High-voltage lithium manganese iron phosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention discloses a high-voltage lithium manganese iron phosphate positive electrode material as well as a preparation method and application thereof, and belongs to the technical field of lithium battery materials. The preparation method comprises the following steps: 1) preparing a mixed solution containing a soluble phosphate compound, soluble bivalent iron salt and soluble bivalent manganese salt; 2) adding a high-boiling-point organic solvent; 3) dropwise adding the lithium hydroxide solution into the mixed solution step by step, carrying out solvothermal reaction under the normal pressure condition, filtering, washing and drying; 4) spray drying and high-temperature calcination treatment; according to the method, the rice-grain-shaped lithium manganese iron phosphate nano-particles with the nano-scale particle size, uniform distribution, good crystallinity and high purity can be successfully prepared under normal pressure, the lithium manganese iron phosphate positive electrode material synthesized by adopting the nano-particles as the precursor can maintain high discharge voltage, so that the energy density of a battery is improved, the synthesis process is simple and novel, and the method is suitable for industrial production. The safety and reliability are high.

Description

Technical Field

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

[0002] As a core energy storage device in the field of modern science and technology, lithium-ion batteries play an irreplaceable supporting role in the conversion and storage of modern energy. With the rapid development of the clean energy storage industry and electric vehicles, the market has put forward higher requirements for the energy density, cycle life, safety and charging speed of lithium-ion batteries. As one of the key materials in the lithium-ion battery system, cathode materials have received widespread attention. Among various cathode materials, olivine-structured lithium manganese iron phosphate has the advantages of being non-toxic, pollution-free, safe, low-cost, high energy density and long cycle life, and has become an important candidate for the next generation of lithium-ion battery cathodes.

[0003] Compared with the commercialized lithium iron phosphate, lithium manganese iron phosphate has lower electronic / ionic conductivity, lower compaction density, more complex phase transition mechanism, structural distortion caused by the Jahn-Teller effect, and Mn dissolution. In order to solve these problems, people have proposed a series of improvement measures, including new synthesis methods, carbon coating and its modification, bulk doping, surface modification, and preparation of spherical particles. These methods have accelerated the lithium diffusion kinetics of the lithium manganese iron phosphate positive electrode to a certain extent, improved the material's tapping and compaction density, stabilized the lattice structure during the lithium insertion and extraction process, and limited the dissolution of Mn. However, there is a generally overlooked defect in the lithium manganese iron phosphate positive electrode, that is, during high-rate discharge, the internal particles will produce significant stress and strain due to the heterogeneity of lithium, resulting in a more serious platform voltage reduction phenomenon, which makes Mn 3+ The discharge voltage deviates from the original Mn 2+ / Mn 3+ The working voltage of the redox reaction; the inventors of the present application have found that this stress and strain will be alleviated to a certain extent as the size of the lithium manganese iron phosphate particles is greatly reduced.

[0004] At present, the main method for synthesizing small-sized lithium manganese iron phosphate particles is the liquid phase method, and its liquid phase system can be divided into pure water, organic or non-aqueous solvents, and mixed solvents of water and organic matter. Among them, the pure water system mainly utilizes a high-temperature and high-pressure hydrothermal environment to promote the dissolution and reaction of various raw materials (such as phosphorus source, iron source, manganese source, etc.), which has the advantages of short synthesis cycle, low cost, and high product purity. However, in order to obtain smaller particles, this system usually requires the addition of a large amount of auxiliary agents or the use of microwaves and other means, and the minimum particle size that can be achieved is still limited; the organic or non-aqueous solvent system is developed based on the pure water system, and the particle product is refined by the regulation of the morphology of organic or non-aqueous solvents. Compared with the pure water system, smaller particles can be obtained in this system, but since the solubility of the initial raw materials in organic or non-aqueous solvents will be greatly reduced, the single synthesis yield will be significantly reduced; the water and organic mixed solvent system combines the advantages of the previous two liquid phase systems, which can not only increase the solubility of the initial raw materials, thereby increasing the yield of a single synthesis, but also obtain small-scale particles, so it is widely used, but this system still relies on a high-pressure closed reaction environment, has high requirements for equipment, low safety, and the positive electrode of the related product is in the Mn 2+ / Mn 3+ The reaction zone still cannot always maintain a high voltage level. Summary of the Invention

[0005] The purpose of the present invention is to provide a high-voltage lithium manganese iron phosphate positive electrode material and a preparation method and application thereof, so as to solve the problems raised in the background technology.

[0006] In order to achieve the above object, the present invention provides the following technical solutions: A first aspect of the present invention provides a method for preparing a high-voltage lithium manganese iron phosphate positive electrode material, comprising the following steps: 1) Under inert atmosphere, a soluble phosphate compound, a soluble divalent manganese salt, and a soluble divalent iron salt are added to deionized water at a molar ratio of P:(Mn+Fe) = 1:1, stirred until completely dissolved, and then an appropriate amount of antioxidant is added to obtain a mixed solution with a total metal salt molar concentration of 1-3 mol / L; 2) adding 1-10 times the volume of deionized water to the mixed solution obtained in step 1) and ultrasonically treating; 3) placing the mixed solution obtained in step 2) in a condensation reflux reactor at atmospheric pressure, slowly adding a lithium hydroxide solution having a concentration of 1-3 mol / L into the reactor, stirring thoroughly at a rate of 50-500 rpm, heating the reactor to 140-180° C., maintaining the temperature until the water evaporates completely, and then maintaining the temperature for condensation reflux reaction for 2-12 hours. After the reaction is completed, filtering the solid product, washing, and drying to obtain lithium manganese iron phosphate nanoparticle powder; 4) The lithium manganese iron phosphate nanoparticle powder prepared in step 3) is dispersed with a carbon source in a solvent, subjected to ultrasonic treatment, spray-dried, and calcined at a high temperature under the protection of an inert gas to obtain a lithium manganese iron phosphate cathode material.

[0007] Preferably, the phosphate salt in step 1) is one or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate or phosphoric acid, the soluble divalent manganese salt is one or more of manganous sulfate, manganous nitrate or manganous chloride, and the soluble divalent iron salt is one or more of ferrous sulfate, ferrous nitrate or ferrous chloride.

[0008] Preferably, in step 1), the molar ratio of Mn:Fe provided by the soluble divalent manganese salt and the soluble divalent iron salt is 1-4:1.

[0009] Preferably, the antioxidant in step 1) is ascorbic acid or citric acid, and the amount of the antioxidant added is 0.5-5% of the total mass of the metal salt.

[0010] Preferably, the boiling point of the high-boiling-point organic solvent in step 2) should be higher than the insulation temperature in step 3).

[0011] More preferably, the high boiling point organic solvent in step 2) is one of ethylene glycol, glycerol, diformate or sulfolane.

[0012] Preferably, the ultrasonic treatment time in step 2) is 3-15 min, and the ultrasonic frequency is 30-50 KHz.

[0013] Preferably, in step 3), lithium hydroxide solution is added to the mixed solution at a molar ratio of Li:P = 3-3.15:1, and the addition rate of the lithium hydroxide solution is 1-10 mL / min.

[0014] Preferably, in step 4), the carbon source is one of glucose, sucrose, phenolic resin or starch, the mass of the carbon source is 1%-20% of the mass of the lithium manganese iron phosphate nanoparticle powder, and the solvent is water or ethanol.

[0015] Preferably, in step 4), the ultrasonic treatment time is 10-60 min, the ultrasonic frequency is 30-50 KHz, the high-temperature calcination temperature is 600-800° C., and the calcination time is 4-8 h.

[0016] The principle of the preparation method of the present invention is as follows: in the initial stage of the reaction, since the solvent system is a mixed solution of water and an organic solvent, the reaction materials first form a manganese ferrous phosphate intermediate. In this stage, by adjusting parameters such as the ratio of water to organic solvent, the concentration of the reactants, and the stirring intensity, the nucleation and growth rates of the manganese ferrous phosphate intermediate can be flexibly adjusted, thereby regulating the particle size and morphology of the final product. In the subsequent condensation reflux reaction stage, the reaction temperature of the system is relatively higher, and the high-temperature solvent thermal effect can enhance the permeability of lithium ions in the solution, prompting them to embed into the manganese ferrous phosphate intermediate to obtain lithium iron manganese phosphate particles, further promoting crystal maturation and improving crystallinity, and ultimately obtaining high-quality small-scale lithium iron manganese phosphate nanoparticles.

[0017] A second aspect of the present invention provides a high-voltage lithium manganese iron phosphate positive electrode material prepared by the above-mentioned preparation method.

[0018] A third aspect of the present invention provides an application of a high-voltage lithium iron manganese phosphate positive electrode material in a lithium-ion battery positive electrode.

[0019] Based on this, the beneficial effects of the present invention are: 1. The present invention overcomes the technical prejudice that lithium manganese iron phosphate must be prepared under high-pressure conditions. By using a mixed solvent of deionized water and a high-boiling-point organic solvent as the carrier in the initial reaction process, the solubility of each reaction raw material is effectively increased, and the yield of lithium manganese iron phosphate nanoparticles per unit volume is relatively increased. Through an innovative reaction method (raising the reaction temperature and completely evaporating the water before heat preservation, condensation and reflux reaction) and optimizing the coordination between various reaction conditions and reaction parameters, the synthesis of lithium manganese iron phosphate nanoparticles under normal pressure is achieved, reducing equipment costs and operational difficulty. The synthesis process is fully visual, safe and reliable; 2. The preparation method of the present invention utilizes the high polarity of deionized water to increase the solubility of each raw material in the early stage of the heating reaction. In the early stage of the heating reaction, the reaction materials first form a manganese ferrous phosphate intermediate. As the temperature increases, the water phase evaporates, the solvent composition in the reaction system gradually changes, the polarity gradually decreases, and the activation energy required for ion binding decreases, thereby promoting the continuous generation of crystal nuclei of the manganese ferrous phosphate intermediate. At the same time, the high viscosity and low dielectric constant of the organic solvent can slow down ion migration, control the growth rate, and avoid particle coarsening, thereby achieving precise control of the size and morphology of the manganese ferrous phosphate intermediate in the gradually changing solvent system. In the condensation reflux reaction stage, the high temperature thermal effect of the single organic solvent can improve the penetration ability of lithium ions in the solution, prompting them to embed into the manganese ferrous phosphate intermediate to obtain lithium iron manganese phosphate particles, and further promote crystal maturation and improve crystallinity, thereby obtaining high-quality small-scale lithium iron manganese phosphate nanoparticles; 3. The present invention uses ultrasonic treatment before the reaction to break up the clusters of metal salt ions, achieve uniform dispersion at the molecular level, and promote uniform nucleation of crystals. At the same time, ultrasound also accelerates the intermolecular mixing of water and organic solvents, promotes the formation of a uniform solvent environment, and ensures the spatial consistency of the subsequent nucleation reaction. By ultrasonically treating the lithium manganese iron phosphate nanoparticle powder and the carbon source, the hard agglomeration of the nanoparticles can be eliminated, the carbon source can be uniformly dispersed, the uniformity of the subsequent carbon coating and the integrity of the electrode microstructure can be ensured, and the voltage decay caused by particle agglomeration can be avoided. By controlling the reaction temperature and holding time, the crystal growth can be further controlled, the particle size and crystallinity can be balanced, and the uniformity of the Mn / Fe distribution can be ensured by appropriate stirring rate and reactant concentration. The coordination between the various reaction conditions and reaction parameters can achieve the preparation of high-quality small-scale lithium manganese iron phosphate nanoparticles under normal pressure conditions. 4. In the present invention, when the reaction enters the termination stage, the reaction solvent system will be converted into a single organic solvent, which is convenient for solvent recovery and reuse through a simple fractionation process, significantly reducing production costs, and also facilitating the enrichment and treatment of lithium-containing waste liquid, thereby improving resource utilization efficiency; 5. The lithium manganese iron phosphate nanoparticles obtained by the preparation method of the present invention are rice-shaped, with a nanometer-scale particle size and uniform distribution, good crystallinity, high purity, and no significant growth during high-temperature sintering. Therefore, the secondary particles of the lithium manganese iron phosphate positive electrode material synthesized using it as a precursor have a microspherical porous morphology, good wettability with the electrolyte, high purity, uniform carbon layer coating, and the primary particle size is consistent with the nanoparticle precursor. The small-sized nanoparticles can not only effectively suppress the occurrence of stress and strain during the discharge process and maintain the high discharge voltage of the positive electrode material, thereby improving the energy density of the battery, but also shorten the path of lithium ion transmission, effectively shorten the distance for lithium ions to diffuse from the surface to the center of the bulk phase, and reduce the charge and discharge polarization, so that the average discharge voltage of manganese in the lithium manganese iron phosphate positive electrode material prepared in this application is greater than 3.9 V, and effectively improve the discharge specific capacity and rate performance of the lithium manganese iron phosphate positive electrode material. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] Figure 1 XRD patterns of lithium iron manganese phosphate nanoparticles LFMP-A1, LFMP-A2, LFMP-A3, and LFMP-B1 in Example 1, Example 2, Example 3, and Comparative Example 1; Figure 2 This is a SEM image of the lithium iron manganese phosphate nanoparticles LFMP-A1 prepared in Example 1; Figure 3 This is a SEM image of lithium iron manganese phosphate nanoparticles LFMP-B1 prepared in Comparative Example 1; Figure 4XRD patterns of lithium iron manganese phosphate positive electrode materials LFMP / C-A1 and LFMP / C-B1 in Example 1 and Comparative Example 1; Figure 5 This is a SEM image of the lithium iron manganese phosphate positive electrode material LFMP / C-A1 prepared in Example 1; Figure 6 This is a SEM image of the lithium iron manganese phosphate positive electrode material LFMP / C-B1 prepared in Comparative Example 1; Figure 7 The charge and discharge curves of test battery 1 and test battery 2 at 2C rate; Figure 8 Figure 2 is the rate performance diagram of test battery 1 and test battery 2. DETAILED DESCRIPTION

[0021] The present invention is described in detail below through examples. It is necessary to point out that the following examples are only used to further illustrate the present invention and cannot be understood as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments to the present invention based on the above content of the present invention. In the following examples, reagents and instruments not specifically described are all commercially available, and experimental operations not specifically described are carried out according to the manufacturer's instructions or conventional techniques in the field. Unless otherwise defined, all professional and scientific terms used in the text have the same meanings as those familiar to those skilled in the art. In addition, any methods and materials similar to or equivalent to those described can be applied to the present invention; the endpoints and any values of the ranges disclosed herein are not limited to the precise ranges or values, and these ranges or values should be understood to include values close to these ranges or values. For numerical ranges, the endpoint values of each range, the endpoint values of each range and the individual point values, and the individual point values can be combined with each other to obtain one or more new numerical ranges, and these numerical ranges should be regarded as specifically disclosed herein; the present invention provides a method for preparing a high-voltage lithium manganese iron phosphate positive electrode material, comprising the following steps: 1) Under inert atmosphere, a soluble phosphate compound, a soluble divalent manganese salt, and a soluble divalent iron salt are added to deionized water at a molar ratio of P:(Mn+Fe) = 1:1, stirred until completely dissolved, and then an appropriate amount of an antioxidant is added to obtain a mixed solution with a total metal salt molar concentration of 1-3 mol / L, wherein the molar ratio of Mn:Fe provided by the soluble divalent manganese salt and the soluble divalent iron salt is 1-4:1; 2) adding 1-10 times the volume of deionized water to the mixed solution obtained in step 1) and ultrasonically treating at a frequency of 30-50 kHz for 3-15 minutes; 3) placing the mixed solution obtained in step 2) in a condensation reflux reactor at atmospheric pressure, slowly adding a lithium hydroxide solution with a concentration of 1-3 mol / L into the reactor at a rate of 1-10 mL / min, stirring thoroughly at a rate of 50-500 rpm, heating the reactor to 140-180° C., maintaining the temperature until the water is completely evaporated, maintaining the temperature and condensing the reflux reaction for 2-12 hours, and filtering the solid product after the reaction, washing, and drying to obtain lithium manganese iron phosphate nanoparticle powder, wherein the molar ratio of Li in the lithium hydroxide solution to P in the mixed solution is 3-3.15:1; 4) The lithium manganese iron phosphate nanoparticle powder prepared in step 3) and 1%-20% of a carbon source by weight are dispersed in a solvent, ultrasonically treated at a frequency of 30-50 kHz for 10-60 min, spray-dried, and calcined at a high temperature of 600-800°C for 4-8 h under inert gas protection to obtain a lithium manganese iron phosphate positive electrode material.

[0022] In some preferred embodiments of the present invention, the total metal salt molar concentration can also be selected as 1.5 mol / L, 2.0 mol / L, or 2.5 mol / L.

[0023] In some preferred embodiments of the present invention, the soluble phosphate compound in step 1) is one or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate or phosphoric acid, the soluble divalent manganese salt is one or more of manganous sulfate, manganous nitrate or manganous chloride, and the soluble divalent iron salt is one or more of ferrous sulfate, ferrous nitrate or ferrous chloride.

[0024] In some preferred embodiments of the present invention, the molar ratio of Mn:Fe provided by the soluble divalent manganese salt and the soluble divalent iron salt in step 1) can also be selected as 1.5:1, 2:1, or 3:1; the molar concentration of the total metal salt can also be selected as 1.5 mol / L, 2.0 mol / L, or 2.5 mol / L.

[0025] In some preferred embodiments of the present invention, the amount of the high-boiling-point organic solvent in step 2) can also be selected to be 2 times, 4 times, 6 times, or 8 times that of deionized water; the ultrasonic treatment time can also be selected to be 5 min, 7 min, 9 min, 11 min, or 13 min; and the ultrasonic frequency can also be selected to be 35 KHz, 40 KHz, or 45 KHz.

[0026] In some preferred embodiments of the present invention, the addition rate of the lithium hydroxide solution in step 3) can also be selected from 3 mL / min, 5 mL / min, 7 mL / min, and 9 mL / min, the concentration of the lithium hydroxide solution can also be selected from 1.5 mol / L, 2.0 mol / L, and 2.5 mol / L, the molar amount of lithium hydroxide can also be selected from 3.03 times, 3.06 times, 3.09 times, and 3.12 times that of the soluble phosphate compound, the stirring rate can also be selected from 100 rpm, 200 rpm, 300 rpm, and 400 rpm, the reaction temperature can also be selected from 150°C, 160°C, and 170°C, and the reaction time can also be selected from 4 h, 6 h, 8 h, and 10 h.

[0027] In some preferred embodiments of the present invention, the mass of the carbon source in step 4) can also be selected as 5%, 10%, or 15% of the mass of the lithium iron manganese phosphate nanoparticle powder, the time of ultrasonic treatment can also be selected as 20 min, 30 min, 40 min, or 50 min, the ultrasonic frequency can also be selected as 35 KHz, 38 KHz, 40 KHz, 42 KHz, 45 KHz, or 48 KHz, the calcination temperature can also be selected as 650 °C, 700 °C, or 750 °C, and the calcination time can also be selected as 5 h, 6 h, or 7 h.

[0028] The present invention also provides lithium manganese iron phosphate nanoparticles prepared by the above preparation method. The nanoparticles are rice-grain-shaped, have good crystallinity, high purity, and uniform size distribution. The product particles are 30-60 nm wide and 50-100 nm long.

[0029] The present invention also provides a high-voltage lithium manganese iron phosphate positive electrode material prepared by the above preparation method. The positive electrode material has a microspherical porous morphology, a uniform carbon layer coating, good wettability with the electrolyte, and excellent rate performance.

[0030] The lithium manganese iron phosphate described in the present invention is used as a positive electrode material for a lithium ion battery, which can enable the lithium ion battery to not only have excellent specific capacity and rate performance, but also maintain a high discharge voltage and have a higher energy density.

[0031] The following are specific embodiments: Example 1 1) In a 500 mL beaker, weigh 20.75 g of 85% phosphoric acid and add 50 mL of deionized water. Under a nitrogen atmosphere, add 15.37 g of manganous sulfate and 25.27 g of ferrous sulfate to the beaker, stir to dissolve, and then add 0.81 g of ascorbic acid. 2) Add 400 mL of ethylene glycol solvent to the solution obtained in step 1) and perform ultrasonic treatment at an ultrasonic frequency of 40 kHz for 5 min; 3) The mixed solution obtained in step 2) was placed in a condensation reflux reactor at normal pressure, 23.11 g of lithium hydroxide was weighed and added to 100 mL of deionized water to fully dissolve to obtain a transparent lithium hydroxide solution; the lithium hydroxide solution was slowly added to the reactor at a stirring rate of 300 rpm at a rate of 5 mL / min; after the addition was completed, the temperature was raised to 180°C and kept warm until the water was completely evaporated, and the reaction was condensed and refluxed for 4 hours. The solution was removed and filtered, and washed with deionized water three times. The washed solid was placed in a blast drying oven and dried at 60°C for 24 hours to obtain lithium manganese iron phosphate nanoparticles LFMP-A1; 4) 5.0 g of the granular powder obtained in step 3) was dispersed with 0.75 g of glucose in 100 mL of deionized water, and the mixture was ultrasonically treated at a frequency of 40 kHz for 30 min. After spray drying, the mixture was calcined at 650 °C for 6 h under an argon atmosphere to obtain the lithium iron manganese phosphate cathode material LFMP / C-A1.

[0032] Example 2 1) In a 500 mL beaker, weigh 20.75 g of 85% phosphoric acid and add 50 mL of deionized water. Under a nitrogen atmosphere, add 15.37 g of manganous sulfate and 25.27 g of ferrous sulfate to the beaker, stir to dissolve, and then add 0.81 g of ascorbic acid. 2) Add 400 mL of ethylene glycol solvent to the solution obtained in step 1) and perform ultrasonic treatment at an ultrasonic frequency of 40 kHz for 5 min; 3) The mixed solution obtained in step 2) was placed in a condensation reflux reactor at normal pressure, 23.11 g of lithium hydroxide was weighed and added to 100 mL of deionized water to fully dissolve to obtain a transparent lithium hydroxide solution; the lithium hydroxide solution was slowly added to the reactor at a stirring rate of 300 rpm at a rate of 5 mL / min; after the addition was completed, the temperature was raised to 170°C and kept warm until the water was completely evaporated, and the reaction was condensed and refluxed for 6 hours. The solution was removed and filtered, and washed with deionized water three times. The washed solid was placed in a blast drying oven and dried at 60°C for 24 hours to obtain lithium iron manganese phosphate nanoparticles LFMP-A2; 4) 5.0 g of the granular powder obtained in step 3) was dispersed with 0.75 g of glucose in 100 mL of deionized water, and the mixture was ultrasonically treated at a frequency of 40 kHz for 30 min. After spray drying, the mixture was calcined at 650 °C for 6 h under an argon atmosphere to obtain the lithium iron manganese phosphate cathode material LFMP / C-A2.

[0033] Example 3 1) In a 500 mL beaker, weigh 20.75 g of 85% phosphoric acid and add 50 mL of deionized water. Under a nitrogen atmosphere, add 15.37 g of manganous sulfate and 25.27 g of ferrous sulfate to the beaker, stir to dissolve, and then add 0.81 g of ascorbic acid. 2) Add 400 mL of ethylene glycol solvent to the solution obtained in step 1) and perform ultrasonic treatment at an ultrasonic frequency of 40 kHz for 5 min; 3) The mixed solution obtained in step 2) was placed in a condensation reflux reactor at normal pressure, 23.11 g of lithium hydroxide was weighed and added to 100 mL of deionized water to fully dissolve to obtain a transparent lithium hydroxide solution; the lithium hydroxide solution was slowly added to the reactor at a stirring rate of 300 rpm at a rate of 5 mL / min; after the addition was completed, the temperature was raised to 160°C and kept warm until the water was completely evaporated, and the reaction was condensed and refluxed for 8 hours. The solution was removed and filtered, and washed with deionized water three times. The washed solid was placed in a blast drying oven and dried at 60°C for 24 hours to obtain lithium manganese iron phosphate nanoparticles LFMP-A3; 4) 5.0 g of the granular powder obtained in step 3) was dispersed with 0.75 g of glucose in 100 mL of deionized water, and ultrasonically treated at a frequency of 40 kHz for 30 min. After spray drying, the mixture was calcined at 650 °C for 6 h under an argon atmosphere to obtain the lithium iron manganese phosphate cathode material LFMP / C-A3.

[0034] Example 4 1) In a 500 mL beaker, weigh 20.75 g of 85% phosphoric acid and add 50 mL of deionized water. Under a nitrogen atmosphere, add 15.37 g of manganous sulfate and 25.27 g of ferrous sulfate to the beaker, stir to dissolve, and then add 0.81 g of ascorbic acid. 2) Add 400 mL of ethylene glycol solvent to the solution obtained in step 1) and perform ultrasonic treatment at an ultrasonic frequency of 40 kHz for 5 min; 3) The mixed solution obtained in step 2) was placed in a condensation reflux reactor at normal pressure, 23.11 g of lithium hydroxide was weighed and added to 100 mL of deionized water to fully dissolve to obtain a transparent lithium hydroxide solution; the lithium hydroxide solution was slowly added to the reactor at a stirring rate of 200 rpm at a rate of 5 mL / min; after the addition was completed, the temperature was raised to 180°C and kept warm until the water was completely evaporated, and the reaction was condensed and refluxed for 4 hours. The solution was removed and filtered, and washed with deionized water three times. The washed solid was placed in a blast drying oven and dried at 60°C for 24 hours to obtain lithium iron manganese phosphate nanoparticles LFMP-A4; 4) 5.0 g of the granular powder obtained in step 3) was dispersed with 0.75 g of glucose in 100 mL of deionized water, and ultrasonically treated at a frequency of 40 kHz for 30 min. After spray drying, the mixture was calcined at 650 °C for 6 h under an argon atmosphere to obtain the lithium iron manganese phosphate cathode material LFMP / C-A4.

[0035] Example 5 1) In a 500 mL beaker, weigh 20.75 g of 85% phosphoric acid and add 50 mL of deionized water. Under a nitrogen atmosphere, add 15.37 g of manganous sulfate and 25.27 g of ferrous sulfate to the beaker, stir to dissolve, and then add 0.81 g of ascorbic acid. 2) adding 400 mL of glycerol solvent to the solution obtained in step 1) and ultrasonically treating the solution at an ultrasonic frequency of 40 kHz for 5 min; 3) The mixed solution obtained in step 2) was placed in a condensation reflux reactor at normal pressure, 23.11 g of lithium hydroxide was weighed and added to 100 mL of deionized water to fully dissolve to obtain a transparent lithium hydroxide solution; the lithium hydroxide solution was slowly added to the reactor at a stirring rate of 300 rpm at a rate of 5 mL / min; after the addition was completed, the temperature was raised to 180°C and kept warm until the water was completely evaporated, and the reaction was condensed and refluxed for 4 hours. The solution was removed and filtered, and washed with deionized water three times. The washed solid was placed in a blast drying oven and dried at 60°C for 24 hours to obtain lithium iron manganese phosphate nanoparticles LFMP-A5; 4) 5.0 g of the granular powder obtained in step 3) and 0.75 g of glucose were dispersed in 100 mL of deionized water, ultrasonically treated at a frequency of 40 kHz for 30 min, spray-dried, and calcined at 650 °C for 6 h under an argon atmosphere to obtain the lithium iron manganese phosphate cathode material LFMP / C-A5.

[0036] Example 6 1) In a 500 mL beaker, weigh 20.92 g of ammonium dihydrogen phosphate and add 50 mL of deionized water. Under a nitrogen atmosphere, add 17.98 g of manganous chloride and 18.07 g of ferrous chloride to the beaker, stir until dissolved, and then add 0.4 g of ascorbic acid. 2) adding 400 mL of glycerol solvent to the solution obtained in step 1) and ultrasonically treating the solution at an ultrasonic frequency of 40 kHz for 5 min; 3) The mixed solution obtained in step 2) was placed in a condensation reflux reactor at normal pressure, 23.11 g of lithium hydroxide was weighed and added to 100 mL of deionized water to fully dissolve to obtain a transparent lithium hydroxide solution; the lithium hydroxide solution was slowly added to the reactor at a stirring rate of 300 rpm at a rate of 5 mL / min; after the addition was completed, the temperature was raised to 180°C and kept warm until the water was completely evaporated, and the reaction was condensed and refluxed for 4 hours. The solution was removed and filtered, and washed with deionized water three times. The washed solid was placed in a blast drying oven and dried at 60°C for 24 hours to obtain lithium manganese iron phosphate nanoparticles LFMP-A6; 4) 5.0 g of the granular powder obtained in step 3) was dispersed with 0.75 g of glucose in 100 mL of deionized water. The mixture was ultrasonically treated at a frequency of 40 kHz for 30 min. After spray drying, the mixture was calcined at 650 °C for 6 h under an argon atmosphere to obtain the lithium iron manganese phosphate cathode material LFMP / C-A6.

[0037] Comparative Example 1 1) In a 500 mL beaker, weigh 20.75 g of 85% phosphoric acid and add 50 mL of deionized water. Under a nitrogen atmosphere, add 15.37 g of manganous sulfate and 25.27 g of ferrous sulfate to the beaker, stir to dissolve, and then add 0.81 g of ascorbic acid. 2) Add 400 mL of ethylene glycol solvent to the solution obtained in step 1) and perform ultrasonic treatment at an ultrasonic frequency of 40 kHz for 5 min; 3) The mixed solution obtained in step 2) was placed in a condensation reflux reactor at normal pressure, 23.11 g of lithium hydroxide was weighed and added to 100 mL of deionized water to fully dissolve to obtain a transparent lithium hydroxide solution; the lithium hydroxide solution was slowly added to the reactor at a stirring rate of 300 rpm at a rate of 5 mL / min; after the addition was completed, the temperature was raised to 130°C and kept warm until the water was completely evaporated, and the reaction was condensed and refluxed for 4 hours. The solution was removed and filtered, and washed with deionized water three times. The washed solid was placed in a blast drying oven and dried at 60°C for 24 hours to obtain lithium manganese iron phosphate nanoparticles LFMP-B1; 4) 5.0 g of the granular powder obtained in step 3) was dispersed with 0.75 g of glucose in 100 mL of deionized water, and ultrasonically treated at a frequency of 40 kHz for 30 min. After spray drying, the mixture was calcined at 650 °C for 6 h under an argon atmosphere to obtain the lithium iron manganese phosphate positive electrode material LFMP / C-B1.

[0038] Comparative Example 2 This comparative example is a conventional hydrothermal method; 1) In a 100 mL beaker, weigh 3.46 g of 85% phosphoric acid and add 30 mL of deionized water. Under a nitrogen atmosphere, add 2.56 g of manganous sulfate and 4.21 g of ferrous sulfate to the beaker, stir to dissolve, and then add 0.14 g of ascorbic acid. 2) Weigh 3.86 g of lithium hydroxide and add it to 30 mL of deionized water to fully dissolve it to obtain a lithium hydroxide aqueous solution; slowly add the lithium hydroxide solution to the mixed solution obtained in step 1) at a rate of 5 mL / min under a stirring rate of 500 rpm and stir for 15 minutes; pour the obtained suspension into a 100 mL polytetrafluoroethylene reactor, react at 180°C for 12 hours, then remove the solution and filter it. The pressure during the reaction process is 0.5-1.5 MPa. Wash it with deionized water three times. Place the washed solid in a blast drying oven and dry it at 60°C for 24 hours to obtain lithium iron manganese phosphate nanoparticles LFMP-B2; 3) 5.0 g of the granular powder obtained in step 3) and 0.75 g of glucose were dispersed in 100 mL of deionized water, ultrasonically treated at an ultrasonic frequency of 40 kHz for 30 min, spray-dried, and calcined at 650 °C for 6 h under an argon atmosphere to obtain the lithium iron manganese phosphate positive electrode material LFMP / C-B2.

[0039] Comparative Example 3 This comparative example is a conventional pure solvothermal method; 1) In a 100 mL beaker, weigh 1.73 g of 85% phosphoric acid and add 30 mL of ethylene glycol. Under a nitrogen atmosphere, add 1.28 g of manganous sulfate and 2.11 g of ferrous sulfate to the beaker, stir to dissolve, and then add 0.07 g of ascorbic acid. 2) Weigh 1.93 g of lithium hydroxide and add it to 30 mL of ethylene glycol. After fully dissolving, obtain a lithium hydroxide alcohol solution; slowly add the lithium hydroxide solution to the mixed solution obtained in step 1) at a rate of 5 mL / min under a stirring rate of 500 rpm and stir for 15 min; pour the obtained suspension into a 100 mL polytetrafluoroethylene reactor, react at 180°C for 12 h, remove the solution and filter it. The pressure during the reaction process is 0.5-1.5 MPa. Wash it with deionized water three times. Place the washed solid in a blast drying oven and dry it at 60°C for 24 h to obtain lithium iron manganese phosphate nanoparticles LFMP-B3; 3) 5.0 g of the granular powder obtained in step 3) and 0.75 g of glucose were dispersed in 100 mL of deionized water, ultrasonically treated at a frequency of 40 kHz for 30 min, spray-dried, and calcined at 650 °C for 6 h under an argon atmosphere to obtain the lithium iron manganese phosphate positive electrode material LFMP / C-B3.

[0040] Comparative Example 4 This comparative example is a mixed solvent thermal method; 1) Prepare a binary solvent in a 1 / 2 ratio of water to ethylene glycol. In a 100 mL beaker, weigh 2.59 g of 85% phosphoric acid and add 30 mL of the binary solvent. Under a nitrogen atmosphere, add 1.92 g of manganous sulfate and 3.16 g of ferrous sulfate to the beaker, stir to dissolve, and then add 0.1 g of ascorbic acid. 2) Weigh 2.89 g of lithium hydroxide and add it to 30 mL of binary solvent. After fully dissolving, obtain a lithium hydroxide alcohol solution; slowly add the lithium hydroxide solution to the mixed solution obtained in step 1) at a rate of 5 mL / min under a stirring rate of 500 rpm and stir for 15 min; pour the obtained suspension into a 100 mL polytetrafluoroethylene reactor, react at 180°C for 12 h, remove the solution and filter it. The pressure during the reaction process is 0.5-1.5 MPa. Wash with deionized water three times. Place the washed solid in a blast drying oven and dry it at 60°C for 24 h to obtain lithium iron manganese phosphate nanoparticles LFMP-B4; 3) 5.0 g of the granular powder obtained in step 3) and 0.75 g of glucose were dispersed in 100 mL of deionized water, ultrasonically treated at a frequency of 40 kHz for 30 min, spray-dried, and calcined at 650 °C for 6 h under an argon atmosphere to obtain the lithium iron manganese phosphate positive electrode material LFMP / C-B4.

[0041] Experimental Example 1 The lithium iron manganese phosphate nanoparticles LFMP-A1, LFMP-A2, LFMP-A3 prepared in Example 1, Example 2, and Example 3 and the lithium iron manganese phosphate nanoparticles LFMP-A1 prepared in Comparative Example 1 were subjected to X-ray diffraction (XRD) characterization analysis. The results are as follows: Figure 1 shown.

[0042] Depend on Figure 1It can be seen that the powder diffraction peaks of the lithium iron manganese phosphate nanoparticles LFMP-A1, LFMP-A2, and LFMP-A3 prepared in Example 1, Example 2, and Example 3 all correspond to standard olivine-type lithium iron manganese phosphate, and the peak shapes are symmetrical and sharp, indicating that the lithium iron manganese phosphate nanoparticles prepared by the present invention are pure phase, and there are no other miscellaneous substances in the system or the impurity content is extremely low; however, the diffraction peak intensity corresponding to the lithium iron manganese phosphate nanoparticles LFMP-B1 prepared in Comparative Example 1 is relatively low, and a small amount of lithium phosphate impurity phase is present.

[0043] Experimental Example 2 The lithium iron manganese phosphate nanoparticles LFMP-A1 prepared in Example 1 and the lithium iron manganese phosphate nanoparticles LFMP-B1 prepared in Comparative Example 1 were observed by scanning electron microscopy (SEM). Figure 2 and Figure 3 shown.

[0044] Depend on Figure 2 It can be seen that the lithium iron manganese phosphate nanoparticles LFMP-A1 prepared in Example 1 are in the shape of rice grains, with a width of 30-50 nm and a length of 50-100 nm, and the particle size distribution is uniform; Figure 3 It can be seen that the lithium iron manganese phosphate nanoparticles LFMP-B1 prepared in Comparative Example 1 are still rice-grain-shaped, but the particle size of each particle is relatively large, with a width of 100-200 nm and a length of 150-300 nm, and many small particles are filled between the large particles.

[0045] Experimental Example 3 The lithium iron manganese phosphate positive electrode material LFMP / C-A1 prepared in Example 1 and the lithium iron manganese phosphate positive electrode material LFMP / C-B1 prepared in Comparative Example 1 were subjected to X-ray diffraction (XRD) characterization analysis. The results are as follows: Figure 4 shown.

[0046] Depend on Figure 4 It can be seen that the powder diffraction peak of the lithium iron manganese phosphate positive electrode material LFMP / C-A1 prepared in Example 1 corresponds to the standard olivine-type lithium iron manganese phosphate, indicating that the lithium iron manganese phosphate prepared by the present invention is a pure phase of the positive electrode material, and there are no other miscellaneous items in the system or the impurity content is extremely low. The lithium iron manganese phosphate positive electrode material LFMP / C-B1 prepared in Comparative Example 1 also presents a standard diffraction peak, indicating that the diffusion of metal elements occurs during the calcination process, thereby causing the impurity phase in the product to disappear or the content to be significantly reduced.

[0047] Experimental Example 4 The lithium iron manganese phosphate positive electrode material LFMP / C-A1 prepared in Example 1 and the lithium iron manganese phosphate positive electrode material LFMP / C-B1 prepared in Comparative Example 1 were observed by scanning electron microscopy (SEM). Figure 5and Figure 6 shown.

[0048] Depend on Figure 5 and Figure 6 It can be seen that the lithium iron manganese phosphate positive electrode materials LFMP / C-A1 and LFMP / C-B1 prepared in Example 1 and Comparative Example 1 are both composed of secondary microspheres formed by primary particles. The size of the primary particles is not significantly increased compared with their respective precursor lithium iron manganese phosphate particles, indicating that the lithium iron manganese phosphate material has good carbon layer coating during high-temperature sintering.

[0049] Experimental Example 5 The lithium iron manganese phosphate positive electrode material LFMP / C-A1 prepared in Example 1 of the present invention and the lithium iron manganese phosphate positive electrode material LFMP / C-B1 prepared in Comparative Example 1 were used as the positive electrode of the lithium-ion battery to assemble a CR2032 button half-cell, with the negative electrode being a metal lithium sheet, and the battery was subjected to charge and discharge tests using the Xinwei system.

[0050] 1. Preparation of button cells: Lithium manganese iron phosphate cathode material, Super P, and polyvinylidene fluoride (PVDF) were dispersed in an N-methylpyrrolidone (NMP) solvent at a mass ratio of 8:1:1 to form a uniform slurry. The slurry was then coated onto a carbon-containing aluminum foil current collector and dried overnight in a vacuum oven at 80°C. Circular electrode sheets with a diameter of 12 mm were then cut using a sheet punch. Finally, CR2032 button cells were assembled in an argon-filled glove box using a Celgard 2500 membrane as the separator, a lithium metal sheet as the anode, and a 1 mol / L LiPF6 / dimethyl carbonate (DMC) / ethyl carbonate (EMC) / ethylene carbonate (EC) (volume ratio 1:1:1) as the electrolyte.

[0051] Among them, the battery using the lithium iron manganese phosphate positive electrode material LFMP / C-A1 prepared in Example 1 as the battery positive electrode is test battery 1, and the battery using the lithium iron manganese phosphate positive electrode material LFMP / C-B1 prepared in Comparative Example 1 as the battery positive electrode is test battery 2.

[0052] 2. Performance testing and results (1) Test battery 1 and test battery 2 were respectively subjected to constant current charge and discharge tests at a 2C rate, with a voltage range of 2.0-4.5 V (vs. Li + / Li), the results are as follows Figure 7 shown.

[0053] Depend on Figure 7 As shown in the discharge curves of LFMP / C-A1 and LFMP / C-B1, there are two pairs of charge-discharge platforms near 4.1 V and 3.45 V, corresponding to Mn 2+ / Mn3+ and Fe 2+ / Fe 3+ However, the discharge curve of LFMP / C-B1 shows a rapid decrease in the working voltage, and Mn 3+ The discharge voltage of the battery is reduced to about 3.65 V. Although the specific capacity is still considerable, the battery energy density is reduced. In the discharge curve of LFMP / C-A1, Mn 3+ The voltage platform is maintained very stably, allowing the battery to maintain a high discharge voltage and thus have a higher energy density.

[0054] (2) Test battery 1 and test battery 2 were charged and discharged at currents of 0.2C, 0.5C, 1C, 2C, 5C, and 10C (1C = 170 mAh / g) respectively. The results are as follows: Figure 8 shown.

[0055] Depend on Figure 8 It can be seen that the discharge specific capacity of test battery 1 at 0.2C, 0.5C, 1C, 2C, 5C, and 10C rates is 150.42 mAh g −1 、146.62 mAh g −1 、142.68 mAh g −1 、138.04 mAh g −1 、129.92 mAh g −1 、121.88 mAh g −1 The discharge capacity of test battery 2 at 0.2C, 0.5C, 1C, 2C, 5C, and 10C rates are 148.85 mAh g −1 、144.44 mAh g −1 、139.21 mAh g −1 、131.96 mAh g −1 、115.37 mAh g −1 , 95.05 mAh g −1 .

[0056] From the above test results, it can be seen that test battery 1 and test battery 2 have similar discharge specific capacities at low rates, which indicates that the physical phases of the lithium iron manganese phosphate positive electrode material LFMP / C-A1 prepared by Example 1 and the lithium iron manganese phosphate positive electrode material LFMP / C-B1 prepared by Comparative Example 1 have high purity. However, as the charge and discharge rate increases, the difference in discharge specific capacity between the two becomes larger. Compared with test battery 2, test battery 1 has better rate performance. This is because the primary particles constituting LFMP / C-A1 are finer, and Li + The diffusion path is shorter.

[0057] Experimental Example 6 According to the experimental method in Experimental Example 5, the lithium iron manganese phosphate positive electrode material LFMP / CA prepared in Examples 1-6 and the lithium iron manganese phosphate positive electrode material LFMP / CB prepared in Comparative Examples 1-4 were used as lithium-ion battery positive electrodes to assemble test cells, and then constant current charge and discharge tests were performed at a 2C rate, with a voltage range of 2.0-4.5 V (vs. Li + / Li), test the average discharge voltage of manganese in lithium manganese iron phosphate positive electrode material, that is, Mn 2+ / Mn 3+ The average value of the voltage within the reaction area is shown in Table 1; The particle size of the lithium iron manganese phosphate nanoparticles LFMP / CA prepared in Example 1-6 and the lithium iron manganese phosphate particles LFMP-B prepared in Comparative Example 1-5 was tested using a particle size tester. The results are shown in Table 1. Table 1 Particle size, average discharge voltage and yield of the materials obtained in Examples 1-6 and Comparative Examples 1-5 Taking Example 1 as an example, the yield calculation formula per 100 mL is as follows: Where m represents the mass of phosphoric acid, M Li Relative molecular weight of the representative product lithium manganese iron phosphate, V m represents the volume of the organic solvent, M P Represents the relative molecular weight of phosphoric acid.

[0058] As can be seen from Table 1, the particle size width of the lithium manganese iron phosphate nanoparticles prepared by the preparation method of the present invention can be maintained in the nanoscale range of 30-60 nm, and the average discharge voltage of manganese in the lithium manganese iron phosphate positive electrode material synthesized using it as a precursor is greater than 3.9 V, which is at least about 80 mV higher than that of the comparative example. The polarization during the charge and discharge process is also smaller. The lower polarization degree and higher voltage are conducive to reducing heat loss during terminal use and improving the energy density per unit volume of the battery. In the pure hydrothermal system, the high polarity of the pure water solvent increases the initial solubility of the raw material, resulting in a higher single yield, but the obtained lithium manganese iron phosphate particle size is larger and the discharge voltage of the positive electrode material is significantly reduced. In the pure organic solvent system, due to the solubility of the initial raw material, the total concentration of the reaction material is limited, the single yield is low, and the obtained lithium manganese iron phosphate particle size is also larger. In the mixed solvent system, although the initial solubility of the raw material is improved compared with the pure organic solvent system, its solubility is still lower than that of the present invention, so the final yield is still lower, and the obtained lithium manganese iron phosphate particle size is also larger.

[0059] In summary, it should be noted that the above is only a preferred embodiment of the present invention and does not overly limit the present invention. Although the present invention has been described in detail with reference to the aforementioned embodiments, those skilled in the art can still make some simple deductions, replacements, or equivalent replacements of some of the technical features of the technical solutions described in the aforementioned embodiments without departing from the concept of the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing a high-voltage lithium manganese iron phosphate positive electrode material, characterized in that: The following steps are involved: 1) Under inert atmosphere, a soluble phosphate compound, a soluble divalent manganese salt, and a soluble divalent iron salt are added to deionized water at a molar ratio of P:(Mn+Fe) = 1:1, stirred until completely dissolved, and then an appropriate amount of antioxidant is added to obtain a mixed solution with a total metal salt molar concentration of 1-3 mol / L; 2) adding 1-10 times the volume of deionized water to the mixed solution obtained in step 1) and ultrasonically treating; 3) placing the mixed solution obtained in step 2) in a condensation reflux reactor at atmospheric pressure, slowly adding a lithium hydroxide solution having a concentration of 1-3 mol / L into the reactor, stirring thoroughly at a rate of 50-500 rpm, heating the reactor to 140-180° C., maintaining the temperature until the water is completely evaporated, and then maintaining the temperature for condensation reflux reaction for 2-12 hours. After the reaction is completed, filtering the solid product, washing, and drying to obtain lithium manganese iron phosphate nanoparticle powder; 4) The lithium manganese iron phosphate nanoparticle powder prepared in step 3) is dispersed with a carbon source in a solvent, subjected to ultrasonic treatment, spray-dried, and calcined at a high temperature under the protection of an inert gas to obtain a lithium manganese iron phosphate cathode material.

2. The method for preparing a high-voltage lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The soluble phosphate compound in step 1) is one or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate or phosphoric acid, the soluble divalent manganese salt is one or more of manganous sulfate, manganous nitrate or manganous chloride, and the soluble divalent iron salt is one or more of ferrous sulfate, ferrous nitrate or ferrous chloride. The molar ratio of Mn:Fe provided by the soluble divalent manganese salt and the soluble divalent iron salt is 1-4:

1.

3. The method for preparing a high-voltage lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The antioxidant in step 1) is ascorbic acid or citric acid, and the amount of the antioxidant added is 0.5-5% of the total mass of the metal salt.

4. The method for preparing a high-voltage lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In the step 2), the high boiling point organic solvent is one of ethylene glycol, glycerol, diformate or sulfolane.

5. The method for preparing a high-voltage lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The ultrasonic treatment time in step 2) is 3-15 min, and the ultrasonic frequency is 30-50 KHz.

6. The method for preparing a high-voltage lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step 3), lithium hydroxide solution is added to the mixed solution at a molar ratio of Li:P = 3-3.15:1, and the addition rate of the lithium hydroxide solution is 1-10 mL / min.

7. The method for preparing a high-voltage lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step 4), the carbon source is one of glucose, sucrose, phenolic resin or starch, the mass of the carbon source is 1%-20% of the mass of the lithium manganese iron phosphate nanoparticle powder, and the solvent is water or ethanol.

8. The method for preparing a high-voltage lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In the step 4), the ultrasonic treatment time is 10-60 min, the ultrasonic frequency is 30-50 KHz, the high-temperature calcination temperature is 600-800° C., and the calcination time is 4-8 h.

9. A high-voltage lithium manganese iron phosphate positive electrode material, characterized in that: Prepared by the method according to any one of claims 1 to 8.

10. A use of the high-voltage lithium manganese iron phosphate positive electrode material according to claim 9, characterized in that: Used to prepare the positive electrode of lithium-ion batteries.

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