Positive electrode material and preparation method and application thereof

Abstract

The present invention relates to a positive electrode material, the positive electrode material comprises a lithium manganese iron phosphate core, and a conductive polymer layer and a carbon coating layer sequentially coating the surface of the core, and the carbon coating layer comprises a metal organic framework (MOF) and a carbon nanotube (CNT). The positive electrode material is stable in structure, manganese ions can be effectively prevented from being dissolved out, the conductivity of the material is improved, the capacity, the rate capability and the cycling stability of the modified lithium manganese iron phosphate material are improved, and the positive electrode material has great industrial application value.

Description

Technical Field

[0001] The present invention relates to the technical field of lithium ion batteries, and in particular to a positive electrode material and a preparation method and application thereof. Background Art

[0002] The core of lithium-ion battery operation is the embedding and de-embedding of lithium ions. The cathode material, as the source of lithium ions, has become the key to improving battery performance. Lithium manganese iron phosphate material (LMFP) is an upgraded version of lithium iron phosphate (LFP). Although the properties and preparation processes of the two are similar, LMFP has higher voltage, higher energy density and better low-temperature performance than LFP. Although LFMP and LFP have the same theoretical gram capacity, compared with LFP, the voltage of LMFP can reach about 4.1V, while that of LFP is about 3.4-3.5V. Therefore, under the same conditions, the theoretical energy density of LMFP with a higher voltage is 15-20% higher than that of LFP, which can break the current upper limit of battery energy density.

[0003] Although in theory, lithium manganese iron phosphate has the safety and cycle advantages of lithium iron phosphate, and has a higher energy density, lithium manganese iron phosphate has a hexagonal close-packed structure, FeO6 and MnO6 are located on the octahedron, and are cross-connected by PO4 tetrahedrons. There is no continuous FeO6 (MnO6) octahedral network, which makes it very poor in conductivity. At the same time, the PO4 tetrahedron is located between the FeO6 (MnO6) octahedrons, blocking the lithium ion diffusion channel, limiting its movement in one-dimensional channels, resulting in a relatively low lithium ion diffusion rate and poor rate performance. These shortcomings make it impossible for lithium manganese iron phosphate materials to fully exert their electrochemical properties, and therefore limit their further large-scale application. In addition, due to the presence of two elements, Fe and Mn, two different operating voltages are brought about, and the dual voltage platform is unstable. Manganese ions will also be affected by the Jahn-Teller effect, resulting in dissolution, deposition on the negative electrode surface, and damage to the SEI film. These shortcomings are the main factors restricting the large-scale application of LMFP in the market.

[0004] This has led to a series of modification measures to improve the electrochemical performance of LMFP and promote its commercial application. The main methods are: (1) Improving the conductivity and cycle performance of the material through carbon coating. (2) Changing the conductivity and ion diffusion properties of the material from the inside of the lattice through ion doping. Doping ions can cause defects in the lattice and inhibit the John-Teller effect, thereby improving the material performance. (3) Reducing the crystal particle size of the material through nano-scaling, improving the rate performance and other electrochemical properties. (4) Comprehensively improving the material performance by combining LMFP with other materials.

[0005] Although LMFP has a higher energy density than LFP, and the current modification scheme of LMFP has also improved the conductivity, manganese precipitation and crystal particle size of LMFP to a certain extent, the carbon coating still has the problem of uneven coating. The dissolution of transition metal manganese will inevitably occur in LMFP during the charge and discharge process. On the one hand, the dissolution of manganese will affect the structure of the positive electrode material and the lithium storage capacity of the positive electrode; on the other hand, the dissolved manganese will be deposited on the negative electrode, affecting the diffusion of lithium ions. At the same time, the deposited manganese will catalyze the decomposition of the electrolyte, resulting in the consumption of active lithium and the formation of a thicker solid electrolyte interface film. In addition, the lithium manganese iron phosphate battery has the disadvantages of large expansion force during the reaction process, difficult to control the self-discharge level, and the electrolyte is easy to react chemically with manganese compounds to dissolve, which is easy to cause manganese precipitation and poor cycle storage performance, resulting in the inability of lithium manganese iron phosphate batteries to develop on a large scale.

[0006] Coating lithium iron manganese with carbon materials is a common method to improve its conductivity. However, simple carbon material coating is prone to some problems. Too much carbon will reduce the tap density of the material, and too little carbon will not form an effective conductive network. In addition, coating lithium iron manganese with conductive polymers is also an effective method to improve its conductivity. Based on this, it is currently necessary to develop a new type of modified lithium iron manganese phosphate material and its preparation method and application. Summary of the invention

[0007] Problem that the invention aims to solve

[0008] Based on the above problems existing in the prior art, the purpose of the present invention is to provide a positive electrode material and a preparation method, so that the positive electrode material has more excellent rate performance, and can also effectively inhibit the precipitation of manganese ions, improve the electrochemical performance, and increase the cycle life.

[0009] Solutions for solving problems

[0010] The present invention provides a positive electrode material, which comprises: a lithium manganese iron phosphate core and a conductive polymer layer and a carbon coating layer sequentially coated on the surface of the core, wherein the carbon coating layer comprises: a metal organic framework (MOF) and a carbon nanotube (CNT).

[0011] Preferably, the particle size of the lithium manganese iron phosphate core is less than 80 nm;

[0012] And / or, the thickness of the conductive polymer layer is 2 to 5 nm;

[0013] And / or, the carbon coating layer has a thickness of 0.1 to 2 nm.

[0014] Preferably, the mass ratio of the MOF to the CNT is (0.01-1):1.

[0015] The present invention also provides a method for preparing the positive electrode material, the method comprising the following steps:

[0016] (1) subjecting a reaction solution containing a phosphorus source, a manganese source, an iron source and a lithium source to a hydrothermal reaction to obtain a lithium iron manganese phosphate precursor;

[0017] (2) mixing the lithium manganese iron phosphate precursor obtained in step (1) with a conductive polymer monomer to obtain a premixed solution;

[0018] (3) adding an oxidant to the premixed solution obtained in step (2) to carry out a polymerization reaction to obtain lithium manganese iron phosphate coated with a conductive polymer;

[0019] (4) mixing a metal salt, an organic ligand and a solvent to react to obtain a MOF, and mixing the MOF with CNTs and grinding them to obtain a MOF-CNT mixture;

[0020] (5) Mixing the conductive polymer-coated lithium manganese iron phosphate obtained in step (3) with the MOF-CNT mixture obtained in step (4), drying to obtain a powder, and sintering the powder under a protective atmosphere to obtain the positive electrode material.

[0021] Preferably, in step (1), the mass ratio of the phosphorus source, manganese source, iron source and lithium source is (1-1.1):(0.6-0.9):(0.1-0.4):1, the solvent of the reaction solution is water, the temperature of the hydrothermal reaction is 100-150° C., and the time is 6-10 h;

[0022] Preferably, the phosphorus source is one or more of diammonium hydrogen phosphate, ammonium phosphate, phosphorus pentoxide, phosphoric acid, and phosphorous acid; the manganese source is one or more of manganese carbonate, manganese phosphate, manganous phosphate, manganese sulfate, manganese oxalate, manganese acetate, manganese chloride, manganese trioxide, and manganese tetraoxide; the iron source is one or more of ferrous phosphate, ferrous oxalate, ferrous chloride, ferrous nitrate, ferrous oxide, ferrous sulfate, ferric chloride, ferric nitrate, ferric sulfate, ferric oxide, ferric oxide, and ferric phosphate; and the lithium source is one or more of lithium carbonate, lithium bicarbonate, lithium acetate, lithium chloride, lithium bromide, lithium hydroxide, lithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate, lithium oxalate, and lithium sulfate.

[0023] Preferably, in step (2), the mass ratio of the lithium manganese iron phosphate precursor to the conductive polymer monomer is 50:1 to 200:1, and the conductive polymer monomer is selected from one or more of fluorene, carbazole and their derivatives.

[0024] Preferably, in step (3), the polymerization reaction temperature is 20-60° C., the reaction time is 2-8 h, and the oxidant is one or more of ammonium persulfate, ferric chloride, potassium permanganate, potassium dichromate, and hydrogen peroxide.

[0025] Preferably, in step (4), the mass ratio of the metal salt, the organic ligand, and the solvent is (0.5-2):(1-1.1):(10-15), the reaction temperature is 90-150° C., the reaction time is 8-12 h, and the mass ratio of the MOF to the CNT is (0.01-1):1;

[0026] And / or, the metal salt is selected from one or more of zinc chloride, zinc sulfate, and zinc nitrate, the organic ligand is selected from one or more of 2-methylimidazole, trimesic acid, phthalic acid, and thiophene-2.5-dicarboxylic acid, the solvent is selected from one or more of methanol, acetone, and anhydrous ethanol, and the CNT has an electrical conductivity higher than 10Ωcm -1 of single-walled carbon nanotubes.

[0027] Preferably, in step (5), the mass ratio of the conductive polymer-coated lithium manganese iron phosphate to the MOF-CNT mixture is 500:1 to 2000:1;

[0028] And / or, the drying temperature is 200-300° C., the sintering temperature is 600-900° C., the heating rate is 2-5° C. / min, and the protective atmosphere is an inert gas.

[0029] The present invention also provides a lithium ion battery, the positive electrode material of the lithium ion battery, or the positive electrode material prepared by the preparation method.

[0030] Effects of the Invention

[0031] 1. The positive electrode material of the present invention has a specific core-shell structure, which is stable. Compared with the prior art, it can more effectively avoid the dissolution of manganese ions, improve the conductivity of the material, and enhance the capacity, rate performance and cycle stability of the modified lithium manganese iron phosphate material, and has great industrial application value.

[0032] 2. The positive electrode material of the present invention adopts a hydrothermal method to synthesize the lithium iron manganese phosphate core, which makes it easier to control the morphology of the lithium iron manganese phosphate and prepare a lithium iron manganese phosphate precursor with a smaller morphology and uniform particles, which is beneficial to shorten the diffusion path of lithium ions, improve the migration efficiency of lithium ions, and improve the rate performance of the material.

[0033] 3. The positive electrode material of the present invention uses a conductive polymer layer to cover the lithium iron manganese phosphate core, which can not only effectively reduce or inhibit the side reactions on the contact surface between the lithium iron manganese phosphate and the electrolyte and reduce manganese precipitation, but also can act as a lithium ion conductor to better transfer lithium ions and improve the long cycle performance of the material.

[0034] 4. The outermost layer of the positive electrode material of the present invention is coated with a carbon coating layer obtained from a composite carbon source of MOF and CNT. The material effectively reduces the resistivity of the positive electrode material through a thin layer coating, improves the conductivity, prevents the growth and agglomeration of particles, thereby maintaining the nanostructure of the particles, effectively reducing the diffusion distance of Li+ inside the active particles, and improving the rate performance of the material. At the same time, it can also effectively inhibit the dissolution of manganese ions, improve electrochemical performance, and increase cycle life. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 Schematic diagram of the structure of lithium manganese iron phosphate positive electrode material.

[0036] Figure 2 This is the SEM image of the lithium manganese iron phosphate positive electrode material. The particle size is about 80nm, which is relatively small.

[0037] Figure 3 This is the TEM image of lithium manganese iron phosphate positive electrode material. DETAILED DESCRIPTION

[0038] In order to make the technical scheme and beneficial effects of the present invention more obvious and easy to understand, the following is a detailed description by listing specific embodiments. Wherein, the drawings are not necessarily drawn to scale, and local features can be enlarged or reduced to more clearly show the details of local features. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of the present invention. The experimental methods in the following examples that do not specify specific conditions are usually based on conventional experimental conditions. Reagents and raw materials used in the present invention are commercially available unless otherwise specified.

[0039] In the following description, a large number of specific details are given to provide a more thorough understanding of the present application. However, it is obvious to those skilled in the art that the present application can be implemented without one or more of these details. In other examples, in order to avoid confusion with the present application, some technical features known in the art are not described; that is, all features of the actual embodiments are not described here, and well-known functions and steps are not described in detail.

[0040] The purpose of the terms used herein is only to describe specific embodiments and is not intended to be limiting of the present application. When used herein, the singular forms "one", "an" and "said / the" are also intended to include plural forms, unless the context clearly indicates otherwise. It should also be understood that the terms "consisting of" and / or "comprising", when used in this specification, determine the presence of the features, integers, steps, operations, elements and / or parts, but do not exclude the presence or addition of one or more other features, integers, steps, operations, elements, parts and / or groups. When used herein, the term "and / or" includes any and all combinations of the relevant listed items.

[0041] In order to thoroughly understand the present application, detailed steps and detailed structures will be presented in the following description to illustrate the technical solution of the present application. The preferred embodiments of the present application are described in detail below, but in addition to these detailed descriptions, the present application may also have other implementation methods.

[0042] Unless otherwise defined, technical and scientific terms used in this application have the same meanings as those in the technical field to which this application belongs.

[0043] If no specific techniques or conditions are specified in the following examples, the conventional techniques or conditions described in the literature in the art, or the conditions recommended by the product instructions and the manufacturer are generally used. The numerical ranges in the following examples all include the endpoint values.

[0044] The present invention provides a positive electrode material, which comprises: a lithium manganese iron phosphate core and a conductive polymer layer and a carbon coating layer sequentially coated on the surface of the core, wherein the carbon coating layer comprises: a metal organic framework (MOF) and a carbon nanotube (CNT).

[0045] In certain embodiments, double-layer coating of a conductive polymer layer and a carbon coating layer can obtain a positive electrode material that has both energy density and long cycle performance. The synergistic effect of the double-layer coating can be used to optimize the long cycle performance of the positive electrode material.

[0046] In certain embodiments, the particle size of the lithium iron manganese phosphate core is less than 80 nm.

[0047] In some embodiments, controlling the particle size of the manganese iron lithium material is convenient for processing on the one hand, and for cost control on the other hand. The thickness of the conductive polymer layer is 2-5nm. If the thickness is too thin, it is easy to cause uneven coating and poor conductivity. If the thickness is too thick, on the one hand, the preparation process is complicated, and it will cause the capacity of lithium manganese iron phosphate to decrease. The thickness of the carbon coating layer is 0.1-2nm. If the thickness of the carbon layer is too thick, it will not only affect the ion transmission, but also affect the capacity of the battery due to the increase of inactive substances. If the thickness of the carbon layer is too thin, on the one hand, it is easy to have incomplete coating, and on the other hand, during the charge and discharge cycle, the too thin carbon layer is prone to breakage, affecting the stability of the battery.

[0048] In some embodiments, the particle size of the lithium manganese iron phosphate core is 79nm, or 78nm, or 77nm, or 76nm, or 75nm, or 74nm, or 73nm, or 72nm, or 71nm, or 70nm, or 69nm, or 68nm, or 67nm, or 66nm, or 65nm, or 64nm, or 63nm, or 62nm, or 61nm, or 60nm, or 59nm, or 58nm, or 57nm, or 56nm, or 55nm, or 54nm, or 53nm, or 52nm, or 51nm, or 50nm.

[0049] In certain embodiments, the thickness of the conductive polymer layer is 2-5 nm.

[0050] In certain embodiments, the thickness of the conductive polymer layer is 2 nm, or 2.1 nm, or 2.2 nm, or 2.3 nm, or 2.4 nm, or 2.5 nm, or 2.6 nm, or 2.7 nm, or 2.8 nm, or 2.9 nm, or 3 nm, or 3.1 nm, or 3.2 nm, or 3.3 nm, or 3.4 nm, or 3.5 nm, or 3.6 nm, or 3.7 nm, or 3.8 nm, or 3.9 nm, or 4 nm, or 4.1 nm, or 4.2 nm, or 4.3 nm, or 4.4 nm, or 4.5 nm, or 4.6 nm, or 4.7 nm, or 4.8 nm, or 4.9 nm, or 5 nm.

[0051] In certain embodiments, the conductive polymer layer has a thickness of 2 nm.

[0052] In certain embodiments, the carbon coating layer has a thickness of 0.1 to 2 nm.

[0053] In certain embodiments, the thickness of the carbon coating layer is 0.1 nm, or 0.2 nm, or 0.3 nm, or 0.4 nm, or 0.5 nm, or 0.6 nm, or 0.7 nm, or 0.8 nm, or 0.9 nm, or 1 nm, or 1.1 nm, or 1.2 nm, or 1.3 nm, or 1.4 nm, or 1.5 nm, or 1.6 nm, or 1.7 nm, or 1.8 nm, or 1.9 nm, or 2 nm.

[0054] In certain embodiments, the carbon coating layer has a thickness of 0.5 nm.

[0055] In certain embodiments, the mass ratio of the MOF to the CNT is (0.01-1):1.

[0056] In certain embodiments, the mass ratio of the MOF to the CNT is 0.01:1, or 0.02:1, or 0.03:1, or 0.04:1, or 0.05:1, or 0.06:1, or 0.07:1, or 0.08:1, or 0.09:1, or 0.1:1, or 0.15:1, or 0.2:1, or 0.25:1, or 0.3:1, or 0.35:1, or 0.4:1, or 0.45:1, or 0.5:1, or 0.55:1, or 0.6:1, or 0.65:1, or 0.7:1, or 0.75:1, or 0.8:1, or 0.85:1, or 0.9:1, or 0.95:1, or 1:1.

[0057] In certain embodiments, the mass ratio of the MOF to the CNT is (0.1-1):1.

[0058] In certain embodiments, the mass ratio of the MOF to the CNT is (0.2-0.8):1.

[0059] In certain embodiments, the mass ratio of the MOF to the CNT is 0.2:1, or 0.6:1, or 0.8:1.

[0060] In certain embodiments, the mass ratio of the MOF to the CNT is 0.2:1.

[0061] In certain embodiments, the mass ratio of the MOF to the CNT is 0.6:1.

[0062] In certain embodiments, the mass ratio of the MOF to the CNT is 0.8:1.

[0063] The present invention also provides a method for preparing the positive electrode material, the method comprising the following steps:

[0064] (1) subjecting a reaction solution containing a phosphorus source, a manganese source, an iron source and a lithium source to a hydrothermal reaction to obtain a lithium iron manganese phosphate precursor;

[0065] (2) mixing the lithium manganese iron phosphate precursor obtained in step (1) with a conductive polymer monomer to obtain a premixed solution;

[0066] (3) adding an oxidant to the premixed solution obtained in step (2) to carry out a polymerization reaction to obtain lithium manganese iron phosphate coated with a conductive polymer;

[0067] (4) mixing a metal salt, an organic ligand and a solvent to react to obtain a MOF, and mixing the MOF with CNTs and grinding them to obtain a MOF-CNT mixture;

[0068] (5) Mixing the conductive polymer-coated lithium manganese iron phosphate obtained in step (3) with the MOF-CNT mixture obtained in step (4), drying to obtain a powder, and sintering the powder under a protective atmosphere to obtain the positive electrode material.

[0069] In certain embodiments, in step (1), the mass ratio of the phosphorus source, manganese source, iron source and lithium source is (1-1.1):(0.6-0.9):(0.1-0.4):1.

[0070] In certain embodiments, in step (1), the mass ratio of the phosphorus source, manganese source, iron source and lithium source is 1:0.6:0.1:1, or 1:0.6:0.2:1, or 1:0.6:0.3:1, or 1:0.6:0.4:1, or 1:0.7:0.1:1, or 1:0.7:0.2:1, or 1:0.7:0.3:1, or 1:0.7:0.4:1, or 1:0.8:0.1:1, or 1:0.8:0.2:1, or 1:0.8:0.3:1, or 1:0.8:0.4:1, or 1:0.9:0.1:1, or 1:0.9:0.2:1, or 1:0.9:0.3:1, or 1:0.9:0.4:1. , or 1.1:0.6:0.1:1, or 1.1:0.6:0.2:1, or 1.1:0.6:0.3:1, or 1.1:0.6:0.4:1, or 1.1:0.7:0.1:1, or 1.1:0.7:0.2:1, or 1.1:0.7:0.3:1, or 1.1:0.7:0.4:1, or 1.1:0.8:0.1:1, or 1.1:0.8:0.2:1, or 1.1:0.8:0.3:1, or 1.1:0.8:0.4:1, or 1.1:0.9:0.1:1, or 1.1:0.9:0.2:1, or 1.1:0.9:0.3:1, or 1.1:0.9:0.4:1.

[0071] In certain embodiments, in step (1), the mass ratio of the phosphorus source, manganese source, iron source and lithium source is 1:0.6:0.4:1.

[0072] In certain embodiments, in step (1), the solvent of the reaction solution is water.

[0073] In certain embodiments, in step (1), the temperature of the hydrothermal reaction is 100-150°C.

[0074] In certain embodiments, in step (1), the temperature of the hydrothermal reaction is 100°C, or 105°C, or 110°C, or 115°C, or 120°C, or 125°C, or 130°C, or 135°C, or 140°C, or 145°C, or 150°C.

[0075] In certain embodiments, in step (1), the temperature of the hydrothermal reaction is 130-150°C.

[0076] In certain embodiments, in step (1), the temperature of the hydrothermal reaction is 150°C.

[0077] In certain embodiments, in step (1), the hydrothermal reaction time is 6 to 10 hours.

[0078] In certain embodiments, in step (1), the hydrothermal reaction time is 6 h, or 6.5 h, or 7 h, or 7.5 h, or 8 h, or 8.5 h, or 9 h, or 9.5 h, or 10 h.

[0079] In certain embodiments, in step (1), the hydrothermal reaction time is 6 to 7 hours.

[0080] In certain embodiments, in step (1), the hydrothermal reaction time is 6 hours.

[0081] In certain embodiments, the hydrothermal method is used to prepare the lithium iron manganese phosphate core, which makes it easier to control the morphology of the lithium iron manganese phosphate and prepare a lithium iron manganese phosphate precursor with smaller morphology and uniform particles. This is beneficial to shorten the diffusion path of lithium ions, improve the migration efficiency of lithium ions, and improve the rate performance of the material.

[0082] In certain embodiments, in step (1), the phosphorus source is one or more of diammonium hydrogen phosphate, ammonium phosphate, phosphorus pentoxide, phosphoric acid, and phosphorous acid.

[0083] In certain embodiments, in step (1), the phosphorus source is ammonium phosphate.

[0084] In certain embodiments, in step (1), the manganese source is one or more of manganese carbonate, manganese phosphate, manganous phosphate, manganese sulfate, manganese oxalate, manganese acetate, manganese chloride, manganese trioxide, and manganese tetraoxide.

[0085] In certain embodiments, in step (1), the manganese source is manganese carbonate.

[0086] In certain embodiments, in step (1), the iron source is one or more of ferrous phosphate, ferrous oxalate, ferrous chloride, ferrous nitrate, ferrous oxide, ferrous sulfate, ferric chloride, ferric nitrate, ferric sulfate, ferric oxide, ferric oxide, ferric tetroxide, and ferric phosphate.

[0087] In certain embodiments, in step (1), the iron source is ferrous phosphate.

[0088] In certain embodiments, in step (1), the lithium source is one or more of lithium carbonate, lithium bicarbonate, lithium acetate, lithium chloride, lithium bromide, lithium hydroxide, lithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate, lithium oxalate, and lithium sulfate.

[0089] In certain embodiments, in step (1), the lithium source is lithium carbonate.

[0090] In certain embodiments, in step (2), the mass ratio of the lithium manganese iron phosphate precursor to the conductive polymer monomer is 50:1 to 200:1.

[0091] In certain embodiments, in step (2), the mass ratio of the lithium manganese iron phosphate precursor to the conductive polymer monomer is 50:1, or 60:1, or 70:1, or 80:1, or 90:1, or 100:1, or 110:1, or 120:1, or 130:1, or 140:1, or 150:1, or 160:1, or 170:1, or 180:1, or 190:1, or 200:1.

[0092] In certain embodiments, in step (2), the mass ratio of the lithium manganese iron phosphate precursor to the conductive polymer monomer is 50:1 to 100:1.

[0093] In certain embodiments, in step (2), the mass ratio of the lithium manganese iron phosphate precursor to the conductive polymer monomer is 50:1.

[0094] In certain embodiments, in step (2), the conductive polymer monomer is selected from one or more of fluorene, carbazole and their derivatives.

[0095] In certain embodiments, in step (2), the conductive polymer monomer is fluorene.

[0096] In certain embodiments, the lithium iron manganese phosphate core is coated with a conductive polymer layer, which is to form a uniform coating layer on the surface of the lithium iron manganese phosphate by an in-situ polymerization reaction of the conductive polymer monomer. The positive electrode material with the coating layer can effectively inhibit the side reaction between the positive electrode material itself and the electrolyte, inhibit the dissolution of transition metals from the positive electrode material, and improve the structural stability of the positive electrode material, thereby improving the cycle performance and safety of the battery using the positive electrode material, and because the conductive polymer itself has conductivity and can contribute to capacity, it can ensure the conductivity of the electrode material and is conducive to improving the rate performance of the battery.

[0097] In certain embodiments, in step (3), the polymerization reaction temperature is 20 to 60°C.

[0098] In certain embodiments, in step (3), the polymerization reaction temperature is 20°C, or 25°C, or 30°C, or 35°C, or 40°C, or 45°C, or 50°C, or 55°C, or 60°C.

[0099] In certain embodiments, in step (3), the polymerization reaction temperature is 40-60°C.

[0100] In certain embodiments, in step (3), the polymerization reaction temperature is 60°C.

[0101] In certain embodiments, in step (3), the polymerization reaction time is 2 to 8 hours.

[0102] In certain embodiments, when the polymerization reaction time of the conductive polymer layer is within the preferred range provided by the present invention, it is beneficial to obtain a positive electrode material that has both energy density and long cycle performance.

[0103] In certain embodiments, in step (3), the polymerization reaction time is 2 h, or 2.5 h, or 3 h, or 3.5 h, or 4 h, or 4.5 h, or 5 h, or 5.5 h, or 6 h, or 6.5 h, or 7 h, or 7.5 h, or 8 h.

[0104] In certain embodiments, in step (3), the polymerization reaction time is 3 to 7 hours.

[0105] In certain embodiments, in step (3), the polymerization reaction time is 3 h, or 5 h, or 7 h.

[0106] In certain embodiments, in step (3), the polymerization reaction time is 3 hours.

[0107] In certain embodiments, in step (3), the polymerization reaction time is 5 hours.

[0108] In certain embodiments, in step (3), the polymerization reaction time is 7 hours.

[0109] In certain embodiments, in step (3), the oxidant is one or more of ammonium persulfate, ferric chloride, potassium permanganate, potassium dichromate, and hydrogen peroxide.

[0110] In certain embodiments, in step (3), the oxidant is hydrogen peroxide.

[0111] In certain embodiments, in step (4), the mass ratio of the metal salt, the organic ligand, and the solvent is (0.5-2):(1-1.1):(10-15).

[0112] In certain embodiments, in step (4), the mass ratio of the metal salt, the organic ligand, and the solvent is 0.5:1:10, or 0.5:1:11, or 0.5:1:12, or 0.5:1:13, or 0.5:1:14, or 0.5:1:15, or 0.5:1.1:10, or 0.5:1.1:11, or 0.5:1.1:12, or 0.5:1.1:13, or 0.5:1.1:14, or 0.5:1.1:15, or 0.6:1:10, or 0.6:1:11, or 0.6:1:12, or 0.6:1:13, or 0.6:1:14, or 0.6:1:15, or 0.6:1:10, or 0.6:1:11, or 0.6:1:12, or 0.6:1:13, or 0.6:1:14, or 0.6:1:15, or 0.6:1.1:10, or 0.6:1.1:11, or 0.6:1.1:12, or 0.6:1.1:13, or 0.6:1.1:14, or 0.6:1.1:15, or 0.7:1:10, or 0.7:1:11, or 0.7:1:12, or 0.7:1:13, or 0.7:1:14, or 0.7:1:15, or 0.7:1.1:10, or 0.7 :1.1:11, or 0.7:1.1:12, or 0.7:1.1:13, or 0.7:1.1:14, or 0.7:1.1:15, or 0.8:1:10, or 0.8:1:11, or 0.8:1:12, or 0.8:1:13, or 0.8:1:14, or 0.8:1:15, or 0.8:1.1: 10, or 0.8:1.1:11, or 0.8:1.1:12, or 0.8:1.1:13, or 0.8:1.1:14, or 0.8:1.1:15, or 0.9:1:10, or 0.9:1:11, or 0.9:1:12, or 0.9:1:13, or 0.9:1:14, or 0.9:1:15, or 0.9:1.1:10, or 0.9:1.1:11, or 0.9:1.1:12, or 0.9:1.1:13, or 0.9:1.1:14, or 0.9:1.1:15, or 1:1:10, or 1:1:11, or 1:1:12, or 1:1:13, or 1:1:14, or 1:1:15, or 1:1.1 :10, or 1:1.1:11, or 1:1.1:12, or 1:1.1:13, or 1:1.1:14, or 1:1.1:15, or 1.5:1:10, or 1.5:1:11, or 1.5:1:12, or 1.5:1:13, or 1.5:1:14, or 1.5:1:15, or 1.5:1.1:1 0, or 1.5:1.1:11, or 1.5:1.1:12, or 1.5:1.1:13, or 1.5:1.1:14, or 1.5:1.1:15, or 2:1:10, or 2:1:11, or 2:1:12, or 2:1:13, or 2:1:14, or 2:1:15, or 2:1.1:10, or 2:1.1:11, or 2:1.1:12, or 2:1.1:13, or 2:1.1:14, or 2:1.1:15. .

[0113] In certain embodiments, in step (4), the mass ratio of the metal salt, the organic ligand, and the solvent is 0.5:1:10.

[0114] In certain embodiments, in step (4), the reaction temperature is 90-150°C.

[0115] In certain embodiments, in step (4), the reaction temperature is 90°C, or 95°C, or 100°C, or 105°C, or 110°C, or 115°C, or 120°C, or 125°C, or 130°C, or 135°C, or 140°C, or 145°C, or 150°C.

[0116] In certain embodiments, in step (4), the reaction temperature is 130-150°C.

[0117] In certain embodiments, in step (4), the reaction temperature is 150°C.

[0118] In certain embodiments, in step (4), the reaction time is 8 to 12 hours.

[0119] In certain embodiments, in step (4), the reaction time is 8 h, or 8.5 h, or 9 h, or 9.5 h, or 10 h, or 10.5 h, or 11 h, or 11.5 h, or 12 h.

[0120] In certain embodiments, in step (4), the reaction time is 10 h.

[0121] In certain embodiments, in step (4), the mass ratio of the MOF to the CNT is (0.01-1):1.

[0122] In certain embodiments, when the mass ratio of MOF to CNT is within the preferred range provided by the present invention, it is beneficial to obtain a positive electrode material that takes both energy density and long cycle performance into consideration.

[0123] In certain embodiments, in step (4), the mass ratio of the MOF to the CNT is 0.01:1, or 0.02:1, or 0.03:1, or 0.04:1, or 0.05:1, or 0.06:1, or 0.07:1, or 0.08:1, or 0.09:1, or 0.1:1, or 0.15:1, or 0.2:1, or 0.25:1, or 0.3:1, or 0.35:1, or 0.4:1, or 0.45:1, or 0.5:1, or 0.55:1, or 0.6:1, or 0.65:1, or 0.7:1, or 0.75:1, or 0.8:1, or 0.85:1, or 0.9:1, or 0.95:1, or 1:1.

[0124] In certain embodiments, in step (4), the mass ratio of the MOF to the CNT is (0.05-1):1.

[0125] In certain embodiments, in step (4), the mass ratio of the MOF to the CNT is (0.2-0.8):1.

[0126] In certain embodiments, in step (4), the mass ratio of the MOF to the CNT is 0.2:1, or 0.6:1, or 0.8:1.

[0127] In certain embodiments, in step (4), the mass ratio of the MOF to the CNT is 0.2:1.

[0128] In certain embodiments, in step (4), the mass ratio of the MOF to the CNT is 0.6:1.

[0129] In certain embodiments, in step (4), the mass ratio of the MOF to the CNT is 0.8:1.

[0130] In certain embodiments, in step (4), the metal salt is selected from one or more of zinc chloride, zinc sulfate, and zinc nitrate.

[0131] In certain embodiments, in step (4), the metal salt is zinc chloride.

[0132] In certain embodiments, in step (4), the organic ligand is selected from one or more of 2-methylimidazole, trimesic acid, phthalic acid, and thiophene-2,5-dicarboxylic acid.

[0133] In some embodiments, in step (4), the organic ligand is 2-methylimidazole. In some embodiments, in step (4), the solvent is selected from one or more of methanol, acetone, and anhydrous ethanol.

[0134] In certain embodiments, in step (4), the solvent is methanol.

[0135] In certain embodiments, in step (4), the CNT has an electrical conductivity higher than 10Ωcm -1 of single-walled carbon nanotubes.

[0136] In certain embodiments, in step (4), the CNT is FT6000 series carbon nanotube powder produced by Tiannai Technology.

[0137] In certain embodiments, in step (4), the CNT average diameter is 5-11 nm, 7-12 nm; purity is ≥95%, ≥98%, ≥99.9%; length is 50-250 nm; ash content is ≤5%, ≤2%, ≤0.1%; specific surface area is 250-350 m 2 / g; Tap density: 0.005-0.25g / cm 3 ; Moisture content: <1000ppm.

[0138] In certain embodiments, in step (5), the mass ratio of the conductive polymer-coated lithium manganese iron phosphate to the MOF-CNT mixture is 500:1 to 2000:1.

[0139] In certain embodiments, in step (5), the mass ratio of the conductive polymer-coated lithium manganese iron phosphate to the MOF-CNT mixture is 500:1, or 550:1, or 600:1, or 650:1, or 700:1, or 750:1, or 800:1, or 850:1, or 900:1, or 950:1, or 1000:1, or 1050:1, or 1100:1, or 1200:1. 1150:1, or 1200:1, or 1250:1, or 1300:1, or 1350:1, or 1400:1, or 1450:1, or 1500:1, or 1550:1, or 1600:1, or 1650:1, or 1700:1, or 1750:1, or 1800:1, or 1850:1, or 1900:1, or 1950:1, or 2000:1.

[0140] In certain embodiments, in step (5), the mass ratio of the conductive polymer-coated lithium manganese iron phosphate to the MOF-CNT mixture is 500:1.

[0141] In certain embodiments, in step (5), the drying temperature is 200-300°C.

[0142] In certain embodiments, in step (5), the drying temperature is 200°C, or 210°C, or 220°C, or 230°C, or 240°C, or 250°C, or 260°C, or 270°C, or 280°C, or 290°C, or 300°C.

[0143] In certain embodiments, in step (5), the drying temperature is 200°C.

[0144] In certain embodiments, in step (5), the sintering temperature is 600-900°C.

[0145] In certain embodiments, in step (5), the sintering temperature is 600°C, or 650°C, or 700°C, or 750°C, or 800°C, or 850°C, or 900°C.

[0146] In certain embodiments, in step (5), the sintering temperature is 900°C.

[0147] In certain embodiments, in step (5), the heating rate of the sintering is 2 to 5° C. / min.

[0148] In certain embodiments, in step (5), the sintering heating rate is 2°C / min, or 2.5°C / min, or 3°C / min, or 3.5°C / min, or 4°C / min, or 4.5°C / min, or 5°C / min.

[0149] In certain embodiments, in step (5), the heating rate of the sintering is 5° C. / min.

[0150] In certain embodiments, in step (5), the protective atmosphere is an inert gas.

[0151] In certain embodiments, in step (5), the protective atmosphere is argon.

[0152] The present invention also provides a lithium ion battery, the positive electrode material of the lithium ion battery, or the positive electrode material prepared by the preparation method.

[0153] In some embodiments, the lithium-ion battery further includes a negative electrode sheet and a separator. Typically, a lithium-ion battery includes a positive electrode sheet, a negative electrode sheet, an electrolyte and a separator, and the separator is disposed between the positive electrode sheet and the negative electrode sheet. During the battery charge and discharge process, active ions are embedded and removed back and forth between the positive electrode sheet and the negative electrode sheet. The electrolyte plays the role of conducting ions between the positive electrode sheet and the negative electrode sheet. The separator is disposed between the positive electrode sheet and the negative electrode sheet, and mainly plays the role of preventing the positive / negative electrode from short-circuiting, while allowing ions to pass through.

[0154] The negative electrode sheet includes a negative electrode current collector and a negative electrode active material layer disposed on the negative electrode current collector. The negative electrode current collector may be aluminum foil, copper foil, titanium foil, nickel foil, iron foil, zinc foil, etc. The negative electrode active material layer includes a negative electrode active material. The negative electrode active material may include a material capable of reversibly inserting / deinserting lithium ions, lithium metal, lithium metal alloy, a material capable of doping / dedoping lithium, or a transition metal oxide. Examples of negative electrode active materials include lithium metal, structured lithium metal, graphite (e.g., natural graphite, artificial graphite), mesophase carbon balls, hard carbon, soft carbon, silicon, silicon-oxygen materials (e.g., silicon dioxide, silicon oxide), silicon-carbon composite materials (Si / C composite materials), Li-Sn alloys, Li-Sn-O alloys, spinel-structured lithiated TiO2-Li4Ti5O12, Li-Al alloys, etc. In certain embodiments, the negative electrode active material comprises graphite.

[0155] In certain embodiments, the negative electrode active material layer may further include one or two selected from a conductive agent and a binder. The conductive agent is used to improve the conductivity of the electrode. Examples of negative electrode conductive agents include conductive carbon black, conductive graphite, vapor deposited carbon fiber (VGCF), carbon nanotubes, graphene, etc. The negative electrode binder improves the bonding performance between the negative electrode active material particles and between the negative electrode active material particles and the current collector. Examples of negative electrode binders include polyvinylidene fluoride (PVDF), styrene-butadiene rubber (SBR), sodium carboxymethyl cellulose (CMC-Na), cyclodextrin, gelatin, polyvinyl alcohol, polyacrylate, acrylonitrile multipolymer, etc.

[0156] In some embodiments, the conductive agent in the negative electrode active material layer is acetylene black, and the binder is styrene-butadiene rubber and sodium carboxymethyl cellulose. The mass ratio of each component in the negative electrode active material layer can be conventional.

[0157] In some embodiments, the negative electrode active material layer is obtained by coating the negative electrode slurry containing the components of the negative electrode active material layer and a solvent onto the negative electrode current collector, and then rolling and slitting. The solvent of the negative electrode slurry can be a conventional solvent in the art, such as deionized water.

[0158] In certain embodiments, the separator can be a polymer porous separator, an inorganic porous separator or a polymer-inorganic composite porous separator. The polymer porous separator includes a single-layer polymer porous separator and a multi-layer polymer porous separator.

[0159] In certain embodiments, the lithium-ion battery of the present application further includes a packaging shell for accommodating a positive electrode sheet, a separator, a negative electrode sheet and an electrolyte, as well as other components known in the art in the lithium-ion battery, and the present application does not limit the above-mentioned other components. The present application does not specifically limit the packaging shell, and it can be a packaging shell known in the art, as long as it can achieve the purpose of the present application.

[0160] The present invention has no special restrictions on the preparation method of the lithium ion battery, and the technical scheme for preparing the negative electrode material into a lithium ion battery such as a secondary battery well known to those skilled in the art can be adopted.

[0161] It should be understood that, since the lithium-ion battery provided in the present application includes the positive electrode material described in the present invention, the beneficial effects of the positive electrode material described in any of the above embodiments are applicable to the lithium-ion battery.

[0162] Example 1: Preparation of positive electrode material (polymerization reaction time 3h, MOF:CNT=0.2:1)

[0163] This embodiment provides a modified lithium manganese iron phosphate material and a preparation method and application thereof, and the polymerization reaction time is 3h. The present invention performs double-layer coating and modification on the lithium manganese iron phosphate core (particle size ≤ 80nm), and the double-layer coating includes a first coating layer (thickness 2nm) of a conductive polymer and a second coating layer (thickness 0.5nm) of a MOF and CNT composite material, wherein the mass ratio of MOF to CNT is 0.2:1.

[0164] (1) Weigh 10 g of ammonium phosphate, manganese carbonate, ferrous phosphate and lithium carbonate in a mass ratio of 1:0.6:0.4:1, and prepare a mixed solution with 30 ml of solvent water. Mix the mixed solution evenly with a magnetic stirrer and add it into a polytetrafluoroethylene reactor to perform a hydrothermal reaction at 150° C. for 6 h. After the hydrothermal reaction is completed, perform a first solid-liquid separation and wash it alternately with water and ethanol three times. After drying, a lithium iron manganese phosphate precursor is obtained.

[0165] (2) The obtained lithium manganese iron phosphate precursor was mixed with 0.2 g of a conductive polymer monomer fluorene at a mass ratio of 50:1 to obtain a premixed solution, and 0.05 g of an oxidant hydrogen peroxide was added to the premixed solution to cause the polymer monomer to undergo a polymerization reaction. The reaction temperature was 60° C. and the reaction time was 3 h to obtain a conductive polymer-coated lithium manganese iron phosphate material.

[0166] (3) 5 g of zinc chloride, 2-methylimidazole and methanol were weighed in a mass ratio of 0.5:1:10, stirred evenly and transferred to a reactor, placed in an oven at 150°C for 10 h, and then centrifuged at 800 r for 30 min and dried at 120°C to obtain the MOF material.

[0167] (4) The MOF material weighed at a mass ratio of 0.2:1 and the conductivity was higher than 10Ωcm -1A total amount of 0.02 g of single-walled carbon nanotubes CNT (FT6000 series carbon nanotube powder of Tiannai Technology) was uniformly mixed and ball-milled for 1 hour. The 0.02 g ball-milled mixture and 10 g of the product in step (2) were ultrasonically dispersed at 0°C for 30 minutes to obtain a uniform suspension. The suspension was then spray-dried at 200°C for 1 hour to obtain modified lithium iron manganese phosphate powder.

[0168] (5) The obtained powder was sintered in a tubular furnace with an inert gas argon, with a heating rate of 5°C / min, a sintering temperature of 900°C, and a heat preservation time of 2h. After crushing and grinding, a modified lithium manganese iron phosphate material coated with a double layer of a conductive polymer and a carbon layer was obtained. The SEM image is shown in FIG. Figure 2 The modified lithium manganese iron phosphate material was subjected to transmission TEM testing to observe and detect the thickness of the coating layer. The results are shown in Figure 3 shown.

[0169] Example 2: Preparation of positive electrode material (polymerization reaction time 5 h, MOF:CNT=0.6:1)

[0170] This embodiment provides a modified lithium manganese iron phosphate material and a preparation method and application thereof, and the polymerization reaction time is 5h. The present invention performs double-layer coating and modification on the lithium manganese iron phosphate core (particle size ≤ 80nm), and the double-layer coating includes a first coating layer (thickness 2nm) of a conductive polymer and a second coating layer (thickness 0.5nm) of a MOF and CNT composite material, wherein the mass ratio of MOF to CNT is 0.6:1.

[0171] (1) Weigh 10 g of ammonium phosphate, manganese carbonate, ferrous phosphate and lithium carbonate in a ratio of 1:0.6:0.4:1, and prepare a mixed solution with 30 ml of solvent water. Mix the mixed solution evenly with a magnetic stirrer and add it into a polytetrafluoroethylene reactor to perform a hydrothermal reaction at 150° C. for 6 h. After the hydrothermal reaction is completed, perform a first solid-liquid separation and wash it alternately with water and ethanol three times. After drying, a lithium iron manganese phosphate precursor is obtained.

[0172] (2) The obtained lithium manganese iron phosphate precursor was mixed with 0.2 g of a conductive polymer monomer fluorene at a mass ratio of 50:1 to obtain a premixed solution, and 0.05 g of an oxidant hydrogen peroxide was added to the premixed solution to cause the polymer monomer to undergo a polymerization reaction. The reaction temperature was 60° C. and the reaction time was 5 h to obtain a conductive polymer-coated lithium manganese iron phosphate material.

[0173] (3) 5 g of zinc chloride, 2-methylimidazole and methanol were weighed in a mass ratio of 0.5:1:10, stirred evenly and transferred to a reactor, placed in an oven at 150°C for 10 h, and then centrifuged at 800 r for 30 min and dried at 120°C to obtain the MOF material.

[0174] (4) The MOF material weighed at a mass ratio of 0.6:1 and the conductivity was higher than 10Ωcm -1 A total amount of 0.02 g of single-walled carbon nanotubes CNT (FT6000 series carbon nanotube powder of Tiannai Technology) was uniformly mixed and ball-milled for 1 hour. The 0.02 g ball-milled mixture and 10 g of the product in step (2) were ultrasonically dispersed at 0°C for 30 minutes to obtain a uniform suspension. The suspension was then spray-dried at 200°C for 1 hour to obtain modified lithium iron manganese phosphate powder.

[0175] (5) The obtained powder is sintered in a tubular furnace with an inert gas argon at a heating rate of 5°C / min and a sintering temperature of 900°C. The powder is kept warm for 2 hours. After crushing and grinding, a modified lithium manganese iron phosphate material coated with a double layer of a conductive polymer and a carbon layer is obtained.

[0176] Example 3: Preparation of positive electrode material (polymerization reaction time 7h, MOF:CNT=0.8:1)

[0177] This embodiment provides a modified lithium manganese iron phosphate material and a preparation method and application thereof, and the polymerization reaction time is 7h. The present invention performs double-layer coating and modification on the lithium manganese iron phosphate core (particle size ≤ 80nm), and the double-layer coating includes a first coating layer (thickness 2nm) of a conductive polymer and a second coating layer (thickness 0.5nm) of a MOF and CNT composite material, wherein the mass ratio of MOF to CNT is 0.8:1.

[0178] (1) Weigh 10 g of ammonium phosphate, manganese carbonate, ferrous phosphate and lithium carbonate in a mass ratio of 1:0.6:0.4:1, and prepare a mixed solution with 30 ml of solvent water. Mix the mixed solution evenly with a magnetic stirrer and add it to a polytetrafluoroethylene reactor to perform a hydrothermal reaction at 150° C. for 6 h. After the hydrothermal reaction is completed, perform a first solid-liquid separation and wash it alternately with water and ethanol three times. After drying, a lithium manganese iron phosphate precursor is obtained.

[0179] (2) The obtained lithium manganese iron phosphate precursor was mixed with 0.2 g of a conductive polymer monomer fluorene at a mass ratio of 50:1 to obtain a premixed solution, and 0.05 g of an oxidant hydrogen peroxide was added to the premixed solution to cause the polymer monomer to undergo a polymerization reaction. The reaction temperature was 60° C. and the reaction time was 7 h to obtain a conductive polymer-coated lithium manganese iron phosphate material.

[0180] (3) 5 g of zinc chloride, 2-methylimidazole and methanol were weighed in a mass ratio of 0.5:1:10, stirred evenly and transferred to a reactor, placed in an oven at 150°C for 10 h, and then centrifuged at 800 r for 30 min and dried at 120°C to obtain the MOF material.

[0181] (4) The MOF material weighed at a mass ratio of 0.8:1 and the conductivity was higher than 10Ωcm -1 A total amount of 0.02 g of single-walled carbon nanotubes CNT (FT6000 series carbon nanotube powder of Tiannai Technology) was uniformly mixed and ball-milled for 1 hour. The 0.02 g ball-milled mixture and 10 g of the product in step (2) were ultrasonically dispersed at 0°C for 30 minutes to obtain a uniform suspension. The suspension was then spray-dried at 200°C for 1 hour to obtain modified lithium iron manganese phosphate powder.

[0182] (5) The obtained powder is sintered in a tubular furnace with an inert gas argon at a heating rate of 5°C / min and a sintering temperature of 900°C. The powder is kept warm for 2 hours. After crushing and grinding, a modified lithium manganese iron phosphate material coated with a double layer of a conductive polymer and a carbon layer is obtained.

[0183] Comparative Example 1: Preparation of positive electrode material (without coating)

[0184] This comparative example provides a lithium manganese iron phosphate positive electrode material, the particle size of which is ≤80nm and is not coated.

[0185] Weigh a total of 10g of ammonium phosphate, manganese carbonate, ferrous phosphate and lithium carbonate in a mass ratio of 1:0.6:0.4:1, and prepare a mixed solution with 30ml of solvent water. Mix the mixed solution evenly with a magnetic stirrer and add it to a polytetrafluoroethylene reactor for hydrothermal reaction at 150°C for 6h. After the hydrothermal reaction is completed, perform the first solid-liquid separation and wash it alternately with water and ethanol three times. After drying, grind and sieve to obtain a lithium manganese iron phosphate precursor.

[0186] Comparative Example 2: Preparation of positive electrode material (coated with conductive polymer, without carbon layer coating)

[0187] This comparative example provides a method for modifying the inner core of lithium manganese iron phosphate by coating with a conductive polymer, without coating with a carbon layer.

[0188] (1) Weigh 10 g of ammonium phosphate, manganese carbonate, ferrous phosphate and lithium carbonate in a mass ratio of 1:0.6:0.4:1, and prepare a mixed solution with 30 ml of solvent water. Mix the mixed solution evenly with a magnetic stirrer and add it to a polytetrafluoroethylene reactor to perform a hydrothermal reaction at 150° C. for 6 h. After the hydrothermal reaction is completed, perform a first solid-liquid separation and wash it alternately with water and ethanol three times. After drying, grind and sieve to obtain a lithium manganese iron phosphate precursor.

[0189] (2) The obtained lithium manganese iron phosphate precursor was mixed with 0.2 g of a conductive polymer monomer fluorene at a mass ratio of 50:1 to obtain a premixed solution, and 0.05 g of an oxidant hydrogen peroxide was added to the premixed solution to cause the polymer monomer to undergo a polymerization reaction. The reaction temperature was 60° C. and the reaction time was 5 h to obtain a conductive polymer-coated lithium manganese iron phosphate material.

[0190] Comparative Example 3: Preparation of positive electrode material (carbon layer coating, no conductive polymer coating)

[0191] This comparative example provides a method for modifying the inner core lithium manganese iron phosphate by coating it with a conductive carbon layer, wherein the carbon layer is a composite material of MOF and CNT, and is not coated with a conductive polymer.

[0192] (1) Weigh 10 g of ammonium phosphate, manganese carbonate, ferrous phosphate and lithium carbonate in a mass ratio of 1:0.6:0.4:1, and prepare a mixed solution with 30 ml of solvent water. Mix the mixed solution evenly with a magnetic stirrer and add it to a polytetrafluoroethylene reactor to perform a hydrothermal reaction at 150° C. for 6 h. After the hydrothermal reaction is completed, perform a first solid-liquid separation and wash it alternately with water and ethanol three times. After drying, grind and sieve to obtain a lithium manganese iron phosphate precursor.

[0193] (2) 5 g of zinc chloride, 2-methylimidazole and methanol were weighed in a mass ratio of 0.5:1:10, stirred evenly and transferred to a reactor, placed in an oven at 150°C for reaction for 10 h, and then centrifuged at 800 r for 30 min and dried at 120°C to obtain the MOF material.

[0194] (3) The MOF material weighed at a mass ratio of 0.6:1 and the conductivity was higher than 10Ωcm -1 A total amount of 0.02 g of single-walled carbon nanotubes CNT (FT6000 series carbon nanotube powder of Tiannai Technology) was uniformly mixed and ball-milled for 1 hour. The 0.02 g ball-milled mixture and 10 g of the product in step (2) were ultrasonically dispersed at 0°C for 30 minutes to obtain a uniform suspension. The suspension was then spray-dried at 200°C for 1 hour to obtain modified lithium iron manganese phosphate powder.

[0195] (4) The obtained powder is sintered in a tubular furnace filled with inert gas argon at a sintering temperature of 900° C., a heating rate of 5° C. / min, and a heat preservation time of 2 h. After crushing and grinding, a lithium manganese iron phosphate material coated with a conductive carbon layer is obtained.

[0196] Comparative Example 4: Preparation of positive electrode material (polymerization reaction time 10 h, MOF:CNT=1.2:1)

[0197] This comparative example provides a method for modifying the inner core lithium manganese iron phosphate by double-layer coating, including a first coating layer of conductive polymer and a second coating layer of MOF and CNT composite material layer.

[0198] (1) Weigh 10 g of ammonium phosphate, manganese carbonate, ferrous phosphate and lithium carbonate in a mass ratio of 1:0.6:0.4:1, and prepare a mixed solution with 30 ml of solvent water. Mix the mixed solution evenly with a magnetic stirrer and add it to a polytetrafluoroethylene reactor to perform a hydrothermal reaction at 150° C. for 6 h. After the hydrothermal reaction is completed, perform a first solid-liquid separation and wash it alternately with water and ethanol three times. After drying, grind and sieve to obtain a lithium manganese iron phosphate precursor.

[0199] (2) The obtained lithium manganese iron phosphate precursor was mixed with 0.2 g of a conductive polymer monomer fluorene at a mass ratio of 50:1 to obtain a premixed solution, and 0.05 g of an oxidant hydrogen peroxide was added to the premixed solution to cause the polymer monomer to undergo a polymerization reaction. The reaction temperature was 60° C. and the reaction time was 10 h to obtain a conductive polymer-coated lithium manganese iron phosphate material.

[0200] (3) 5 g of zinc chloride, 2-methylimidazole and methanol were weighed in a mass ratio of 0.5:1:10, stirred evenly and transferred to a reactor, placed in an oven at 150°C for 10 h, and then centrifuged at 800 r for 30 min and dried at 120°C to obtain the MOF material.

[0201] (4) The MOF material weighed at a mass ratio of 1.2:1 and the conductivity was higher than 10Ωcm -1 A total amount of 0.02 g of single-walled carbon nanotubes CNT (FT6000 series carbon nanotube powder of Tiannai Technology) was uniformly mixed and ball-milled for 1 hour. The 0.02 g ball-milled mixture and 10 g of the product in step (2) were ultrasonically dispersed at 0°C for 30 minutes to obtain a uniform suspension. The suspension was then spray-dried at 200°C for 1 hour to obtain modified lithium iron manganese phosphate powder.

[0202] (5) The obtained powder is sintered in a tubular furnace with an inert gas argon at a heating rate of 5°C / min and a sintering temperature of 900°C. The powder is kept warm for 2 hours. After crushing and grinding, a modified lithium manganese iron phosphate material coated with a double layer of a conductive polymer and a carbon layer is obtained.

[0203] Example 4: Preparation of battery

[0204] The lithium manganese iron phosphate prepared in Examples 1-3 and Comparative Examples 1-4 were used as positive electrode materials to prepare positive electrode sheets, and button cells were assembled to perform electrochemical performance test and analysis, as follows:

[0205] 1. Preparation of positive electrode sheet

[0206] The positive electrode materials obtained in Examples 1-3 and Comparative Examples 1-4 were dissolved in N-methylpyrrolidone (NMP) with conductive carbon (super-p) and PVDF (polyvinylidene fluoride) at a mass ratio of 8:1:1 and stirred to form a uniform and stable slurry. The slurry was then coated on the surface of the carbon-coated aluminum foil by a doctor blade method with a coating thickness of about 100 μm. The slurry was dried and punched into discs with a diameter of 14 mm using a slicer to obtain positive electrode sheets. The sheets were weighed, sealed and placed in a drying dish for later use.

[0207] 2. Assemble the battery

[0208] The metal lithium sheet was used as the negative electrode sheet, and the prepared positive electrode sheet was assembled with the CR2032 button cell in a glove box filled with high-purity argon atmosphere. The electrolyte was 1M LiPF6, and the solvent was a mixed solvent composed of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 1:1:1. The diaphragm used was a PP diaphragm.

[0209] 3. Performance testing

[0210] 3.1 Discharge capacity

[0211] 0.1C discharge capacity: 0.1C constant current and constant voltage charge to 4.3V, the cut-off current is 0.05C, stand for 30 minutes, then discharge at 0.1C to 2.5V, stand for 30 minutes. The capacity measured in this step is the 0.1C discharge capacity.

[0212] 0.33C discharge capacity: 0.33C constant current and constant voltage charge to 4.3V, the cut-off current is 0.05C, and after standing for 60 minutes, 0.33C discharge to 2.5V and stand for 60 minutes. The capacity measured in this step is the 0.33C discharge capacity (recorded as C0).

[0213] 3.2x rate performance

[0214] Discharge capacity at XC rate: 0.33 constant current constant voltage charge to 4.3V, cut-off current is 0.05C, stand for 60min, XC (X=1, 2, 3, 5) discharge to 2.5V, stand for 60min, obtain the discharge capacity of different rate cells (denoted as CX), the rate performance under XC is specified as CX / C0

[0215] 3.3 Cycle performance

[0216] Charging: 1C constant current and constant voltage charging to 4.3V, cut-off current is 0.05C, and stand for 60min; Discharging: 1C constant current discharge to 2.5V, stand for 30min, and completing one charge and discharge process is considered to complete one cycle. Discharge capacity after 1C operation for the first cycle: 1C constant current and constant voltage charging to 4.3V, cut-off current is 0.05C, 1C constant current discharge to 2.5V, the discharge capacity measured in this step is the discharge capacity of the first cycle of 1C operation (recorded as C3).

[0217] Discharge capacity when 1C runs to the 200th cycle: 1C constant current and constant voltage charging to 4.3V, the cut-off current is 0.05C, and 1C constant current discharge to 2.5V. The discharge capacity measured in this step is the discharge capacity when 1C runs to 200 cycles (recorded as C4).

[0218] After running 200 cycles at 1C, the capacity retention rate is: C4 / C3.

[0219] The assembled CR2032 button cell was subjected to constant current charge / discharge test using the LAND CT2001A battery test system. The charge / discharge voltage window was 2.0 to 4.3 V. The results are shown in Table 1.

[0220] Table 1: Battery performance test results

[0221]

[0222] (1) It can be seen from the results of Examples 1-3 and Comparative Examples 1-2 that the lithium manganese iron phosphate core is coated with a conductive polymer and a carbon layer to obtain a positive electrode material that takes into account both energy density and long cycle performance, and the synergistic effect of the double-layer coating can be used to optimize the long cycle performance of the positive electrode material. In Examples 1-3, the lithium manganese iron is coated with a first layer of conductive polymer and a second layer of carbon layer, which improves the specific capacity and cycle performance of the composite positive electrode material, among which the performance in Example 2 is the best, that is, when the polymerization reaction time in the first coating layer is 5h and the MOF:CNT ratio in the second coating layer is 0.6:1, the discharge capacity at 0.1C reaches 159.8mAh / g, the discharge capacity at 0.33C reaches 157.9mAh / g, the discharge capacity at 3C reaches more than 90% of the 0.33C capacity, and the discharge capacity at 5C can be maintained at more than 85% of the 0.33C capacity. At the same time, the capacity retention rate still reaches 99.3% after 200 cycles.

[0223] (2) It can be seen from the results of Examples 1-3 and Comparative Example 2 that in Examples 1-3, a conductive polymer layer is used to coat the lithium iron manganese phosphate core, and the conductive polymer monomer is subjected to an in-situ polymerization reaction to form a uniform coating layer on the surface of the lithium iron manganese phosphate. The lithium iron manganese phosphate positive electrode material having the coating layer can effectively inhibit the side reaction between the positive electrode material itself and the electrolyte, inhibit the dissolution of transition metals from the positive electrode material, and improve the structural stability of the positive electrode material, thereby improving the cycle performance and safety of the battery using the positive electrode material. In addition, since the conductive polymer itself is conductive and can contribute to capacity, it can ensure the conductivity of the electrode material and is conducive to improving the rate performance of the battery.

[0224] (3) It can be seen from the results of Examples 1-3 and Comparative Example 3 that in Examples 1-3, MOF and CNT composite carbon layers are used to coat the lithium manganese iron phosphate core. The present invention prepares a lithium manganese iron phosphate modified material coated with a porous carbon thin layer by preparing a MOF and CNT composite material material and cooperating with a lithium manganese iron phosphate precursor. The material effectively reduces the resistivity of the positive electrode material through a thin layer of carbon coating, improves the conductivity, and prevents the particles from growing and agglomerating, thereby maintaining the nanostructure of the particles, effectively reducing the diffusion distance of Li+ inside the active particles, and making the material have more outstanding rate performance. At the same time, it can also effectively inhibit the precipitation of manganese ions, improve electrochemical performance, and increase cycle life. At the same time, the lithium manganese iron phosphate precursor material is mixed with a carbon source and ground and then calcined. The MOF-derived carbon source will dissolve first at high temperature. The dissolved material has good fluidity and can be evenly infiltrated and coated on the surface of the material to form a thin layer of carbon coating.

[0225] (4) It can be seen from the results of Examples 1-3 and Comparative Example 4 that when the polymerization reaction time of the conductive polymer layer and the ratio of MOF to CNT in the carbon layer are not within the preferred range provided by the present invention, it is not conducive to obtaining a positive electrode material that takes into account both energy density and long cycle performance.

[0226] It should be understood that the above embodiments are exemplary and are not intended to include all possible implementations included in the claims. Various modifications and changes may be made on the basis of the above embodiments without departing from the scope of the present disclosure. Similarly, the various technical features of the above embodiments may be arbitrarily combined to form other embodiments of the present invention that may not be explicitly described. Therefore, the above embodiments only express several implementations of the present invention and do not limit the scope of protection of the patent of the present invention.

Claims

1. A positive electrode material, characterized in that: The positive electrode material comprises: a lithium manganese iron phosphate core and a conductive polymer layer and a carbon coating layer sequentially coated on the surface of the core, and the carbon coating layer comprises: a metal organic framework (MOF) and a carbon nanotube (CNT).

2. The positive electrode material according to claim 1, characterized in that The particle size of the lithium iron manganese phosphate core is less than 80nm; And / or, the thickness of the conductive polymer layer is 2 to 5 nm; And / or, the carbon coating layer has a thickness of 0.1 to 2 nm.

3. The positive electrode material according to claim 1, characterized in that The mass ratio of the MOF to the CNT is (0.01-1):

1.

4. A method for preparing the positive electrode material according to any one of claims 1 to 3, characterized in that: The preparation method comprises the following steps: (1) subjecting a reaction solution containing a phosphorus source, a manganese source, an iron source and a lithium source to a hydrothermal reaction to obtain a lithium iron manganese phosphate precursor; (2) mixing the lithium manganese iron phosphate precursor obtained in step (1) with a conductive polymer monomer to obtain a premixed solution; (3) adding an oxidant to the premixed solution obtained in step (2) to carry out a polymerization reaction to obtain lithium manganese iron phosphate coated with a conductive polymer; (4) mixing a metal salt, an organic ligand and a solvent to react to obtain a MOF, and mixing the MOF with CNTs and grinding them to obtain a MOF-CNT mixture; (5) Mixing the conductive polymer-coated lithium manganese iron phosphate obtained in step (3) with the MOF-CNT mixture obtained in step (4), drying to obtain a powder, and sintering the powder under a protective atmosphere to obtain the positive electrode material.

5. The preparation method according to claim 4, characterized in that: In step (1), the mass ratio of the phosphorus source, manganese source, iron source and lithium source is (1-1.1):(0.6-0.9):(0.1-0.4):1, the solvent of the reaction solution is water, the temperature of the hydrothermal reaction is 100-150° C., and the time is 6-10 h; Preferably, the phosphorus source is one or more of diammonium hydrogen phosphate, ammonium phosphate, phosphorus pentoxide, phosphoric acid, and phosphorous acid; the manganese source is one or more of manganese carbonate, manganese phosphate, manganous phosphate, manganese sulfate, manganese oxalate, manganese acetate, manganese chloride, manganese trioxide, and manganese tetraoxide; the iron source is one or more of ferrous phosphate, ferrous oxalate, ferrous chloride, ferrous nitrate, ferrous oxide, ferrous sulfate, ferric chloride, ferric nitrate, ferrous sulfate, ferric oxide, ferric oxide, and ferric phosphate; and the lithium source is one or more of lithium carbonate, lithium bicarbonate, lithium acetate, lithium chloride, lithium bromide, lithium hydroxide, lithium phosphate, dilithium hydrogen phosphate, lithium dihydrogen phosphate, lithium oxalate, and lithium sulfate.

6. The preparation method according to claim 4, characterized in that: In step (2), the mass ratio of the lithium manganese iron phosphate precursor to the conductive polymer monomer is 50:1 to 200:1, and the conductive polymer monomer is selected from one or more of fluorene, carbazole and their derivatives.

7. The preparation method according to claim 4, characterized in that: In step (3), the polymerization reaction temperature is 20-60° C., the reaction time is 2-8 hours, and the oxidant is one or more of ammonium persulfate, ferric chloride, potassium permanganate, potassium dichromate, and hydrogen peroxide.

8. The preparation method according to claim 4, characterized in that: In step (4), the mass ratio of the metal salt, the organic ligand, and the solvent is (0.5-2):(1-1.1):(10-15), the reaction temperature is 90-150° C., the reaction time is 8-12 h, and the mass ratio of the MOF to the CNT is (0.01-1):1; And / or, the metal salt is selected from one or more of zinc chloride, zinc sulfate, and zinc nitrate, the organic ligand is selected from one or more of 2-methylimidazole, trimesic acid, phthalic acid, and thiophene-2.5-dicarboxylic acid, the solvent is selected from one or more of methanol, acetone, and anhydrous ethanol, and the CNT has an electrical conductivity higher than 10Ωcm -1 of single-walled carbon nanotubes.

9. The preparation method according to claim 4, characterized in that: In step (5), the mass ratio of the conductive polymer-coated lithium manganese iron phosphate to the MOF-CNT mixture is 500:1 to 2000:1; And / or, the drying temperature is 200-300° C., the sintering temperature is 600-900° C., the heating rate is 2-5° C. / min, and the protective atmosphere is an inert gas.

10. A lithium ion battery, characterized in that: The lithium-ion battery comprises the positive electrode material according to any one of claims 1 to 3, or the positive electrode material prepared by the preparation method according to any one of claims 4 to 9.

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