1) 400 g of hexadecyltrimethoxysilane (HDTMS) was added to 2 kg of deionized water, and an appropriate amount of acetic acid was added to adjust the pH to 6, and the mixture was stirred at 30°C for 1 hour to hydrolyze the hexadecyltrimethoxysilane and obtain a uniform dispersion.

[0213] (2) 4 kg of iron phosphate was dispersed in 7 kg of water at room temperature for 40 minutes to obtain an iron phosphate dispersion, and the dispersion obtained in step (1) was added to the iron phosphate dispersion and stirred to obtain a mixture, wherein the mass ratio of the silane coupling agent (hexadecyltrimethoxysilane) to the iron source (iron phosphate) was 0.1:1.

[0214] (3) 983 g of lithium carbonate, 330 g of glucose, and 250 g of PEG were added to the mixture obtained in step (2) and stirred at 30°C for 1 hour. The molar ratio of total iron element to total phosphorus element was 1:1; the molar ratio of total iron element to lithium element was 1:1.003; the mass of the carbon source (glucose and PEG) accounted for 14.5% of the mass of the iron source (iron phosphate); and the solid content of the slurry was 39.85%. The dispersed slurry was ground, and the grinding was stopped when the slurry particle size Dv50 was 0.5 μm. The ground slurry was then spray dried to obtain a precursor;

[0215] (4) The precursor obtained in step (3) was sintered under a nitrogen atmosphere (oxygen content <5 ppm). The temperature was raised to 780°C at a rate of 5°C / min, and the temperature was maintained at this temperature for 10 hours, and then naturally cooled to room temperature. After air flow crushing, a co-coated lithium iron phosphate positive electrode material was obtained.

[0216] The positive electrode material of Comparative Example 4 included a lithium iron phosphate matrix and a coating layer on the surface of the lithium iron phosphate matrix, the coating layer comprising carbon and silicon dioxide; the weight of the silicon dioxide accounted for 1.5% of the total weight of the positive electrode material, the weight of the carbon accounted for 2.5% of the total weight of the positive electrode material, and the Dv50 particle size of the lithium iron phosphate matrix was 0.85 μm, Figure 11 The TEM image of the positive electrode material of Comparative Example 4 is shown in Figure 4. Figure 11 It can be seen that the thickness of the coating layer in Comparative Example 4 is about 15 nm.

[0217] The powder resistivity of the lithium iron phosphate positive electrode materials prepared in Examples 1-7 and Comparative Examples 1-4 was tested.

[0218] The lithium iron phosphate positive electrode materials prepared in the above examples and comparative examples were used as positive electrode active materials, and were assembled into button cells for electrochemical performance testing, and the test results are shown in Table 1.

[0219] Table 1 Performance test data of the positive electrode materials of the examples and comparative examples

[0220]

[0221] As can be seen from Table 1, the co-coated lithium iron phosphate positive electrode materials of Examples 1-7 have low powder resistivity, high rate performance and excellent cycle stability, and the comprehensive performance is significantly better than that of Comparative Examples 1-4.

[0222] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, but not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that they can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part or all of the technical features thereof; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the examples of the present application.

[0223] In addition, those skilled in the art can understand that although some of the embodiments herein include certain features rather than others included in other embodiments, the combination of features of different embodiments means that it is within the scope of the present application and forms different embodiments. For example, in the above claims, any one of the claimed embodiments can be used in any combination. The information disclosed in the background section is only intended to deepen the understanding of the overall background of the present application, and should not be regarded as acknowledging or implying in any form that the information constitutes prior art known to those skilled in the art.

Claims

1. A co-coated lithium iron phosphate cathode material, characterized in that, The lithium iron phosphate-based material comprises a lithium iron phosphate base and a coating layer on the surface of the lithium iron phosphate base, wherein the coating layer comprises carbon and silicon dioxide. The weight of the silicon dioxide accounts for 0.1%-5% of the total weight of the positive electrode material. The weight of the carbon accounts for 0.1%-6% of the total weight of the positive electrode material. The Dv50 particle size of the lithium iron phosphate base is 0.1-20 μm. The thickness of the coating layer is 1-20 nm. 2.The co-coated lithium iron phosphate cathode material of claim 1, characterized in that, The weight of the silicon dioxide accounts for 0.3-1.2% of the total weight of the positive electrode material. The weight of the carbon accounts for 0.5-2% of the total weight of the positive electrode material. The Dv50 particle size of the lithium iron phosphate base is 0.9-1.2 μm. The thickness of the coating layer is 2-18 nm.

3. A method for preparing the co-coated lithium iron phosphate cathode material according to claim 1 or 2, characterized in that, The method comprises the following steps: Mixing a silane coupling agent with water to perform a hydrolysis reaction to obtain a silane coupling agent dispersion liquid; Dispersing an iron source in water to obtain an iron source dispersion liquid, mixing the silane coupling agent dispersion liquid with the iron source dispersion liquid to obtain a mixed liquid; Mixing the mixed liquid with a lithium source, a carbon source and an optional phosphorus source to obtain a slurry, grinding and drying the slurry to obtain a precursor; Sintering the precursor in an inert atmosphere to obtain a co-coated lithium iron phosphate positive electrode material.

4. The method for preparing the co-coated lithium iron phosphate cathode material according to claim 3, characterized in that, The silane coupling agent has a structural formula Y 4-n -Si-(OR) n wherein n is 1-3, Y includes at least one of alkenyl, amino, epoxy, methacryloyloxy, mercapto, phenyl, alkyl, aminoalkyl, fluoroalkyl, and R includes alkyl. The iron source comprises at least one of an iron salt, a ferrous salt and an iron oxide, and the iron salt comprises iron phosphate. The lithium source comprises at least one of lithium hydroxide, lithium carbonate, lithium acetate, lithium dihydrogen phosphate and lithium nitrate. The phosphorus source comprises at least one of ammonium dihydrogen phosphate, ammonium phosphate, iron phosphate, lithium dihydrogen phosphate and phosphoric acid. In the step of preparing the silane coupling agent dispersion liquid, the mass ratio of the silane coupling agent to water is 1: (1-10). The method further comprises adding an alcohol solvent to the hydrolysis reaction system. The alcohol solvent comprises at least one of methanol, ethanol, ethylene glycol and isopropyl alcohol. The volume ratio of the water to the alcohol solvent is 1: (1-3). The method further comprises adding a pH regulator to the hydrolysis reaction system to make the pH of the hydrolysis reaction system be 3-7. The pH regulator comprises at least one of formic acid and acetic acid. The temperature of the hydrolysis reaction is 20-60 °C. The time of the hydrolysis reaction is 0.5-6 hours. The solid content of the slurry is 30%-70%. The carbon source comprises at least one of a sugar compound, an acid compound and an alcohol compound, the sugar compound comprises at least one of glucose, sucrose and starch, the acid compound comprises ascorbic acid, and the alcohol compound comprises polyethylene glycol.

5. The method for preparing the co-coated lithium iron phosphate cathode material according to claim 4, characterized in that, The silane coupling agent comprises at least one of hexadecyl trimethoxysilane, octyl trimethoxysilane, vinyl trimethoxysilane, γ-aminopropyl triethoxysilane, phenyl triethoxysilane, methyl trimethoxysilane and fluoroalkyl trimethoxysilane.

6. The method for preparing the co-coated lithium iron phosphate cathode material according to claim 3, characterized in that, The mass ratio of the silane coupling agent to the iron source is 0.001-0.08:

1. And / or, the molar ratio of Li in the lithium source, Fe in the iron source and P in the raw material is (0.95-1.05):(0.95-1.05):1, wherein the P in the raw material comes from the iron source and / or the phosphorus source. And / or, the mass of the carbon source accounts for 5-14% of the mass of the iron source.

7. The method for preparing the co-coated lithium iron phosphate cathode material according to claim 3, characterized in that, The Dv50 particle size of the grinded slurry is 0.1-10 μm.

8. The method for preparing the co-coated lithium iron phosphate cathode material according to any one of claims 3-7, characterized in that, The heating rate of the sintering is 1-8 ℃ / min. And / or, the temperature of the sintering is 700-830 ℃. And / or, the time of the sintering is 4-10 h. And / or, the inert atmosphere comprises at least one of nitrogen and argon, and the oxygen content of the inert atmosphere is <30 ppm.

9. A secondary battery characterized by comprising: The secondary battery comprises the co-coated lithium iron phosphate positive electrode material of claim 1 or 2 or the co-coated lithium iron phosphate positive electrode material prepared by the preparation method of any one of claims 3-8.

10. An electric device, characterized by The electric device comprises the secondary battery of claim 9.

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