A lithium iron manganese phosphate positive electrode material and preparation method thereof

Abstract

The present invention provides a lithium iron phosphate (LFP) cathode material and a preparation method thereof. A metal salt and an organic carbon source are uniformly coated on the surface of the LFP material using a liquid phase method, and carbonized at high temperature to form a metal / carbon composite coating on the surface of the LFP material. Organic small molecules bearing Lewis acid / base groups are then modified on the surfaces of LFP particles of varying particle sizes through coordination bonds, and after drying, surface-modified LFP particles are obtained. Finally, the LFP particles of varying particle sizes are mixed in proportion to obtain a LFP cathode material having an appropriate particle size and uniform distribution. The preparation process of the present invention is simple, and a uniform distribution of large and small particles can be achieved without the need for multiple sintering steps. The LFP cathode material obtained by the present invention can optimize filling performance and significantly increase the compaction density of the cathode material.

Description

Technical Field

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

[0002] Lithium iron phosphate (LFP) has a similar theoretical capacity of 170 mAh / g, similar to LFP. However, due to its operating voltage of 4.1V, its theoretical energy density approaches 700 Wh / kg, a 15-20% increase over LFP. This higher energy density makes LFP cathode material potentially capable of exceeding the current upper limit of battery range. Furthermore, LFP material costs approximately 28% less than LFP. Considering LFP's low-temperature performance, safety, and cycling performance, its use as a positive electrode material for power batteries has become a growing trend.

[0003] Lithium manganese iron phosphate (LMP) materials have an olivine structure that maintains structural stability during the insertion and extraction of lithium ions. However, LMP materials suffer from drawbacks such as easy manganese ion dissolution, poor conductivity, and low compaction density, which significantly limit their practical electrochemical performance. Currently available LMP materials generally have a compaction density of 2.1-2.2 g / cm³ and a gram capacity of 135-150 mAh / g, which makes them unable to meet the demand for high-energy-density power batteries. Summary of the Invention

[0004] In order to solve the problems described in the background technology, the present invention provides a lithium manganese iron phosphate positive electrode material and a preparation method thereof.

[0005] The lithium iron phosphate positive electrode material of the present invention has a metal / carbon composite coating layer on its surface; the positive electrode material includes a small-particle lithium iron phosphate material modified with an organic small molecule having a Lewis acid group and a large-particle lithium iron phosphate material modified with an organic small molecule having a Lewis base group; the particle size of the small-particle lithium iron phosphate material is 0.4-3 μm, and the particle size of the large-particle lithium iron phosphate material is 5-15 μm.

[0006] The preparation method of the present invention comprises the following steps:

[0007] Step 1: uniformly coating the surface of the lithium manganese iron phosphate material with a metal inorganic salt and an organic carbon source by a liquid phase method, and carbonizing the surface at a high temperature to obtain lithium manganese iron phosphate material particles with a metal / carbon composite coating layer;

[0008] Step 2: dividing the lithium manganese iron phosphate material particles of the metal / carbon composite coating layer into small-particle lithium manganese iron phosphate material and large-particle lithium manganese iron phosphate material;

[0009] Step 3: mixing the organic small molecule solution with Lewis acid groups with the small-particle lithium iron manganese phosphate material and drying the mixture to obtain a small-particle mixed material; mixing the organic small molecule solution with Lewis base groups with the large-particle lithium iron manganese phosphate material and drying the mixture to obtain a large-particle mixed material;

[0010] Step 4: Mix the small-particle mixed material and the large-particle mixed material in a certain proportion and stir them evenly to obtain a lithium manganese iron phosphate positive electrode material.

[0011] Furthermore, in step 1, the metal inorganic salt includes one of zirconium oxychloride, aluminum chloride, magnesium nitrate, cobalt nitrate, nickel nitrate or zinc acetate.

[0012] Furthermore, in step 1, the organic carbon source includes one of ethylenediaminetetraacetic acid, aminotriacetic acid, diethylenetriaminepentaacetic acid, citric acid, tartaric acid, gluconic acid, hydroxyethylethylenediaminetriacetic acid or dihydroxyethylglycine.

[0013] Furthermore, in the step one, the metal inorganic salt and the organic carbon source are first weighed, an appropriate amount of deionized water is added and stirred to mix and react fully, and then the lithium iron manganese phosphate material is added and stirred to mix evenly to obtain a mixed solution, and the mixture obtained after the mixed solution is dried is pyrolyzed and carbon-coated under a nitrogen atmosphere to obtain lithium iron manganese phosphate material particles with a metal / carbon composite coating layer; wherein the mass ratio of the metal inorganic salt, the organic carbon source and the lithium iron manganese phosphate material is (1-100): (1-200): 1000.

[0014] Furthermore, in step 1, the mixing temperature of the metal inorganic salt, the organic carbon source and the deionized water is 25-30°C; the drying temperature of the mixed solution is 80-100°C; the temperature of the pyrolytic carbon coating is 450-500°C, and the time is 18-24 hours.

[0015] Furthermore, in step three, the organic small molecule solution with a Lewis acid group is a mixture of an amino small molecule and anhydrous ethanol in a mass ratio of (1-100):1000, and the amino small molecule includes one of ethylenediamine, dodecanediamine, 1,11-undecanediamine, dodecaethylene glycol diamine, and 1,18-octadecanediamine.

[0016] Furthermore, in the step three, the organic small molecule solution with a Lewis base group is a mixture of a carboxyl small molecule and anhydrous ethanol in a mass ratio of (1-100):1000, and the carboxyl small molecule includes one of ethylenediaminetetraacetic acid, trans-1,2-cyclohexanediaminetetraacetic acid, diethylenetriaminepentaacetic acid, ethylene glycol bis(2-aminoethyl ether)tetraacetic acid, and N-(2-hydroxy)ethylenediamine-N,N',N'-triacetic acid.

[0017] Furthermore, in the step three, the mixing conditions of the organic small molecule solution with Lewis acid groups and the small particle lithium manganese iron phosphate material and the organic small molecule solution with Lewis base groups and the large particle lithium manganese iron phosphate material are: weight ratio (1-100): 1000, mixing temperature 30-50°C, mixing stirring rate 2500-3000r / s, mixing stirring time 1-10h, and drying temperature 80-100°C.

[0018] Furthermore, in the step 4, the mixing weight ratio of the small particle mixed material and the large particle mixed material is (3-6):(4-7), and the mixing temperature is 30-35°C.

[0019] Compared with the prior art, the present invention utilizes a liquid phase method to uniformly coat metal salts and organic carbon sources on the surface of a lithium iron manganese phosphate material, and carbonizes the material at high temperature to form a metal / carbon composite coating layer on the surface of the lithium iron manganese phosphate material; then, organic small molecules with Lewis acid / base groups are modified on the surfaces of lithium iron manganese phosphate material particles of different particle sizes through coordination bonds, and surface-modified lithium iron manganese phosphate particle materials are obtained after drying; finally, lithium iron manganese phosphate material particles of different particle sizes are mixed in proportion, and after the proportionate mixing, the large and small particles of the lithium iron manganese phosphate material can self-assemble through Lewis acid-base interaction, thereby obtaining a lithium iron manganese phosphate positive electrode material with appropriate particle size and uniform distribution. In the present invention, lithium manganese iron phosphate material particles of different particle sizes can self-assemble through the interaction between surface-modified Lewis acid / base groups, inducing regular arrangement during the mixing process, thereby obtaining a lithium manganese iron phosphate positive electrode material with uniform distribution of large and small particles. The preparation process is simple and can achieve uniform distribution of large and small particles without the need for multi-step sintering. The lithium manganese iron phosphate positive electrode material prepared by the present invention can optimize filling performance and greatly improve the compaction density of the positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 This is a SEM photograph of the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0021] In order to make the technical problems, technical solutions and beneficial effects to be solved by this application more clearly understood, this application is further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are only used to explain this application and are not intended to limit this application.

[0022] In the lithium manganese iron phosphate positive electrode material of the present invention, the surface of the lithium manganese iron phosphate material has a metal / carbon composite coating layer; the positive electrode material includes a small-particle lithium manganese iron phosphate material modified with an organic small molecule having a Lewis acid group and a large-particle lithium manganese iron phosphate material modified with an organic small molecule having a Lewis base group; the particle size of the small-particle lithium manganese iron phosphate material is 0.4-3 μm, and the particle size of the large-particle lithium manganese iron phosphate material is 5-15 μm.

[0023] The preparation method of the lithium manganese iron phosphate positive electrode material of the present invention is as follows:

[0024] Step 1: uniformly coat the metal inorganic salt and the organic carbon source on the surface of the lithium manganese iron phosphate material by a liquid phase method, and carbonize it at a high temperature to obtain a lithium manganese iron phosphate material with a metal / carbon composite coating layer.

[0025] Specifically, the metal inorganic salt includes one of zirconium oxychloride, aluminum chloride, magnesium nitrate, cobalt nitrate, nickel nitrate or zinc acetate, and the organic carbon source includes one of ethylenediaminetetraacetic acid, aminotriacetic acid, diethylenetriaminepentaacetic acid, citric acid, tartaric acid, gluconic acid, hydroxyethylethylenediaminetriacetic acid or dihydroxyethylglycine.

[0026] More specifically, first weigh the above-mentioned metal inorganic salt, organic carbon source and lithium iron manganese phosphate positive electrode material in a mass ratio of (1-100): (1-200): 1000; then add an appropriate amount of deionized water to the metal inorganic salt and the organic carbon source, stir at 25-30°C for 1-3h, and the stirring rate is 1500-2500r / s. After the metal inorganic salt and the organic carbon source are fully reacted, the lithium iron manganese phosphate positive electrode material is added, and stirred at 25-30°C for 3-5h, and the stirring rate is 3000-4500r / s to obtain a mixed solution; the mixed solution is placed in a drying oven at 80-100°C to dry to obtain a mixture; the mixture is heated to 450-500°C under nitrogen atmosphere and maintained for 18-24h for pyrolysis carbon coating to obtain lithium iron manganese phosphate material particles with a metal / carbon composite coating layer.

[0027] Step 2: The lithium manganese iron phosphate material particles of the metal / carbon composite coating layer are divided into small-particle lithium manganese iron phosphate material and large-particle lithium manganese iron phosphate material; wherein the particle size of the small-particle lithium manganese iron phosphate material is 0.4-3 μm, and the particle size of the large-particle lithium manganese iron phosphate material is 5-15 μm.

[0028] Step 3: Mix the organic small molecule solution with Lewis acid groups with the small-particle lithium manganese iron phosphate material and dry it to obtain a small-particle mixed material; mix the organic small molecule solution with Lewis base groups with the large-particle lithium manganese iron phosphate material and dry it to obtain a large-particle mixed material. Specifically, the organic small molecule solution with a Lewis acid group is a mixture of an amino small molecule and anhydrous ethanol in a mass ratio of (1-100):1000, and the amino small molecule includes one of ethylenediamine, dodecanediamine, 1,11-undecanediamine, dodecaethylene glycol diamine, and 1,18-octadecanediamine; the organic small molecule solution with a Lewis base group is a mixture of a carboxyl small molecule and anhydrous ethanol in a mass ratio of (1-100):1000, and the carboxyl small molecule includes one of ethylenediaminetetraacetic acid, trans-1,2-cyclohexanediaminetetraacetic acid, diethylenetriaminepentaacetic acid, ethylene glycol bis(2-aminoethyl ether)tetraacetic acid, and N-(2-hydroxy)ethylenediamine-N,N',N'-triacetic acid.

[0029] More specifically, first, amino small molecules, carboxyl small molecules and corresponding weights of anhydrous ethanol are weighed according to the above mass ratio; corresponding weights of anhydrous ethanol are added to the amino small molecules and carboxyl small molecules, respectively, and stirred at 25-30° C. for 1-2 hours at a stirring rate of 1000-2000 r / s to obtain an organic small molecule solution with a Lewis acid group and an organic small molecule solution with a Lewis base group; then the organic small molecule solution with a Lewis acid group and the small particle lithium manganese iron phosphate material are mixed in a weight ratio of (1-100): 1000 The mixture is stirred at 30-50°C for 5-10 hours at a stirring rate of 2500-3000 r / s, and after mixing evenly, it is placed in a drying oven at 80-100°C for drying to obtain a small-particle mixed material; the organic small molecule solution with a Lewis base group is mixed with the large-particle lithium manganese iron phosphate material in a weight ratio of (1-100): 1000, and the mixture is stirred at 30-50°C for 5-10 hours at a stirring rate of 2500-3000 r / s. After mixing evenly, it is placed in a drying oven at 80-100°C for drying to obtain a small-particle mixed material.

[0030] Step 4: Mix the small particle mixed material and the large particle mixed material in a weight ratio of (3-6): (4-7), stir at 30-35° C. for 1-3 h at a stirring rate of 2000-3000 r / s to obtain a lithium manganese iron phosphate positive electrode material.

[0031] The present invention is further described below in conjunction with specific embodiments. It must be noted that only a part of the various materials in the preparation method are taken in the specific embodiments, and not all materials in the preparation method are listed. The specific embodiments are only used to explain the present invention more clearly and further reflect the beneficial effects achieved by the present invention.

[0032] Example 1

[0033] The metal inorganic salt zirconium oxychloride, the organic carbon source ethylenediaminetetraacetic acid and the lithium iron manganese phosphate positive electrode material are weighed in a mass ratio of 80:150:1000; an appropriate amount of deionized water is added to the zirconium oxychloride and ethylenediaminetetraacetic acid, respectively, and stirred at 25°C for 2 hours at a stirring rate of 2000r / s; after the metal inorganic salt and the organic carbon source are fully reacted, the lithium iron manganese phosphate positive electrode material is added, and then the lithium iron manganese phosphate positive electrode material is added, and stirred at 25°C for 4 hours at a stirring rate of 4000r / s to obtain a mixed solution; the mixed solution is placed in a 90°C drying oven and dried to obtain a mixture; the mixture is heated to 480°C under nitrogen atmosphere protection and maintained for 20 hours for pyrolytic carbon coating to obtain lithium iron manganese phosphate material particles with a metal / carbon composite coating layer.

[0034] The lithium manganese iron phosphate material particles of the metal / carbon composite coating layer are divided into small-particle lithium manganese iron phosphate material and large-particle lithium manganese iron phosphate material, wherein the particle size of the small-particle lithium manganese iron phosphate material is 0.4-3μm, and the particle size of the large-particle lithium manganese iron phosphate material is 5-15μm.

[0035] Weigh amino small molecule ethylenediamine and carboxyl small molecule ethylenediaminetetraacetic acid, add appropriate amount of anhydrous ethanol respectively, wherein the mass ratio of amino small molecule to anhydrous ethanol is 50:1000, and the mass ratio of carboxyl small molecule to anhydrous ethanol is 50:1000; stir at 25°C for 2h at a stirring rate of 1500r / s to obtain an organic small molecule solution with Lewis acid and an organic small molecule solution with Lewis base group.

[0036] Then, the organic small molecule solution with Lewis acid and the small-particle lithium manganese iron phosphate material were mixed and stirred at a weight ratio of 50:1000 at 40°C for 8 hours at a stirring rate of 2500r / s. After mixing evenly, the mixture was placed in a 90°C drying oven to dry and obtain a small-particle mixed material. The organic small molecule solution with Lewis base groups and the large-particle lithium manganese iron phosphate material were mixed and stirred at a weight ratio of 50:1000 at 40°C for 8 hours at a stirring rate of 2500r / s. After mixing evenly, the mixture was placed in a 90°C drying oven to dry and obtain a small-particle mixed material.

[0037] The small-particle mixed material and the large-particle mixed material were mixed in a weight ratio of 3:7, and stirred at 35° C. for 2 h at a stirring rate of 2000 r / s to obtain a lithium manganese iron phosphate positive electrode material.

[0038] Example 2

[0039] The metal inorganic salt aluminum chloride, the organic carbon source aminotriacetic acid and the lithium iron manganese phosphate positive electrode material were weighed in a mass ratio of 80:150:1000; appropriate amounts of deionized water were added to the aluminum chloride and aminotriacetic acid, respectively, and stirred at 30°C for 1 hour at a stirring rate of 1500r / s; after the metal inorganic salt and the organic carbon source were fully reacted, the lithium iron manganese phosphate positive electrode material was added, and stirred at 30°C for 3 hours at a stirring rate of 4500r / s to obtain a mixed solution; the mixed solution was placed in a 100°C drying oven to dry to obtain a mixture; the mixture was heated to 500°C under nitrogen atmosphere protection and maintained for 18 hours for pyrolytic carbon coating to obtain lithium iron manganese phosphate material particles with a metal / carbon composite coating layer.

[0040] The lithium manganese iron phosphate material particles of the metal / carbon composite coating layer are divided into small-particle lithium manganese iron phosphate material and large-particle lithium manganese iron phosphate material, wherein the particle size of the small-particle lithium manganese iron phosphate material is 0.4-3μm, and the particle size of the large-particle lithium manganese iron phosphate material is 5-15μm.

[0041] Weigh the amino small molecule dodecanediamine and the carboxyl small molecule trans-1,2-cyclohexanediaminetetraacetic acid, and add appropriate amounts of anhydrous ethanol respectively, wherein the mass ratio of the amino small molecule to anhydrous ethanol is 50:1000, and the mass ratio of the carboxyl small molecule to anhydrous ethanol is 50:1000; stir at 30°C for 1h at a stirring rate of 1000r / s to obtain an organic small molecule solution with a Lewis acid group and an organic small molecule solution with a Lewis base group.

[0042] Then, the organic small molecule solution with Lewis acid and the small-particle lithium manganese iron phosphate material were mixed and stirred at a weight ratio of 50:1000 at 50°C for 5 hours at a stirring rate of 3000r / s. After mixing evenly, the mixture was placed in a 100°C drying oven for drying to obtain a small-particle mixed material. The organic small molecule solution with Lewis base groups and the large-particle lithium manganese iron phosphate material were mixed and stirred at a weight ratio of 50:1000 at 50°C for 5 hours at a stirring rate of 2800r / s. After mixing evenly, the mixture was placed in a 100°C drying oven for drying to obtain a small-particle mixed material.

[0043] The small-particle mixed material and the large-particle mixed material were mixed in a weight ratio of 4:6, and stirred at 30° C. for 3 h at a stirring rate of 3000 r / s to obtain a lithium manganese iron phosphate positive electrode material.

[0044] Example 3

[0045] The metal inorganic salt magnesium nitrate, the organic carbon source aminodiethylenetriaminepentaacetic acid and the lithium iron manganese phosphate positive electrode material were weighed in a mass ratio of 80:150:1000; appropriate amounts of deionized water were added to the magnesium nitrate and diethylenetriaminepentaacetic acid, respectively, and stirred at 28°C for 3 hours at a stirring rate of 2500r / s; after the metal inorganic salt and the organic carbon source were fully reacted, the lithium iron manganese phosphate positive electrode material was added, and stirred at 28°C for 5 hours at a stirring rate of 3000r / s to obtain a mixed solution; the mixed solution was placed in an 800°C drying oven and dried to obtain a mixture; the mixture was heated to 450°C under nitrogen atmosphere protection and maintained for 24 hours for pyrolytic carbon coating to obtain lithium iron manganese phosphate material particles with a metal / carbon composite coating layer.

[0046] The lithium manganese iron phosphate material particles of the metal / carbon composite coating layer are divided into small-particle lithium manganese iron phosphate material and large-particle lithium manganese iron phosphate material, wherein the particle size of the small-particle lithium manganese iron phosphate material is 0.4-3μm, and the particle size of the large-particle lithium manganese iron phosphate material is 5-15μm.

[0047] Weigh the amino small molecule 1,11-undecanediamine and the carboxyl small molecule diethylenetriaminepentaacetic acid, and add appropriate amounts of anhydrous ethanol respectively, wherein the mass ratio of the amino small molecule to anhydrous ethanol is 50:1000, and the mass ratio of the carboxyl small molecule to anhydrous ethanol is 50:1000; stir at 28°C for 1.5h at a stirring rate of 2000r / s to obtain an organic small molecule solution with a Lewis acid group and an organic small molecule solution with a Lewis base group.

[0048] Then, the organic small molecule solution with Lewis acid and the small-particle lithium manganese iron phosphate material were mixed and stirred at a weight ratio of 50:1000 at 30°C for 10 hours at a stirring rate of 2800 r / s. After the mixture was evenly mixed, the mixture was placed in an 80°C drying oven to dry and obtain a small-particle mixed material. The organic small molecule solution with Lewis base groups and the large-particle lithium manganese iron phosphate material were mixed and stirred at a weight ratio of 50:1000 at 30°C for 10 hours at a stirring rate of 2800 r / s. After the mixture was evenly mixed, the mixture was placed in an 80°C drying oven to dry and obtain a small-particle mixed material.

[0049] The small-particle mixed material and the large-particle mixed material were mixed in a weight ratio of 5:5, and stirred at 32° C. for 1 h at a stirring rate of 2500 r / s to obtain a lithium manganese iron phosphate positive electrode material.

[0050] Comparative Example 1

[0051] A metal inorganic salt and an organic carbon source are uniformly coated on the surface of a lithium iron manganese phosphate material by a liquid phase method, and carbonized at high temperature to obtain a lithium iron manganese phosphate material with a metal / carbon composite coating layer. The metal inorganic salt includes one of zirconium oxychloride, aluminum chloride, magnesium nitrate, cobalt nitrate, nickel nitrate, or zinc acetate, and the organic carbon source includes one of ethylenediaminetetraacetic acid, aminotriacetic acid, diethylenetriaminepentaacetic acid, citric acid, tartaric acid, gluconic acid, hydroxyethylethylenediaminetriacetic acid, or dihydroxyethylglycine.

[0052] More specifically, the above-mentioned metal inorganic salt, organic carbon source and lithium iron manganese phosphate positive electrode material are weighed in a mass ratio of 80:150:1000; the metal inorganic salt taken in this comparative example is zirconium oxychloride, and the organic carbon source is ethylenediaminetetraacetic acid. An appropriate amount of deionized water is added to the metal inorganic salt and the organic carbon source, and the mixture is stirred at 28°C for 3 hours at a stirring rate of 2500r / s. After the metal inorganic salt and the organic carbon source are fully reacted, the lithium iron manganese phosphate positive electrode material is added, and the mixture is stirred at 28°C for 4 hours at a stirring rate of 4000r / s to obtain a mixed solution; the mixed solution is placed in a drying oven at 90°C and dried to obtain a mixture; the mixture is heated to 450°C under nitrogen atmosphere and maintained for 24 hours for pyrolysis carbon coating to obtain lithium iron manganese phosphate material particles with a metal / carbon composite coating layer.

[0053] The lithium manganese iron phosphate material particles with the metal / carbon composite coating layer were stirred at 32° C. for 2 h at a stirring rate of 2500 r / s to obtain a lithium manganese iron phosphate positive electrode material.

[0054] Comparative Example 2

[0055] The difference from Comparative Example 1 is that the lithium manganese iron phosphate material particles with a metal / carbon composite coating layer are mixed with an organic small molecule solution with a Lewis acid, wherein the organic small molecule with a Lewis acid is an amino small molecule ethylenediamine, in a weight ratio of 1000:50 at 40°C for 8 hours, with a stirring rate of 2500r / s. After mixing evenly, the mixture is placed in a drying oven at 90°C and dried to obtain a lithium manganese iron phosphate positive electrode material.

[0056] Comparative Example 3

[0057] The difference from Comparative Example 1 is that the lithium manganese iron phosphate material particles with a metal / carbon composite coating layer are mixed with an organic small molecule solution with a Lewis base, wherein the organic small molecule with a Lewis base is a carboxyl small molecule ethylenediaminetetraacetic acid, in a weight ratio of 1000:50, and stirred at 40°C for 8 hours with a stirring rate of 2500r / s. After mixing evenly, the mixture is placed in a drying oven at 90°C and dried to obtain a lithium manganese iron phosphate positive electrode material.

[0058] The SEM photo of the lithium manganese iron phosphate positive electrode material prepared in Example 1 of the present invention is as follows: Figure 1As shown, the particles are evenly distributed in different sizes.

[0059] The lithium iron manganese phosphate cathode materials obtained in the above examples and comparative examples were used as the cathode active material to assemble button cells according to the following method: the cathode active material, polyvinylidene fluoride (PVDF), and superconducting carbon black (SP) were mixed in a mass ratio of 90:5:5 and stirred in NMP. The slurry was evenly coated on aluminum foil and dried in a vacuum oven at 80°C for 2 hours. A circular electrode piece with a diameter of 14 mm was cut using a punch to serve as the positive working electrode. R2032 button cells were assembled in a glove box according to a specific assembly process, using a lithium metal sheet as the counter electrode, a Celgard 2400 porous polypropylene (PP) film as the separator, and a 1M lithium hexafluorophosphate (LiPF6) solution as the electrolyte. After assembly, the cells were allowed to stand for 3 hours to allow the electrolyte and electrode materials to fully penetrate.

[0060] The electrode compaction density, 0.1C discharge capacity, and volume energy density of the assembled button battery were tested. The test results are shown in Table 1. It can be seen that the lithium manganese iron phosphate positive electrode material prepared by the present invention is used as the active material of the positive electrode material. Not only is the electrode compaction density significantly higher than that of the comparative example, the 0.1C discharge capacity is equivalent to or slightly higher than that of the comparative example, and its volume energy density is also significantly higher than that of the comparative example.

[0061] Table 1 Summary of button battery test results

[0062]

[0063] The above-described embodiments are only used to illustrate the technical solutions of the present application, rather than to limit them. Although the present application has been described in detail with reference to the aforementioned embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the aforementioned embodiments, or make equivalent replacements for some of the technical features therein. These modifications or replacements do not deviate the essence of the corresponding technical solutions from the spirit and scope of the technical solutions of the various embodiments of the present application, and should all be included in the scope of protection of the present application.

Claims

1. A lithium manganese iron phosphate positive electrode material, characterized in that: The surface of the lithium iron manganese phosphate positive electrode material has a metal / carbon composite coating layer; the positive electrode material includes a small-particle lithium iron manganese phosphate material modified with an organic small molecule having a Lewis acid group and a large-particle lithium iron manganese phosphate material modified with an organic small molecule having a Lewis base group; the particle size of the small-particle lithium iron manganese phosphate material is 0.4-3 μm, and the particle size of the large-particle lithium iron manganese phosphate material is 5-15 μm; The surface of a lithium iron phosphate material is uniformly coated with a metal inorganic salt and an organic carbon source by a liquid phase method, and carbonized at high temperature to obtain lithium iron phosphate material particles with a metal / carbon composite coating layer; the lithium iron phosphate material particles with the metal / carbon composite coating layer are divided into small-particle lithium iron phosphate material and large-particle lithium iron phosphate material; an organic small molecule solution with a Lewis acid group is mixed with the small-particle lithium iron phosphate material, and then dried to obtain a small-particle mixed material; an organic small molecule solution with a Lewis base group is mixed with the large-particle lithium iron phosphate material, and then dried to obtain a large-particle mixed material; the small-particle mixed material and the large-particle mixed material are mixed and stirred uniformly in a certain proportion to obtain a lithium iron phosphate positive electrode material; The mixing weight ratio of the small particle mixed material and the large particle mixed material is (3-6):(4-7).

2. A method for preparing the lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The following steps are involved: Step 1: uniformly coating the surface of the lithium manganese iron phosphate material with a metal inorganic salt and an organic carbon source by a liquid phase method, and carbonizing the surface at a high temperature to obtain lithium manganese iron phosphate material particles with a metal / carbon composite coating layer; Step 2: dividing the lithium manganese iron phosphate material particles of the metal / carbon composite coating layer into small-particle lithium manganese iron phosphate material and large-particle lithium manganese iron phosphate material; Step 3: mixing the organic small molecule solution with Lewis acid groups with the small-particle lithium iron manganese phosphate material and drying the mixture to obtain a small-particle mixed material; mixing the organic small molecule solution with Lewis base groups with the large-particle lithium iron manganese phosphate material and drying the mixture to obtain a large-particle mixed material; Step 4: Mix the small-particle mixed material and the large-particle mixed material in a certain proportion and stir them evenly to obtain a lithium manganese iron phosphate positive electrode material.

3. The method for preparing a lithium manganese iron phosphate positive electrode material according to claim 2, wherein: In the step 1, the metal inorganic salt includes one of zirconium oxychloride, aluminum chloride, magnesium nitrate, cobalt nitrate, nickel nitrate or zinc acetate.

4. The method for preparing a lithium iron manganese phosphate positive electrode material according to claim 3, characterized in that: In the step 1, the organic carbon source includes one of ethylenediaminetetraacetic acid, aminotriacetic acid, diethylenetriaminepentaacetic acid, citric acid, tartaric acid, gluconic acid, hydroxyethylethylenediaminetriacetic acid or dihydroxyethylglycine.

5. The method for preparing a lithium iron manganese phosphate positive electrode material according to claim 4, characterized in that: In the step 1, a metal inorganic salt and an organic carbon source are first weighed, an appropriate amount of deionized water is added, stirred and mixed for sufficient reaction, and then a lithium iron manganese phosphate material is added and stirred and mixed evenly to obtain a mixed solution. The mixed solution is dried and the obtained mixture is pyrolyzed and carbon-coated under a nitrogen atmosphere to obtain lithium iron manganese phosphate material particles with a metal / carbon composite coating layer; wherein the mass ratio of the metal inorganic salt, the organic carbon source and the lithium iron manganese phosphate material is (1-100): (1-200): 1000.

6. The method for preparing a lithium iron manganese phosphate positive electrode material according to claim 5, characterized in that: In the step 1, the mixing temperature of the metal inorganic salt, the organic carbon source and the deionized water is 25-30° C.; the drying temperature of the mixed solution is 80-100° C.; the temperature of the pyrolytic carbon coating is 450-500° C., and the time is 18-24 hours.

7. The method for preparing a lithium iron manganese phosphate cathode material according to any one of claims 2 to 6, characterized in that: In the step 3, the organic small molecule solution with a Lewis acid group is a mixture of an amino small molecule and anhydrous ethanol in a mass ratio of (1-100):1000, and the amino small molecule includes one of ethylenediamine, dodecanediamine, 1,11-undecanediamine, dodecaethylene glycol diamine, and 1,18-octadecanediamine.

8. The method for preparing a lithium iron manganese phosphate positive electrode material according to claim 7, characterized in that: In the step three, the organic small molecule solution with a Lewis base group is a mixture of a carboxyl small molecule and anhydrous ethanol in a mass ratio of (1-100):1000, and the carboxyl small molecule includes one of ethylenediaminetetraacetic acid, trans-1,2-cyclohexanediaminetetraacetic acid, diethylenetriaminepentaacetic acid, ethylene glycol bis(2-aminoethyl ether)tetraacetic acid, and N-(2-hydroxy)ethylenediamine-N,N',N'-triacetic acid.

9. The method for preparing a lithium iron manganese phosphate positive electrode material according to claim 8, characterized in that: In the step three, the mixing conditions of the organic small molecule solution with Lewis acid groups and the small particle lithium manganese iron phosphate material and the organic small molecule solution with Lewis base groups and the large particle lithium manganese iron phosphate material are: weight ratio (1-100): 1000, mixing temperature 30-50°C, mixing stirring rate 2500-3000r / s, mixing stirring time 1-10h, and drying temperature 80-100°C.

10. The method for preparing a lithium iron manganese phosphate positive electrode material according to claim 9, characterized in that: In the step 4, the mixing weight ratio of the small particle mixed material and the large particle mixed material is (3-6):(4-7), and the mixing temperature is 30-35°C.

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