Positive electrode material and preparation method thereof, positive electrode plate and secondary battery

Abstract

The invention provides a positive electrode material and a preparation method thereof, a positive electrode plate and a secondary battery, and belongs to the technical field of secondary batteries, the positive electrode material comprises active particles, a carbon coating layer and a polydopamine coating layer, the carbon coating layer coats the surface of the active particles, the polydopamine coating layer coats the surface of the carbon coating layer, and the polydopamine coating layer coats the surface of the active particles. The silver nanoparticles are distributed on the polydopamine coating layer; the ratio of the thickness of the carbon coating layer to the thickness of the polydopamine coating layer is 1: (0.72-0.97); the porosity of the positive electrode material is 10-30%, the average pore size is 16-36 nm, and the specific surface area is 9 m < 2 > / g-26 m < 2 > / g. According to the present invention, the carbon coating layer and the polydopamine coating layer with the distributed silver nanoparticles are sequentially arranged on the surface of the active particle, and the characteristic parameters of the positive electrode material meet the range, such that the conductivity and the compaction density of the positive electrode material can be improved so as to improve the energy density, the rate performance, the cycle performance and other electrochemical performances of the battery.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of secondary batteries, and particularly relates to a positive electrode material and a preparation method thereof, a positive electrode sheet and a secondary battery. BACKGROUND

[0002] Lithium iron phosphate (LiFePO4) is known for its high voltage, high energy density, environmental friendliness and good safety performance as a positive electrode material of a lithium ion battery, and occupies an important position in the electric vehicle market, such as application as a battery in the new energy vehicle field. However, it also faces problems such as poor electrical conductivity, which limits its application in high-rate discharge and high-power density batteries.

[0003] In order to solve these problems, scientists have proposed various improvement strategies, including element doping, reducing particle size, and coating methods. Carbon coating is an effective means, but the current carbon-coated lithium iron phosphate still has problems such as incomplete, uneven carbon coating, poor carbon layer stability, etc. Moreover, simple carbon coating has limited effect on improving electrical conductivity.

[0004] Therefore, a new positive electrode material and a preparation method thereof, a positive electrode sheet and a secondary battery are needed to improve the electrical conductivity of lithium iron phosphate. SUMMARY

[0005] In view of the technical problems in the background art, the application provides a positive electrode material and a preparation method thereof, a positive electrode sheet and a secondary battery, aiming to solve the technical problem of poor electrical conductivity of existing lithium iron phosphate.

[0006] In a first aspect, the application provides a positive electrode material, which comprises active particles, a carbon coating layer and a polydopamine coating layer, the carbon coating layer is coated on the surface of the active particles, the polydopamine coating layer is coated on the surface of the carbon coating layer, and silver nanoparticles are distributed in the polydopamine coating layer; the ratio of the thickness of the carbon coating layer to the thickness of the polydopamine coating layer is 1:(0.72-0.97); the porosity of the positive electrode material is 10%-30%, the average pore size is 16 nm-36 nm, and the specific surface area is 9 m 2 / g-26 m 2 / g.

[0007] In the technical scheme of the embodiment of the present application, the carbon coating layer and the polydopamine coating layer are sequentially arranged on the surface of the active particles, which helps to form a comprehensive, uniform and stable coating on the active particles, and is conducive to the distribution of silver nanoparticles, thereby improving the conductivity and stability of the positive electrode material, and further improving the electrochemical performance such as the capacity performance, rate performance and cycle performance of the positive electrode material. The silver nanoparticles distributed in the polydopamine coating layer help to improve the conductivity and ion diffusion rate of the particles and between the particles in the positive electrode material. When the thickness ratio of the carbon coating layer and the polydopamine coating layer meets the above range, the carbon coating layer and the polydopamine coating layer better cooperate with each other, which is conducive to optimizing the electron transport and ion diffusion path of the positive electrode material, so as to further improve the conductivity and ion diffusion rate of the positive electrode material. The positive electrode material has high porosity, small average pore size and moderate specific surface area, which is more conducive to loading more silver nanoparticles and has moderate contact area with the electrolyte, which is conducive to improving the conductivity and compaction density of the positive electrode material, and further improving the energy density, rate performance and cycle performance of the positive electrode material.

[0008] In some embodiments, the thickness of the carbon coating layer is 1.20 nm to 1.61 nm; the thickness of the polydopamine coating layer is 1.13 nm to 1.26 nm; the ratio of the average particle size of the secondary particles of the active particles to the thickness of the carbon coating layer is 1:(0.0018-0.0062); the mass fraction of nitrogen in the polydopamine coating layer is greater than that in the carbon coating layer; the mass fraction of nitrogen in the polydopamine coating layer is 2.0% to 2.68%; the particle size of the silver nanoparticles in the polydopamine coating layer is 8 nm to 23.5 nm; and the mass fraction of silver in the positive electrode material is 1% to 13%.

[0009] In this embodiment, when the thicknesses of the carbon coating layer and the polydopamine coating layer meet the above ranges, the resistance of the positive electrode material is effectively reduced, a smooth path is provided for ion intercalation and deintercalation, the conductivity and ion diffusion rate of the positive electrode material are improved, and the energy density and power performance are avoided from being excessively sacrificed; when the ratio of the average particle size of the secondary particles of the active particles to the thickness of the carbon coating layer meets the above range, the thickness of the carbon coating layer relative to the active particles is appropriately controlled, the conductivity and ion diffusion rate of the positive electrode material are improved, and the energy density and power performance are avoided from being excessively sacrificed; when the mass fraction of nitrogen in the polydopamine coating layer is greater than the mass fraction of nitrogen in the carbon coating layer, the nitrogen in the polydopamine coating layer introduces additional free electrons, the electron mobility of the carbon coating layer and the polydopamine coating layer is enhanced, and the conductivity of the positive electrode material is improved; when the mass fraction of nitrogen in the polydopamine coating layer is 2.0% to 2.68%, the content of nitrogen is moderate, which is helpful to improve the conductivity of the positive electrode material; when the particle size of the silver nanoparticles in the polydopamine coating layer meets the above range, sufficient active sites are provided, which is helpful to improve the electrochemical performance of the positive electrode material; and when the mass fraction of silver in the positive electrode material meets the above range, the content of silver is moderate, which is helpful to improve the conductivity and ion transmission rate of the positive electrode material, and prevents the agglomeration of silver particles from blocking Li + channels or occupying too many Li + sites, which adversely affects the electrical performance of the positive electrode material.

[0010] In some embodiments, the average particle size of the active particles is 100 nm to 450 nm; and the active particles are at least one of lithium iron phosphate, lithium iron manganese phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminum oxide, lithium manganese oxide, lithium cobalt oxide, and lithium titanate.

[0011] In this embodiment, when the average particle size of the secondary particles of the active particles meets the above range, the secondary particles have a moderate size, which is conducive to the close contact between the positive electrode material particles and the effective diffusion of lithium ions in the positive electrode material, thereby improving the energy density and electrochemical performance of the positive electrode material.

[0012] In some embodiments, the electronic conductivity of the positive electrode material is 2.0 x 10 -5 S / cm to 5.0 x 10 -5 S / cm, and the lithium ion diffusion rate is 3.598 x 10 -10 cm 2 / s to 4.598 x 10 -10 cm 2 / s.

[0013] In this embodiment, the positive electrode material has high electronic conductivity and lithium ion diffusion rate, which is conducive to significantly improving the rate performance, cycle performance, and other electrochemical performance of the positive electrode material.

[0014] In a second aspect, the embodiments of the present application provide a preparation method of a positive electrode material, comprising the following steps: forming a carbon coating layer on the surface of the active particles; forming a polydopamine coating layer on the surface of the carbon coating layer to obtain the positive electrode material; wherein, the silver nanoparticles are distributed in the polydopamine coating layer; the ratio of the thickness of the carbon coating layer to the thickness of the polydopamine coating layer is 1:(0.72-0.97); the porosity of the positive electrode material is 10%-30%, the average pore size is 16 nm-36 nm, and the specific surface area is 9 m 2 / g-26 m 2 / g.

[0015] In the technical scheme of the embodiments of the present application, the carbon coating layer and the polydopamine coating layer distributed with silver nanoparticles are sequentially formed on the surface of the active particles, which improves the coating rate of the active particles, enhances the protection effect of the carbon coating layer and the polydopamine coating layer on the active particles, and thus improves the electrical conductivity and stability of the positive electrode material, and further improves the electrochemical properties such as the capacity performance, rate performance and cycle performance of the positive electrode material. When the ratio of the thickness of the carbon coating layer to the thickness of the polydopamine coating layer meets the above range, the carbon coating layer and the polydopamine coating layer better cooperate with each other, which is beneficial to optimizing the electron transmission and ion diffusion path of the positive electrode material, so as to further improve the electrical conductivity and ion diffusion rate of the positive electrode material. The positive electrode material as a whole has high porosity, small average pore size and moderate specific surface area, which is more conducive to loading more silver nanoparticles and has moderate contact area with the electrolyte, which is beneficial to improving the electrical conductivity and compaction density of the positive electrode material, and thus improving the energy density, rate performance and cycle performance of the positive electrode material.

[0016] In some embodiments, the step of forming a polydopamine coating layer on the surface of the carbon coating layer to obtain the positive electrode material comprises: mixing the active particles with the surface forming the carbon coating layer, dopamine, tris-hydroxymethyl aminomethane and a first solvent, performing a first reaction treatment, and performing a first solid-liquid separation treatment to obtain a first material; mixing the first material, a silver ammonia solution and a second solvent, performing a second reaction treatment, and performing a second solid-liquid separation treatment to obtain the positive electrode material; wherein, the temperature of the first reaction treatment is 20°C-30°C, and the time of the first reaction treatment is 12 h-36 h; the temperature of the second reaction treatment is 40°C-80°C, and the time of the second reaction treatment is 2 h-6 h.

[0017] In the embodiment, the surface of the active particle forming a carbon coating layer, dopamine, tris-hydroxymethyl aminomethane and a first solvent are mixed to perform a first reaction treatment, so that the dopamine is oxidized and polymerized into polydopamine by oxygen in the first solvent, and the polydopamine is coated on the surface of the carbon coating layer, thereby improving the coating rate of the coating layer on the surface of the active particle, and facilitating the improvement of the electronic conductivity and ion diffusion rate of the positive electrode material; the first material, silver amine solution and a second solvent are mixed to perform a second reaction treatment, and the silver ions are reduced by the reducing property of the polydopamine to form silver nanoparticles distributed in the polydopamine coating layer, thereby facilitating the increase of the conductivity between the particles and the particles of the positive electrode material, and improving the electronic conductivity and ion diffusion rate. When the temperature of the first reaction treatment meets the above range, the dopamine can be polymerized and the polydopamine coating layer can be formed, and the waste of energy caused by the too high temperature can be avoided; when the temperature of the second reaction treatment meets the above range, the reduction of the silver ions can be realized, and the problem of the reduction of the conductivity and the hindering of the ion transmission caused by the too high temperature and the agglomeration of the silver particles can be avoided.

[0018] In some embodiments, the step of mixing the first material, silver amine solution and second solvent to perform a second reaction treatment, and obtaining the positive electrode material through a second solid-liquid separation treatment, includes: mixing the first material, silver amine solution and second solvent to perform a first step reaction treatment, and obtaining a first mixture; mixing the first mixture and a reducing agent to perform a second step reaction treatment, and obtaining a second mixture; obtaining the positive electrode material through a second solid-liquid separation treatment of the second mixture; wherein, the temperature of the first step reaction treatment is 40℃-80℃, and the time of the first step reaction treatment is 2h-4h the temperature of the second step reaction treatment is 40℃-80℃, and the time of the second step reaction treatment is 0.5h-2h.

[0019] In the embodiment, the first material, silver amine solution and second solvent are mixed to perform a first step reaction treatment, and the silver amine is reduced by the polydopamine coating layer in the first material to form silver nanoparticles, thereby obtaining a first mixture; the first mixture and a reducing agent are mixed to perform a second step reaction treatment, and the remaining silver amine is further reduced by the reducing agent, so that a sufficient amount of silver nanoparticles are distributed in the polydopamine coating layer in the second mixture.

[0020] In some embodiments, the mass ratio of the surface of the active particle forming a carbon coating layer, dopamine and tris-hydroxymethyl aminomethane is 1:(0.015-0.035):(0.015-0.020); the mass ratio of the first material and silver element in the silver amine solution is 1:(0.03-0.1); the reducing agent is at least one of glucose, ascorbic acid and citric acid; and the amount-of-substance ratio of the silver element in the silver amine solution and the reducing agent is 1:(1-1.2).

[0021] In the embodiment, when the mass ratio of the active particles with the surface forming a carbon coating layer and dopamine to tris (hydroxymethyl) aminomethane meets the above range, it is beneficial to maintain the pH value of the first reaction treatment at weak alkaline (8-8.5), beneficial to the formation of polydopamine and beneficial to the regulation of the thickness of the polydopamine coating layer. When the mass ratio of the first material to silver in the silver amine solution meets the above range, it is beneficial to fully load the silver nanoparticles formed by the reduction of silver ions in the polydopamine coating layer, thereby improving the conductivity and ion transmission rate of the positive electrode material. The present application further uses the above reducing agent, which is beneficial to the sufficient reduction of silver ions that are not reduced by polydopamine, thereby forming sufficient silver nanoparticles. When the amount-of-substance ratio of silver in the silver amine solution to the reducing agent meets the above range, it is beneficial to more fully reduce silver ions and reduce the remaining reducing agent, forming sufficient silver nanoparticles while reducing the use cost of the reducing agent.

[0022] In a third aspect, the embodiments of the present application provide a positive electrode tab, which comprises the positive electrode material or the positive electrode material prepared by the preparation method.

[0023] In the technical solution of the embodiments of the present application, the positive electrode tab comprises the above-mentioned positive electrode material, and therefore has good electrochemical properties such as charge-discharge specific capacity, rate performance, and cycle life.

[0024] In a fourth aspect, the embodiments of the present application provide a secondary battery, which comprises the positive electrode tab.

[0025] In the technical solution of the embodiments of the present application, the secondary battery comprises the above-mentioned positive electrode tab, and therefore has good electrochemical properties such as charge-discharge specific capacity, rate performance, and cycle life.

[0026] The above description is only a summary of the technical solutions of the present application. In order to more clearly understand the technical means of the present application, the embodiments of the present application can be implemented according to the content of the description, and in order to make the above and other purposes, features and advantages of the present application more obvious and easy to understand, the following specific embodiments of the present application are described. BRIEF DESCRIPTION OF DRAWINGS

[0027] In order to more clearly illustrate the technical solutions of the present application, the following will briefly introduce the drawings used in the present application. Obviously, the drawings described below are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creating laborious work.

[0028] Figure 1 is a transmission electron microscope image of LFP@C provided in step (1) of embodiment 1 of the present application; Figure 2is a transmission electron microscope image of LFP@C@PDA-45 provided in step (2) in Example 1 of the present application; Figure 3 is a scanning electron microscope image of LFP@C@PDA-45 provided in step (2) in Example 1 of the present application; Figure 4 is a scanning electron microscope image of LFP@C@PDA-45@Ag-6 provided in step (3) in Example 1 of the present application; Figure 5 is an infrared spectrum of LFP@C, LFP@C@PDA-45@Ag-6, LFP@C@PDA-60@Ag-6, LFP@C@PDA-75@Ag-6, LFP@C@PDA-90@Ag-6 and LFP@C@PDA-105@Ag-6 provided in Comparative Example 1 and Examples 1-5 of the present application; Figure 6 is a flowchart of a preparation method of a positive electrode material provided in the present application. DETAILED DESCRIPTION

[0029] The embodiments of the technical solutions of the present application will be described in detail below with reference to the accompanying drawings. The following examples are only used to more clearly illustrate the technical solutions of the present application, and therefore only serve as examples, and cannot limit the protection scope of the present application.

[0030] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which the present application belongs; the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the present application; the terms "include" and "have" and any variations thereof in the specification and claims of the present application and the above description of drawings are intended to cover non-exclusive inclusion.

[0031] In the description of the embodiments of the present application, the technical terms "first", "second", etc. are only used to distinguish different objects, and cannot be understood as indicating or implying relative importance or implicitly indicating the number, specific order or primary and secondary relationship of the indicated technical features. In the description of the embodiments of the present application, the meaning of "a plurality of" is two or more, unless otherwise explicitly and specifically limited.

[0032] Reference herein to "an embodiment" means that a particular feature, structure, or characteristic described in connection with the embodiment can be included in at least one embodiment of the present application. The appearance of the phrase in various places in the specification does not necessarily all refer to the same embodiment, nor is it necessarily independent or alternative embodiments to other embodiments. It is explicitly and implicitly understood by those skilled in the art that the embodiments described herein can be combined with other embodiments.

[0033] In the description of the embodiments of the present application, the term "and / or" is merely to describe an association relationship of associated objects, which means that there can be three relationships, for example, A and / or B, which can represent the three cases of A existing alone, A and B existing together, and B existing alone. In addition, the character " / " herein generally represents an "or" relationship between the front and rear associated objects.

[0034] In the description of the embodiments of the present application, the term "a plurality of" refers to two or more (including two), and similarly, "a plurality of groups" refers to two or more groups (including two groups), and "a plurality of pieces" refers to two or more pieces (including two pieces).

[0035] In the description of the embodiments of the present application, the solvent or other solvent in the solution is selected from at least one of deionized water, distilled water, secondary water, pure water, and ultrapure water.

[0036] The existing lithium iron phosphate has problems such as poor electrical conductivity. Carbon coating is a common method to improve the electrical conductivity of lithium iron phosphate. However, this method still faces some challenges, such as incomplete coverage of the carbon coating layer, uneven distribution, and insufficient stability of the carbon coating layer itself. These problems limit the effectiveness of the carbon coating layer in improving the electrical conductivity and ion diffusion rate of lithium iron phosphate. Therefore, although carbon coating provides some improvement, its effectiveness is still limited by the above-mentioned deficiencies.

[0037] To solve the technical problems of poor electrical conductivity and low ion diffusion rate of existing lithium iron phosphate, the present application provides a positive electrode material and a preparation method thereof, a positive electrode sheet, and a secondary battery.

[0038] In a first aspect, the embodiments of the present application provide a positive electrode material, which includes active particles, a carbon coating layer, and a polydopamine (PDA) coating layer. The carbon coating layer is coated on the surface of the active particles, the polydopamine coating layer is coated on the surface of the carbon coating layer, and silver nanoparticles are distributed in the polydopamine coating layer. Wherein, The ratio of the thickness of the carbon coating layer to the thickness of the polydopamine coating layer is 1:(0.72-0.97), including but not limited to 1:0.72, 1:0.81, 1:0.87, 1:0.88, 1:0.89, 1:0.90, 1:0.91, 1:0.92, 1:0.93, 1:0.94, 1:0.95, 1:0.96, 1:0.97, etc. Further, the ratio of the thickness of the carbon coating layer to the thickness of the polydopamine coating layer is preferably 1:(0.81-0.91).

[0039] The porosity of the positive electrode material is 10% to 30%, including but not limited to 10%, 12%, 15%, 18%, 20%, 25%, 28%, 30%, and the like, and is further preferably 12% to 28%; the average pore size is 16 nm to 36 nm, including but not limited to 16 nm, 19 nm, 20 nm, 21 nm, 22 nm, 23 nm, 24 nm, 30 nm, 36 nm, and the like, and is further preferably 19 nm to 36 nm; the specific surface area is 9 m 2 / g to 26 m 2 / g, including but not limited to 9 m 2 / g, 10 m 2 / g, 11 m 2 / g, 12 m 2 / g, 13 m 2 / g, 14 m 2 / g, 15 m 2 / g, 16 m 2 / g, 17 m 2 / g, 18 m 2 / g, 19 m 2 / g, 20 m 2 / g, 22 m 2 / g, 26 m 2 / g, and the like, and is further preferably 10 m to 26 m 2 / g.

[0040] In the technical solution of the embodiment of the present application, by sequentially arranging the carbon coating layer and the polydopamine coating layer on the surface of the active particles, it is helpful to form a comprehensive, uniform and stable coating on the active particles, and it is conducive to the distribution of silver nanoparticles, thereby improving the conductivity and stability of the positive electrode material, and further improving the electrochemical performance such as the capacity performance, rate performance and cycle performance of the positive electrode material.

[0041] If only the carbon coating layer is arranged on the surface of the active particles, the carbon coating layer has problems such as incomplete coverage, uneven distribution, and insufficient stability of the carbon coating layer itself, and it is difficult to load materials such as silver nanoparticles that are conducive to improving the conductivity and ion transmission rate, resulting in insufficient improvement of the conductivity, ion transmission rate and stability of the positive electrode material, thereby adversely affecting the electrochemical performance such as the capacity performance, rate performance and cycle performance of the positive electrode material; if only the polydopamine coating layer is arranged on the surface of the active particles, there is a lack of reducing materials for forming active particles from active particle raw materials during the formation of active particles, or other reducing materials need to be introduced for the formation of active particles, resulting in increased difficulty in forming active particles and increased preparation cost.

[0042] The silver nanoparticles are distributed in the dopamine coating layer in a doped and / or coated form, which is conducive to improving the conductivity of the particles and the ion diffusion rate in the positive electrode material.

[0043] When the ratio of the thicknesses of the carbon coating layer and the polydopamine coating layer satisfies the above range, the carbon coating layer and the polydopamine coating layer are better matched, which is conducive to optimizing the electron transport and ion diffusion path of the positive electrode material, so as to further improve the conductivity and ion diffusion rate of the positive electrode material.

[0044] The positive electrode material has a high porosity, a small average pore size, and a moderate specific surface area, which is more conducive to loading more silver nanoparticles and has a moderate contact area with the electrolyte, which is conducive to improving the conductivity, ion diffusion rate, and compaction density of the positive electrode material, and further improving the energy density, rate performance, and cycle performance of the positive electrode material.

[0045] If the coverage of the carbon coating layer and the polydopamine coating layer of the positive electrode material is too low, it is easy to cause insufficient improvement of the conductivity and ion diffusion rate of the positive electrode material, and the carbon coating layer and the polydopamine coating layer are difficult to fully protect the active particles, which makes it difficult to ensure the stability of the positive electrode material; if the coverage of the carbon coating layer and the polydopamine coating layer of the positive electrode material is too high, it is easy to cause the proportion of non-active substances (carbon and polydopamine) to be too high, thereby reducing the energy density of the positive electrode material.

[0046] If the specific surface area of the positive electrode material is too large, it is easy to cause the contact area with the electrolyte to be too large, which is not conducive to the stability of the active particles; if the specific surface area is too small, it is difficult to provide enough active sites, which is not conducive to improving the electrochemical performance such as the charge-discharge performance and rate performance of the positive electrode material.

[0047] Further, in some embodiments, the thickness of the carbon coating layer is 1.20 nm to 1.61 nm, including but not limited to 1.20 nm, 1.33 nm, 1.34 nm, 1.35 nm, 1.36 nm, 1.37 nm, 1.38 nm, 1.39 nm, 1.40 nm, 1.41 nm, 1.42 nm, 1.43 nm, 1.44 nm, 1.48 nm, 1.50 nm, 1.53 nm, 1.55 nm, 1.58 nm, 1.60 nm, 1.61 nm, and the like, and is further preferably 1.33 nm to 1.44 nm.

[0048] Further, in some embodiments, the thickness of the polydopamine coating layer is 1.13 nm to 1.26 nm, including but not limited to 1.13 nm, 1.14 nm, 1.15 nm, 1.16 nm, 1.17 nm, 1.18 nm, 1.19 nm, 1.20 nm, 1.21 nm, 1.22 nm, 1.23 nm, 1.24 nm, 1.25 nm, 1.26 nm, and the like, and is further preferably 1.16 nm to 1.25 nm.

[0049] In the technical solution of the embodiments of the present application, when the thickness of the carbon coating layer and the thickness of the polydopamine coating layer respectively meet the above ranges, the electrical resistance of the positive electrode material is effectively reduced, a smooth path is provided for ion intercalation and deintercalation, the conductivity and ion diffusion rate of the positive electrode material are improved, and the energy density and power performance are avoided from being excessively sacrificed.

[0050] If the carbon coating layer and / or the polydopamine coating layer is too thin, it is difficult to effectively improve the conductivity of the positive electrode material and to fully protect the active particles, and at the same time, the polydopamine coating layer that is too thin is difficult to load a sufficient amount of silver nanoparticles, which is not conducive to the further improvement of the conductivity and ion diffusion rate of the positive electrode material. If the carbon coating layer and / or the polydopamine coating layer is too thick, the electron transport and ion diffusion path of the positive electrode material is lengthened, and the proportion of active particles in the positive electrode material is reduced, which is not conducive to the improvement of the energy density and power performance of the positive electrode material.

[0051] Further, in some embodiments, the ratio of the average particle size of the secondary particles of the active particles to the thickness of the carbon coating layer is 1:(0.0018-0.0062), including but not limited to 1:0.0018, 1:0.0020, 1:0.0022, 1:0.0026, 1:0.0027, 1:0.0028, 1:0.0029, 1:0.0030, 1:0.0031, 1:0.0032, 1:0.0033, 1:0.0034, 1:0.0035, 1:0.0036, 1:0.0040, 1:0.0044, 1:0.0045, 1:0.0050, 1:0.0054, 1:0.0055, 1:0.0060, 1:0.0062, and the like, and is further preferably 1:(0.0018-0.0054).

[0052] In the technical solution of the embodiments of the present application, when the ratio of the average particle size of the secondary particles of the active particles to the thickness of the carbon coating layer meets the above range, it is conducive to controlling the thickness of the carbon coating layer relative to the active particles to be appropriate, improving the conductivity and ion diffusion rate of the positive electrode material, and avoiding excessive sacrifice of energy density and power performance.

[0053] Further, in some embodiments, the mass fraction of nitrogen in the polydopamine coating layer is greater than the mass fraction of nitrogen in the carbon coating layer.

[0054] In the technical solution of the embodiments of the present application, when the mass fraction of nitrogen in the polydopamine coating layer is greater than the mass fraction of nitrogen in the carbon coating layer, the nitrogen in the polydopamine coating layer introduces additional free electrons, enhances the electron mobility of the carbon coating layer and the polydopamine coating layer, and improves the conductivity of the positive electrode material.

[0055] Further, in some embodiments, the mass fraction of nitrogen in the polydopamine coating layer is 2.0% to 2.68%, including but not limited to 2.0%, 2.13%, 2.17%, 2.21%, 2.25%, 2.29%, 2.33%, 2.37%, 2.41%, 2.45%, 2.49%, 2.53%, 2.6%, 2.68%, and the like, and is further preferably 2.33% to 2.51%.

[0056] In the technical solution of the embodiments of the present application, when the mass fraction of nitrogen in the polydopamine coating layer satisfies the above range, the nitrogen content is moderate, which helps to improve the conductivity of the positive electrode material. If the mass fraction of nitrogen in the polydopamine coating layer is too low, the conductivity of the positive electrode material is limitedly improved; if the mass fraction of nitrogen in the polydopamine coating layer is too high, too many defects are easily caused, which in turn reduces the electron mobility and thus reduces the conductivity of the positive electrode material.

[0057] Further, in some embodiments, the particle size of the silver nanoparticles in the polydopamine coating layer is 8nm to 23.5nm, including but not limited to 8nm, 10nm, 12nm, 14nm, 16nm, 18nm, 20nm, 23.5nm, and the like, and is further preferably 12nm to 18nm.

[0058] In the technical solution of the embodiments of the present application, when the particle size of the silver nanoparticles in the polydopamine coating layer satisfies the above range, sufficient active sites are provided, which helps to improve the electrochemical performance of the positive electrode material. If the particle size of the silver nanoparticles is too small, the surface energy of the silver nanoparticles with too small particle size is too high, which is more easily affected by the physical and chemical environment in the long-term cycle process, thereby affecting the stability and cycle life of the positive electrode material; if the particle size of the silver nanoparticles is too large, it is easy to cause insufficient active sites, which is not conducive to improving the electrochemical performance of the positive electrode material.

[0059] Further, in some embodiments, the mass fraction of silver in the positive electrode material is 1% to 13%, for example, can be 1%, 2%, 5%, 8%, 10%, 13%, and the like, and is further preferably 3% to 9%.

[0060] In the technical solution of the embodiments of the present application, when the mass fraction of silver in the positive electrode material meets the above range, the silver content is moderate, which helps to improve the conductivity and ion transmission rate of the positive electrode material, and avoids the agglomeration of silver particles to block Li + channels or occupy too many Li + sites, which adversely affects the electrical properties of the positive electrode material. If the mass fraction of silver in the positive electrode material is too low, it will result in insufficient improvement of the conductivity and ion transmission rate of the positive electrode material; if the mass fraction of silver in the positive electrode material is too high, it will result in the agglomeration of silver particles to block Li + channels or occupy too many Li + sites, thereby reducing the charge specific capacity, discharge specific capacity and other electrochemical properties.

[0061] Further, in some embodiments, the average particle size of the active particles is 100 nm to 450 nm, including but not limited to 100 nm, 150 nm, 200 nm, 250 nm, 30 nm, 350 nm, 400 nm, 450 nm, and the like, and is further preferably 200 nm to 360 nm. The average particle size of the active particles is the average particle size of the secondary particles of the active particles.

[0062] In the technical solution of the embodiments of the present application, when the average particle size of the active particles meets the above range, it has a moderate secondary particle size, which is conducive to the close contact between the positive electrode material particles and the intercalation and deintercalation of lithium ions in the positive electrode material, thereby improving the energy density and electrochemical activity of the positive electrode material. If the particle size of the active particles is too small, it is easy to cause the particles of the positive electrode material to be difficult to pack closely, thereby reducing the compaction density of the positive electrode material, and thus reducing the energy density of the positive electrode material; if the particle size of the active particles is too large, it will result in a longer diffusion path of lithium ions inside the particles of the positive electrode material, increasing the resistance of lithium ions to diffuse between the electrolyte and the active particles, thereby reducing the electrochemical performance of the positive electrode material.

[0063] Further, in some embodiments, the active particles are at least one of lithium iron phosphate, lithium manganese iron phosphate, lithium nickel cobalt manganese phosphate, lithium nickel cobalt aluminum phosphate, lithium manganese phosphate, lithium cobalt phosphate and lithium titanate.

[0064] In the technical solution of the embodiments of the present application, the material selection of the active particles is wide, which is conducive to the positive electrode material to meet different application scenarios, and thus is conducive to the wide application of the positive electrode material.

[0065] Further, in some embodiments, the electronic conductivity of the positive electrode material is 2.0 x 10 -5 S / cm to 5.0 x 10 - 5 S / cm, including but not limited to 2.0 x 10 -5 S / cm, 2.5 x 10 -5S / cm, 3.0*10 -5 S / cm, 3.5*10 -5 S / cm, 4.0*10 -5 S / cm, 4.5*10 -5 S / cm, 5.0*10 -5 S / cm, etc.; the lithium ion diffusion rate is 3.598*10 -10 cm 2 / s, 4.598*10 -10 cm 2 / s, including but not limited to 3.598*10 -10 cm 2 / s, 3.898*10 -10 cm 2 / s, 4.198*10 -10 cm 2 / s, 4.398*10 -10 cm 2 / s, 4.598*10 -10 cm 2 / s, etc.

[0066] In the technical scheme of the embodiment of the present application, the positive electrode material has the characteristics of high electron conductivity and lithium ion diffusion rate, which is beneficial to improving the rate performance, cycle performance and other electrochemical properties of the positive electrode material.

[0067] In a second aspect, the embodiment of the present application provides a preparation method of a positive electrode material, please refer to Figure 6 , the preparation method comprises the following steps: S10, forming a carbon coating layer on the surface of the active particle; S20, forming a polydopamine coating layer on the surface of the carbon coating layer to obtain a positive electrode material; Wherein, the silver nanoparticles are distributed in the polydopamine coating layer; The ratio of the thickness of the carbon coating layer to the thickness of the polydopamine coating layer is 1:(0.72~0.97), including but not limited to 1:0.87, 1:0.88, 1:0.89, 1:0.90, 1:0.91, 1:0.92, 1:0.93, 1:0.94, 1:0.95, 1:0.96, 1:0.97, etc. The porosity of the positive electrode material is 10%~30%, the average pore size is 16nm~36nm, including but not limited to 19nm, 20nm, 21nm, 22nm, 23nm, 24nm, etc., the specific surface area is 9m 2 / g~26m 2 / g, including but not limited to 9m 2 / g, 10m 2 / g, 11m 2 / g、12m 2 / g、13m 2 / g、14m 2 / g, 15m 2 / g、16m 2 / g、17m 2 / g、18m 2 / g、19m 2 / g etc.

[0068] In the technical solution of this application embodiment, a carbon coating layer and a polydopamine coating layer containing distributed silver nanoparticles are sequentially formed on the surface of the active particles. This increases the coating rate of the active particles and enhances the protective effect of the coating layer on the active particles, thereby improving the conductivity and stability of the cathode material, and further improving the capacity, rate performance, and cycle performance of the cathode material. When the thickness ratio of the carbon coating layer to the polydopamine coating layer meets the above-mentioned range, the carbon coating layer and the polydopamine coating layer cooperate better, which is beneficial to optimizing the electron transport and ion diffusion paths of the cathode material, so as to further improve the conductivity and ion diffusion rate of the cathode material. The cathode material as a whole has a high porosity, a small average pore size, and a moderate specific surface area, which is more conducive to loading more silver nanoparticles and has a moderate contact area with the electrolyte, which is beneficial to improving the conductivity, ion diffusion rate, and compaction density of the cathode material, thereby improving the energy density, rate performance, and cycle performance of the cathode material.

[0069] Furthermore, in some embodiments, the step of forming a polydopamine coating layer on the surface of the carbon coating layer to obtain the cathode material includes: Active particles with a carbon coating on their surface, dopamine, tris(hydroxymethyl)aminomethane, and a first solvent are mixed and subjected to a first reaction treatment. After a first solid-liquid separation treatment, a first material is obtained. The first material, silver ammonia solution, and second solvent are mixed and subjected to a second reaction treatment, followed by a second solid-liquid separation treatment to obtain the positive electrode material; wherein, The temperature of the first reaction treatment is 20℃~30℃, including but not limited to 20℃, 22℃, 24℃, 26℃, 28℃, 30℃, etc.; the time of the first reaction treatment is 12h~36h, including but not limited to 12h, 18h, 24h, 30h, 36h, etc. The temperature of the second reaction treatment is 40℃~80℃, including but not limited to 40℃, 50℃, 60℃, 70℃, 80℃, etc.; the time of the second reaction treatment is 2h~6h, including but not limited to 2h, 3h, 4h, 5h, 6h, etc.

[0070] The technical scheme of the embodiment of the present application mixes the active particles with a carbon-coated surface, dopamine, tris-hydroxymethyl aminomethane and a first solvent, performs a first reaction treatment, so that the dopamine is oxidized and polymerized into polydopamine by oxygen in the first solvent and coated on the surface of the carbon-coated layer, thereby improving the coating rate of the coating layer on the surface of the active particles and being conducive to improving the electronic conductivity and ion diffusion rate of the positive electrode material; the first material, silver amine solution and a second solvent are mixed, a second reaction treatment is performed, and the polydopamine is used to reduce silver ions to form silver nanoparticles distributed in the polydopamine coating layer, which is conducive to increasing the conductivity between the particles and the particles of the positive electrode material and improving the electronic conductivity and ion diffusion rate. When the temperature of the first reaction treatment meets the above range, the dopamine can be polymerized and the polydopamine coating layer can be formed, and energy waste caused by excessively high temperature can be avoided; when the temperature of the second reaction treatment meets the above range, the reduction of silver ions can be realized, and the problem of reducing the conductivity and hindering ion transmission caused by excessively high temperature and silver particle agglomeration can be avoided.

[0071] Further, in some embodiments, the active particles with a carbon-coated surface are prepared by a chemical vapor deposition method, a sol-gel method or a ball milling method.

[0072] Further, in some embodiments, the mass ratio of the active particles with a carbon-coated surface to dopamine is 1: (0.015-0.035), including but not limited to 1:0.015, 1:0.02, 1:0.025, 1:0.03, 1:0.035, etc.

[0073] Further, in some embodiments, the mass ratio of the active particles with a carbon-coated surface to tris-hydroxymethyl aminomethane is 1: (0.015-0.020), including but not limited to 1:0.015, 1:0.016, 1:0.017, 1:0.018, 1:0.019, 1:0.020, etc.

[0074] In the technical scheme of the embodiment of the present application, when the mass ratio of the active particles with a carbon-coated surface to dopamine and tris-hydroxymethyl aminomethane meets the above range, it is conducive to maintaining the pH value of the first reaction treatment at weak alkaline (8-8.5), which is conducive to the formation of polydopamine and the regulation of the thickness of the formed polydopamine coating layer.

[0075] Further, in some embodiments, the first solvent is selected from at least one of distilled water, secondary water, deionized water, pure water and ultrapure water.

[0076] Further, in some embodiments, the concentration of the active particles forming the carbon-coated layer on the surface in the first solvent is 0.008-0.012 g / mL, including but not limited to 0.008 g / mL, 0.009 g / mL, 0.01 g / mL, 0.011 g / mL, 0.012 g / mL, etc.

[0077] Further, in some embodiments, the step of mixing the first material, the silver amine solution and the second solvent, performing a second reaction treatment, and performing a second solid-liquid separation treatment to obtain the positive electrode material, includes: mixing the first material, the silver amine solution and the second solvent, and performing a first step reaction treatment to obtain a first mixture; mixing the first mixture with a reducing agent and performing a second step reaction treatment to obtain a second mixture; performing a second solid-liquid separation treatment on the second mixture to obtain the positive electrode material; wherein, the temperature of the first step reaction treatment is 40-80℃, including but not limited to 40℃, 50℃, 60℃, 70℃, 80℃, etc., and the time of the first step reaction treatment is 2-4h, including but not limited to 2h, 2.5h, 3h, 3.5h, 4h, etc. the temperature of the second step reaction treatment is 40-80℃, including but not limited to 40℃, 50℃, 60℃, 70℃, 80℃, etc., and the time of the second step reaction treatment is 0.5-2h, including but not limited to 0.5h, 1h, 1.5h, 2h, etc.

[0078] In the technical solution of the embodiments of the present application, the first material, the silver amine solution and the second solvent are mixed to perform a first step reaction treatment, and the polydopamine coating layer in the first material is used to reduce silver amine to form silver nanoparticles, thereby obtaining a first mixture; the first mixture is mixed with a reducing agent to perform a second step reaction treatment, and the reducing agent is used to further reduce the remaining silver amine, so that the polydopamine coating layer in the second mixture is distributed with sufficient silver nanoparticles.

[0079] Further, in some embodiments, the preparation process of the silver ammine solution comprises: mixing 0.05 mol / L-0.10 mol / L (including but not limited to 0.05 mol / L, 0.06 mol / L, 0.07 mol / L, 0.08 mol / L, 0.09 mol / L, 0.10 mol / L, etc.) silver nitrate solution, 1%-8% (including but not limited to 1%, 2%, 4%, 5%, 7%, 8%, etc.) sodium hydroxide solution, and then adding 0.05 mol / L-0.10 mol / L (including but not limited to 0.05 mol / L, 0.06 mol / L, 0.07 mol / L, 0.08 mol / L, 0.09 mol / L, 0.10 mol / L, etc.) ammonia water to dissolve until clear to obtain the silver ammine solution; the volume ratio of the silver nitrate solution to the sodium hydroxide solution and the ammonia water is 1:(0.2-1.1):(0.8-2.2), including but not limited to 1:0.2:0.8, 1:0.3:1, 1:0.5:1.2, 1:1:1.8, 1:1:2, 1:1:2.2, 1:1.1:1.8, 1:1.1:2, 1:1.1:2.2, etc.

[0080] Further, in some embodiments, the mass ratio of the first material to the silver element in the silver ammine solution is 1:(0.03-0.1), including but not limited to 1:0.03, 1:0.05, 1:0.07, 1:0.07, 1:0.1, etc.

[0081] In the technical scheme of the embodiments of the present application, when the mass ratio of the first material to the silver element in the silver ammine solution meets the above range, it is beneficial to fully load the silver nanoparticles formed by the reduction of silver ions in the polydopamine coating layer, thereby improving the conductivity and ion transmission rate of the positive electrode material.

[0082] Further, in some embodiments, the reducing agent is at least one of glucose, ascorbic acid, and citric acid.

[0083] In the technical scheme of the embodiments of the present application, the selection of the reducing agent is wide, which is beneficial to fully reduce the silver ammine and is beneficial to mass production in industry.

[0084] Further, in some embodiments, the amount-of-substance ratio of the silver element to the reducing agent in the silver ammine solution is 1:(1-1.2), including but not limited to 1:1, 1:1.05, 1:1.1, 1:1.15, 1:1.2, etc.

[0085] In the technical scheme of the embodiments of the present application, when the amount-of-substance ratio of the silver element to the reducing agent in the silver ammine solution meets the above range, it is beneficial to more fully reduce the silver ions and reduce the remaining reducing agent, to reduce the use cost of the reducing agent while forming sufficient silver nanoparticles.

[0086] Further, in some embodiments, the second solvent is selected from at least one of distilled water, secondary water, deionized water, pure water, and ultrapure water.

[0087] Further, in some embodiments, the concentration of the first material in the second solvent is 0.001 g / mL to 0.01 g / mL, including but not limited to 0.001 g / mL, 0.002 g / mL, 0.004 g / mL, 0.006 g / mL, 0.008 g / mL, 0.01 g / mL, and the like.

[0088] In the present application, it should be noted that the solid-liquid separation process includes solid-liquid separation, washing, and drying processes, which are prior art and will not be described here.

[0089] In a third aspect, the embodiments of the present application provide a positive electrode sheet, which comprises the positive electrode material or the positive electrode material prepared by the preparation method.

[0090] In the technical solution of the embodiments of the present application, the positive electrode sheet comprises the above-mentioned positive electrode material, and thus has good electrochemical properties such as specific charge capacity, rate performance, and cycle life.

[0091] In a fourth aspect, the embodiments of the present application provide a secondary battery, which comprises the above-mentioned positive electrode sheet.

[0092] In the technical solution of the embodiments of the present application, the secondary battery comprises the above-mentioned positive electrode sheet, and thus has good electrochemical properties such as specific charge capacity, rate performance, and cycle life.

[0093] Some specific embodiments are listed below. It should be noted that the embodiments described below are exemplary and are used to explain the present application, and cannot be understood as a limitation on the present application. If the specific technology or condition is not specified in the embodiments, the technology or condition described in the literature in the art or according to the product manual is used. If the reagent or instrument is not specified by the manufacturer, it is a conventional product that can be obtained from the market.

[0094] I. Preparation method Example 1 (1) Anhydrous iron phosphate, lithium carbonate, and glucose were dissolved in water according to a mass ratio of 80:20:9 to obtain a mixed slurry, which was then sand-milled and spray-dried (the inlet air temperature was 240 ℃, and the outlet air temperature was 95 ℃); then, the temperature was raised to 800 ℃ at a rate of 3 ℃ / min under a nitrogen atmosphere, and the temperature was maintained for 9 h; after the temperature maintenance, the temperature was lowered to below 100 ℃, and the product was discharged from the furnace and crushed to obtain a carbon-coated lithium iron phosphate positive electrode material, which is denoted as LFP@C.

[0095] (2) Take 54 mg of Tris powder and dissolve it in 300 mL of deionized water, and ultrasonic treatment for 25 minutes to obtain 300 mL of Tris aqueous solution; take 3 g of LFP@C powder and add it to 300 mL of Tris aqueous solution, and ultrasonic treatment for 10 minutes to obtain 300 mL of LFP@C / Tris aqueous solution; take 45 mg of dopamine hydrochloride powder and dissolve it in 300 mL of LFP@C / Tris aqueous solution, and ultrasonic treatment for 1 hour, and then stirring at room temperature for 24 hours to obtain LFP@C / PDA aqueous solution; the 300 mL of LFP@C / PDA aqueous solution is filtered with a cellulose ester membrane with a pore size of 0.22 μm, and then repeatedly washed with double distilled water, and the obtained black solid is dried at 50 ℃ to obtain PDA-coated lithium iron phosphate, which is recorded as LFP@C@PDA-45.

[0096] (3) Take 0.5 g of the prepared LFP@C@PDA-45 powder and disperse it in 100 mL of double distilled water, and ultrasonic oscillation at room temperature for 1 h to obtain a dispersion of LFP@C@PDA-45; according to the volume ratio of 1:0.2:1, add a 5% sodium hydroxide solution by mass fraction to a silver nitrate solution with a concentration of 0.00354 mol / L, and then add an ammonia water with a concentration of 0.00354 mol / L, and then obtain a silver ammonia solution after the system is dissolved to be clear; slowly add the prepared silver ammonia solution to the dispersion of LFP@C@PDA-45, and control the mass ratio of LFP@C to Ag to be 94:6, and mechanically stir at 60 ℃ for 3 h, and then add glucose in an equimolar ratio with AgNO3, and continue to react for 1 h, and then filter the obtained suspension with a cellulose ester membrane with a pore size of 0.22 μm, and repeatedly wash several times with double distilled water, and then vacuum dry the obtained black solid at 60 ℃ to obtain LFP@C@PDA-45 with silver nanoparticles distributed on the surface, which is recorded as LFP@C@PDA-45@Ag-6.

[0097] Example 2 The difference between this example and Example 1 is that the amount of dopamine hydrochloride powder added is 60 mg.

[0098] The PDA-coated lithium iron phosphate prepared in this example is recorded as LFP@C@PDA-60.

[0099] The LFP@C@PDA-60 with silver nanoparticles distributed on the surface prepared in this example is recorded as LFP@C@PDA-60@Ag-6.

[0100] Example 3 The difference between this example and Example 1 is that the amount of dopamine hydrochloride powder added is 75 mg.

[0101] The PDA-coated lithium iron phosphate prepared in this example is denoted as LFP@C@PDA-75.

[0102] The LFP@C@PDA-75 with silver nanoparticles distributed on the surface prepared in this example is denoted as LFP@C@PDA-75@Ag-6.

[0103] Example 4 The difference between this example and Example 1 is that the amount of dopamine hydrochloride powder added is 90 mg.

[0104] The PDA-coated lithium iron phosphate prepared in this example is denoted as LFP@C@PDA-90.

[0105] The LFP@C@PDA-90 with silver nanoparticles distributed on the surface prepared in this example is denoted as LFP@C@PDA-90@Ag-6.

[0106] Example 5 The difference between this example and Example 1 is that the amount of dopamine hydrochloride powder added is 105 mg.

[0107] The PDA-coated lithium iron phosphate prepared in this example is denoted as LFP@C@PDA-105.

[0108] The LFP@C@PDA-105 with silver nanoparticles distributed on the surface prepared in this example is denoted as LFP@C@PDA-105@Ag-6.

[0109] Example 6 The difference between this example and Example 1 is that the amount of dopamine hydrochloride powder added is 120 mg.

[0110] The PDA-coated lithium iron phosphate prepared in this example is denoted as LFP@C@PDA-120.

[0111] The LFP@C@PDA-120 with silver nanoparticles distributed on the surface prepared in this example is denoted as LFP@C@PDA-120@Ag-6.

[0112] Example 7 The difference between this example and Example 1 is that the amount of dopamine hydrochloride powder added is 30 mg.

[0113] The PDA-coated lithium iron phosphate prepared in this example is denoted as LFP@C@PDA-30.

[0114] The LFP@C@PDA-30 with silver nanoparticles distributed on the surface prepared in this example is denoted as LFP@C@PDA-30@Ag-6.

[0115] Example 8 The difference between this embodiment and embodiment 1 is that the mass ratio of LFP@C to Ag is controlled to be 98:2.

[0116] The LFP@C@PDA with silver nanoparticles distributed on the surface prepared in this embodiment is denoted as LFP@C@PDA-45@Ag-2.

[0117] Embodiment 9 The difference between this embodiment and embodiment 1 is that the mass ratio of LFP@C to Ag is controlled to be 100:3.

[0118] The LFP@C@PDA with silver nanoparticles distributed on the surface prepared in this embodiment is denoted as LFP@C@PDA-45@Ag-3.

[0119] Embodiment 10 The difference between this embodiment and embodiment 1 is that the mass ratio of LFP@C to Ag is controlled to be 100:10.

[0120] The LFP@C@PDA with silver nanoparticles distributed on the surface prepared in this embodiment is denoted as LFP@C@PDA-45@Ag-9.

[0121] Embodiment 11 The difference between this embodiment and embodiment 1 is that the mass ratio of LFP@C to Ag is controlled to be 90:10.

[0122] The LFP@C@PDA with silver nanoparticles distributed on the surface prepared in this embodiment is denoted as LFP@C@PDA-45@Ag-10.

[0123] Embodiment 12 The difference between this embodiment and embodiment 1 is that in step (3), the molar ratio of AgNO3 to glucose is 1:0.8.

[0124] Embodiment 13 The difference between this embodiment and embodiment 1 is that in step (3), the molar ratio of AgNO3 to glucose is 1:1.1.

[0125] Embodiment 14 The difference between this embodiment and embodiment 1 is that in step (3), the molar ratio of AgNO3 to glucose is 1:1.2.

[0126] Embodiment 15 The difference between this embodiment and embodiment 1 is that in step (3), the molar ratio of AgNO3 to glucose is 1:1.5.

[0127] Embodiment 16 The difference between this embodiment and embodiment 1 is that: (2) 54 mg of Tris powder was weighed and dissolved in 300 mL of deionized water, and ultrasonic treatment was performed for 20 minutes to obtain 300 mL of a Tris aqueous solution; 3 g of LFP@C powder was weighed and added to the 300 mL of the Tris aqueous solution, and ultrasonic treatment was performed for 5 minutes to obtain 300 mL of an LFP@C / Tris aqueous solution; 45 mg of dopamine hydrochloride powder was weighed and dissolved in 300 mL of the LFP@C / Tris aqueous solution, and ultrasonic treatment was performed for 0.5 hours, and then stirring was performed at 20°C for 12 hours to obtain an LFP@C / PDA aqueous solution; the 300 mL of the LFP@C / PDA aqueous solution was suction-filtered by using a cellulose ester membrane with a pore size of 0.22 μm, and then repeatedly washed with double-distilled water, and the obtained black solid was dried at 40°C to obtain PDA-coated lithium iron phosphate, denoted as LFP@C@PDA-45.

[0128] (3) 0.5 g of the prepared LFP@C@PDA-45 powder was dispersed in 100 mL of double-distilled water, and ultrasonic oscillation was performed at room temperature for 0.5 hours to obtain a dispersion liquid of LFP@C@PDA-45; according to a volume ratio of 1:0.2:1, a 5% sodium hydroxide solution was added dropwise to a silver nitrate solution with a concentration of 0.00354 mol / L, and then ammonia water with a concentration of 0.00354 mol / L was added, and after the system was dissolved to be clear, a silver ammonia solution was obtained; the prepared silver ammonia solution was slowly added dropwise to the dispersion liquid of LFP@C@PDA-45, and the mass ratio of LFP@C to Ag was controlled to be 94:6, and mechanical stirring was performed at 40°C for 2 hours, and then glucose with an equal molar ratio to AgNO3 was added, and the reaction was continued for 0.5 hours, and then the obtained suspension was suction-filtered by using a cellulose ester membrane with a pore size of 0.22 μm, and repeatedly washed with double-distilled water for several times, and the obtained black solid was vacuum dried at 50°C to obtain LFP@C@PDA-45 with silver nanoparticles distributed on the surface, denoted as LFP@C@PDA-45@Ag-6.

[0129] Example 17 The difference between this example and Example 1 is that: (2) Weigh 54 mg of Tris powder and dissolve it in 300 mL of deionized water. Sonicate the solution for 30 minutes to obtain 300 mL of Tris aqueous solution. Weigh 3 g of LFP@C powder and add it to 300 mL of Tris aqueous solution. Sonicate the solution for 15 minutes to obtain 300 mL of LFP@C / Tris aqueous solution. Weigh 45 mg of dopamine hydrochloride powder and dissolve it in 300 mL of LFP@C / Tris aqueous solution. Sonicate the solution for 1.5 hours and then stir it at 30°C for 36 hours to obtain LFP@C / PDA aqueous solution. Filter the 300 mL LFP@C / PDA aqueous solution through a cellulose ester membrane with a pore size of 0.22 μm. Wash the solution repeatedly with double-distilled water. Dry the resulting black solid at 60°C to obtain PDA-coated lithium iron phosphate, denoted as LFP@C@PDA-45.

[0130] (3) Take 0.5 g of the prepared LFP@C@PDA-45 powder and disperse it in 100 mL of double-distilled water. After ultrasonic oscillation at room temperature for 1.5 h, a dispersion of LFP@C@PDA-45 is obtained. Add 5% sodium hydroxide solution to a 0.00354 mol / L silver nitrate solution at a volume ratio of 1:0.2:1, and then add 0.00354 mol / L ammonia solution. After the system dissolves and becomes clear, a silver ammonia solution is obtained. Slowly add the prepared silver ammonia solution to the LFP@C@PDA-45 dispersion, control the mass ratio of LFP@C to Ag to be 94:6, and react with mechanical stirring at 80 °C in a water bath for 4 h. Then add glucose in an equimolar ratio with AgNO3 and continue the reaction for 2 h. Filter the obtained suspension through a 0.22 μm cellulose ester membrane and wash it repeatedly with double-distilled water. Take 70 g of the obtained black solid. LFP@C@PDA-45 with silver nanoparticles distributed on its surface was obtained by vacuum drying at ℃, and was denoted as LFP@C@PDA-45@Ag-6.

[0131] Example 18 The difference between this embodiment and embodiment 1 is that in step (1), the mass ratio of anhydrous iron phosphate: lithium carbonate: glucose is 80:20:8.0.

[0132] Example 19 The difference between this embodiment and embodiment 1 is that in step (1), the mass ratio of anhydrous iron phosphate: lithium carbonate: glucose is 80:20:8.5.

[0133] Example 20 The difference between this embodiment and embodiment 1 is that in step (1), the mass ratio of anhydrous iron phosphate: lithium carbonate: glucose is 80:20:9.5.

[0134] Example 21 The difference between this embodiment and embodiment 1 is that in step (1), the mass ratio of anhydrous iron phosphate: lithium carbonate: glucose is 80:20:10.0.

[0135] Example 22 The difference between this embodiment and Embodiment 1 is that carbon-coated lithium manganese iron phosphate (LFMP@C) is provided in step (1). The preparation process of the carbon-coated lithium manganese iron phosphate is as follows: Anhydrous iron phosphate, lithium carbonate, manganese tetroxide, glucose, and ammonium dihydrogen phosphate were dissolved in water at a mass ratio of 0.4:1.05:0.6:0.15:1.03 and premixed for 30 min to obtain a mixed slurry. The mixed slurry was then sand-milled and spray-dried (inlet air temperature 240 ℃, outlet air temperature 95 ℃). Next, it was heated to 760 ℃ at a rate of 3 ℃ / min under a nitrogen atmosphere and held at that temperature for 9 h. After the holding period, it was cooled to below 100 ℃ and removed from the furnace, then pulverized to obtain carbon-coated lithium manganese iron phosphate cathode material, denoted as LFMP@C.

[0136] Comparative Example 1 This comparative example is the LFP@C obtained in step (1) of Example 1.

[0137] Comparative Example 2 This comparative example is LFP@C@PDA-45 from Example 1.

[0138] Comparative Example 3 The difference between this comparative example and Example 1 is that glucose was not added in step (3).

[0139] Comparative Example 4 This comparative example is LFMP@C from Example 22.

[0140] II. Testing Methods (1) Material property testing 1. TEM testing: The morphology of the material was characterized using transmission electron microscopy, and the results are shown in [Figure number missing]. Figure 1 and 2 .

[0141] 2. SEM testing: The morphology of the material was characterized using field emission scanning electron microscopy. The results are shown in [Figure number missing]. Figure 3 and 4 .

[0142] 3. Infrared spectroscopy testing: The composition of the material was characterized using a Fourier transform infrared spectrometer. The results are shown in [Figure number missing]. Figure 5 .

[0143] 4. Average particle size of active particles: detected using TEM.

[0144] 5. Carbon coating layer thickness and polydopamine coating layer thickness: TEM was used for detection.

[0145] 6. Mass fraction of nitrogen element in polydopamine coating layer: X-ray photoelectron spectroscopy was used for detection.

[0146] 7. Mass fraction of silver element in positive electrode material: X-ray photoelectron spectroscopy was used for detection.

[0147] 8. Particle size of silver nanoparticles: SEM was used for detection.

[0148] 9. Porosity, average pore size and BET specific surface area: gas adsorption BET method was used for determination.

[0149] 10. Electronic conductivity: the resistance (R) of the sample was measured by alternating current impedance spectroscopy, and the conductivity (σ) = L / (R·A), L was the length of the sample in the direction of conduction, and A was the cross-sectional area perpendicular to the direction of conduction. (II) Property test of secondary battery The positive electrode materials prepared in each example and comparative example were mixed with carbon black conductive agent (Super P, SP) and polyvinylidene fluoride in N-methyl pyrrolidone (NMP) according to a mass ratio of 90:5:5, then coated on an aluminum foil, and after drying, the sheet was pressed; lithium sheet was used as the negative electrode, lithium hexafluorophosphate (LiPF6) was used as the lithium salt, and ethylene carbonate (EC), methyl ethyl carbonate (EMC) and diethyl carbonate (DEC) in a volume ratio of 1:1:1 were used as the organic solvent, the lithium salt was dissolved in the organic solvent to prepare an electrolyte with a concentration of 1M, and a button cell was prepared.

[0150] Battery performance test: Wuhan Landian Electronics Technology Co., Ltd. battery performance test system (model: CT3002A) was used, the test temperature was 25℃, the voltage range was 2V~3.75V, and the test was carried out at 0.1C rate and 1C rate, respectively.

[0151] III. Analysis of test results of each example and comparative example Table 1

[0152] Table 2

[0153] Table 3

[0154] Please refer to Figure 1and 2 , by Figure 1 and 2 It can be seen that, after being coated by the polydopamine layer, the increase in the thickness of the coating layer indicates that the polydopamine is successfully coated on the surface of the carbon-coated lithium iron phosphate.

[0155] Referring to Figure 3 and 4 , compared to Figure 3 , after the addition of the silver ammine solution and the reaction, Figure 4 , the silver nanoparticles are adsorbed on the surface of the lithium iron phosphate.

[0156] Referring to Figure 5 , by Figure 5 It can be seen that, in the 1615 cm -1 and 3420 cm -1 of different LFP@C@PDA@Ag, the characteristic absorption peaks of the polydopamine appear, which indicates that the polydopamine is successfully coated on the surface of the LFP@C.

[0157] Referring to Tables 1-3, by Tables 1-3 and in combination with Figures 1-5 It can be seen that the positive electrode material prepared in Examples 1-22 of the present application comprises active particles, a carbon coating layer and a polydopamine coating layer, the carbon coating layer is coated on the surface of the active particles, the polydopamine coating layer is coated on the surface of the carbon coating layer, and the silver nanoparticles are distributed in the polydopamine coating layer; wherein the ratio of the thickness of the carbon coating layer to the thickness of the polydopamine coating layer is 1: (0.72-0.97); the porosity of the positive electrode material is 10%-30%, the average pore size is 16 nm-36 nm, and the specific surface area is 9 m 2 / g-26 m 2 / g; the thickness of the carbon coating layer is 1.20 nm-1.61 nm; the thickness of the polydopamine coating layer is 1.13 nm-1.26 nm; the ratio of the particle size of the active particles to the thickness of the carbon coating layer is 1: (0.0018-0.0062); the mass fraction of the nitrogen element in the polydopamine coating layer is 2.0%-2.68%; the particle size of the silver nanoparticles in the polydopamine coating layer is 8 nm-20 nm; the mass fraction of the silver element in the positive electrode material is 1%-13%; the average particle size of the secondary particles of the active particles is 100 nm-450 nm; and the electronic conductivity of the positive electrode material is 2.0 x 10 -5 S / cm-5.0 x 10 -5 S / cm.

[0158] It can be known by comparing examples 1-7 that when the mass ratio of the active particles for forming the carbon-coated layer on the surface and dopamine is in the range of 1:(0.015-0.035), the porosity, average pore size and specific surface area of the positive electrode material are beneficial to be controlled, and when the parameters are controlled in the appropriate range, the charging specific capacity, discharging specific capacity, rate performance and cycle life of the secondary battery using the positive electrode material are further improved.

[0159] It can be known by examples 1 and 8-11 that the secondary battery using the positive electrode material has high 0.1C, 1C charging specific capacity, discharging specific capacity and capacity retention rate, which indicates that when the mass ratio of LFP@C and Ag is in the range of 1:(0.03-0.1), the charging specific capacity, discharging specific capacity, rate performance and cycle life of the secondary battery using the positive electrode material are improved.

[0160] It can be known by examples 1, 12-15 and comparative example 3 that the battery has high 0.1C, 1C charging specific capacity, discharging specific capacity and capacity retention rate, which indicates that when the molar ratio of AgNO3 and glucose is in the range of 1:(1-1.2), the charging specific capacity, discharging specific capacity, rate performance and cycle life of the secondary battery using the positive electrode material are improved.

[0161] It can be seen from examples 1 and 18-21 that when the thickness of the carbon layer is in the range of 1.20 nm-1.61 nm, the secondary battery using the positive electrode material has high 0.1C, 1C charging specific capacity, discharging specific capacity and capacity retention rate, which indicates that when the thickness of the carbon layer is controlled in the appropriate range by controlling the amount of carbon source, the charging specific capacity, discharging specific capacity, rate performance and cycle life of the secondary battery using the positive electrode material are improved.

[0162] It can be seen from examples 1 and comparative examples 1-2 that introducing a polydopamine coating layer on the surface of LFP@C is beneficial to significantly improve the porosity, average pore size, specific surface area, improve the integrity of the coating layer, improve the conductivity, and then lead to the improvement of the charging specific capacity, discharging specific capacity, rate performance and cycle life; loading silver nanoparticles on the surface of LFP@C@PDA has little effect on the porosity, average pore size and specific surface area, but significantly improves the conductivity, and then leads to the improvement of the charging specific capacity, discharging specific capacity, rate performance and cycle life.

[0163] It can be seen from example 22 and comparative example 4 that the active particle material of the application can also be lithium manganese iron phosphate, which is beneficial to the positive electrode material to meet different application scenarios, and then is beneficial to the wide application of the positive electrode material.

[0164] Note that the present application is not limited to the above-described embodiments. The above-described embodiments are merely examples, and embodiments having substantially the same configuration, function, and effect as the technical idea of the present application are included in the technical scope of the present application. Furthermore, other modes constructed by applying various modifications to the embodiments, or by combining part of the configurations of the embodiments, which can be conceived by those skilled in the art without departing from the spirit of the present application, are also included in the scope of the present application.

Claims

1. A positive electrode material, characterized by, The positive electrode material comprises active particles, a carbon coating layer and a polydopamine coating layer, the carbon coating layer is coated on the surface of the active particles, the polydopamine coating layer is coated on the surface of the carbon coating layer, and silver nanoparticles are distributed in the polydopamine coating layer; wherein, The ratio of the thickness of the carbon coating layer to the thickness of the polydopamine coating layer is 1:(0.72-0.97); The porosity of the positive electrode material is 10% to 30%, the average pore size is 16 nm to 36 nm, and the specific surface area is 9 m 2 / g to 26 m 2 / g.

2. The cathode material of claim 1, wherein, The thickness of the carbon coating layer is 1.20 nm-1.61 nm; and / or, The thickness of the polydopamine coating layer is 1.13 nm-1.26 nm; and / or, The ratio of the particle size of the active particles to the thickness of the carbon coating layer is 1:(0.0018-0.0062); and / or, The mass fraction of nitrogen in the polydopamine coating layer is greater than the mass fraction of nitrogen in the carbon coating layer; and / or, The mass fraction of nitrogen in the polydopamine coating layer is 2.0%-2.68%; and / or, The particle size of the silver nanoparticles in the polydopamine coating layer is 8 nm-23.5 nm; and / or, The mass fraction of silver in the positive electrode material is 1%-13%.

3. The cathode material of claim 1, wherein, The average particle size of the active particles is 100 nm-450 nm; and / or, The active particles are at least one of lithium iron phosphate, lithium iron manganese phosphate, lithium nickel cobalt manganese oxide, lithium nickel cobalt aluminate, lithium manganate, lithium cobaltate and lithium titanate.

4. The cathode material of claim 1, wherein, The electronic conductivity of the positive electrode material is 2.0 x 10 -5 S / cm~5.0 x 10 -5 S / cm, and the lithium ion diffusion rate is 3.598 x 10 -10 cm 2 / s~4.598 x 10 -10 cm 2 / s.

5. A method for producing a positive electrode material, characterized by, The method comprises the following steps: forming a carbon coating layer on the surface of the active particles; forming a polydopamine coating layer on the surface of the carbon coating layer to obtain a positive electrode material; wherein, silver nanoparticles are distributed in the polydopamine coating layer; the ratio of the thickness of the carbon coating layer to the thickness of the polydopamine coating layer is 1:(0.72-0.97); The porosity of the positive electrode material is 10% to 30%, the average pore size is 16 nm to 36 nm, and the specific surface area is 9 m 2 / g to 26 m 2 / g.

6. The method for preparing the cathode material according to claim 5, characterized in that, forming a polydopamine coating layer on the surface of the carbon coating layer to obtain a positive electrode material comprises: mixing the active particles with a carbon coating layer on the surface, dopamine, tris-hydroxymethyl aminomethane and a first solvent, performing a first reaction treatment, and performing a first solid-liquid separation treatment to obtain a first material; mixing the first material, a silver ammine solution and a second solvent, performing a second reaction treatment, and performing a second solid-liquid separation treatment to obtain the positive electrode material; wherein, the temperature of the first reaction treatment is 20-30°C, and the time of the first reaction treatment is 12-36 h; the temperature of the second reaction treatment is 40-80°C, and the time of the second reaction treatment is 2-6 h.

7. The method for preparing the cathode material according to claim 6, characterized in that, mixing the first material, a silver ammine solution and a second solvent, performing a second reaction treatment, and performing a second solid-liquid separation treatment to obtain the positive electrode material comprises: mixing the first material, a silver ammine solution and a second solvent, performing a first step reaction treatment to obtain a first mixture; mixing the first mixture with a reducing agent, performing a second step reaction treatment to obtain a second mixture; the second mixture is subjected to a second solid-liquid separation treatment to obtain the positive electrode material; wherein, the temperature of the first step reaction treatment is 40-80°C, and the time of the first step reaction treatment is 2-4 h; the temperature of the second step reaction treatment is 40-80°C, and the time of the second step reaction treatment is 0.5-2 h.

8. The method for preparing the cathode material according to any one of claims 6 to 7, characterized in that, The mass ratio of the active particles forming the carbon-coated layer, dopamine and tris-hydroxymethyl aminomethane is 1: (0.015-0.035): (0.015-0.020); and / or, The mass ratio of the first material and silver in the silver-ammonia solution is 1: (0.03-0.1); and / or, The reducing agent is at least one of glucose, ascorbic acid and citric acid; and / or, The molar ratio of silver in the silver-ammonia solution and the reducing agent is 1: (1-1.2).

9. A positive electrode sheet characterized by comprising: The positive electrode plate comprises the positive electrode material of any one of claims 1-4 or the positive electrode material prepared by the method of any one of claims 5-8.

10. A secondary battery characterized by comprising: The secondary battery comprises the positive electrode plate of claim 9.

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