Lithium manganese iron phosphate positive electrode material as well as preparation method and application thereof

Abstract

The invention provides a lithium manganese iron phosphate positive electrode material and a preparation method and application thereof. The preparation method of the lithium manganese iron phosphate positive electrode material comprises the following steps: providing or preparing polymer nano-microspheres with different size ranges, wherein the polymer nano-microspheres comprise at least one polar substituent of carboxyl, hydroxyl, aldehyde group, amino and amide; the method comprises the following steps: adding polymer nano-microspheres with different size ranges, a precipitator, a manganese source, an iron source and a phosphorus source into a first solution, carrying out precipitation reaction in an inert atmosphere, and filtering to obtain a precipitation intermediate; and mixing the precipitation intermediate with a lithium source, and sintering in an inert atmosphere to obtain the lithium iron manganese phosphate positive electrode material. According to the embodiment of the invention, the lithium manganese iron phosphate positive electrode material with high compaction density, high electronic conductivity and high ion diffusivity can be prepared, and the electrochemical performance of the lithium manganese iron phosphate positive electrode material can be favorably exerted. Meanwhile, the material is excellent in processability, and the application feasibility of the lithium iron manganese phosphate positive electrode material can be improved.

Description

Technical Field

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

[0002] Lithium-ion batteries dominate the power supply market for lightweight electronic products with their excellent cycle life, energy density and environmental friendliness. In addition, they are also used in energy storage systems, hybrid vehicles and electric vehicles. Lithium iron manganese phosphate cathode material combines the high safety of lithium iron phosphate and the high voltage characteristics of lithium manganese phosphate, and is considered to be the next generation of commercial cathode material with the potential to replace traditional LiFePO4. Lithium iron manganese phosphate cathode material has the advantages of environmental friendliness, high energy density and good cycle stability, and is suitable for electric vehicles and energy storage systems. However, the lithium iron manganese phosphate cathode material produced by the existing preparation method of lithium iron manganese phosphate cathode material has low electronic conductivity and low ion diffusion rate, which hinders the practical application of lithium iron manganese phosphate cathode material. Summary of the invention

[0003] The present application provides a lithium iron manganese phosphate positive electrode material and a preparation method and application thereof, so as to solve the problems of low electronic conductivity, low ion diffusion rate, low compaction density, poor processing performance and poor cycle performance of the lithium iron manganese phosphate positive electrode material.

[0004] In a first aspect, the present application provides a method for preparing a lithium iron manganese phosphate positive electrode material, the method for preparing a lithium iron manganese phosphate positive electrode material comprising:

[0005] Providing or preparing polymer nanoparticles of different size ranges, wherein the polymer nanoparticles include at least one polar substituent selected from the group consisting of carboxyl, hydroxyl, aldehyde, amino, and amide;

[0006] Adding the polymer nanoparticles of different size ranges, a precipitant, a manganese source, an iron source, and a phosphorus source into the first solution, and causing a precipitation reaction under an inert atmosphere, and filtering to obtain a precipitated intermediate;

[0007] The precipitated intermediate is mixed with a lithium source and sintered in an inert atmosphere to obtain the lithium manganese iron phosphate positive electrode material.

[0008] In the present application, polymer nanospheres of different sizes modified by polar substituents such as carboxyl, hydroxyl, aldehyde, amino or amide are introduced as templates and carbon sources, and combined with carburizing heat treatment, lithium iron manganese phosphate positive electrode materials are prepared. During the heat treatment process, the polymer nanospheres can not only construct a conductive carbon layer and optimize the electron transfer path, but also refine the lithium iron manganese phosphate particles and inhibit the aggregation of lithium iron manganese phosphate nanoparticles, thereby improving the processing performance of lithium iron manganese phosphate. Polar substituents such as carboxyl, hydroxyl, aldehyde, amino or amide in polymer nanospheres can effectively improve the aggregation between lithium iron manganese phosphate positive electrode material particles, which is beneficial to improve the processing performance of lithium iron manganese phosphate positive electrode materials. And polymer nanospheres of different size ranges can regulate the particle distribution of lithium iron manganese phosphate, thereby realizing particle grading, obtaining a lithium iron manganese phosphate positive electrode material with a high compaction density, and improving the problem of reduced compaction density caused by particle refinement to a certain extent. The interconnected conductive carbon coating formed after carbonization of the polymer nanospheres significantly improves the diffusion coefficient of lithium ions in the lithium iron manganese phosphate positive electrode material, which helps to improve the discharge specific capacity and cycle performance of the lithium iron manganese phosphate positive electrode material. In one embodiment, the size ranges of the polymer nanospheres do not overlap.

[0009] The method for preparing the lithium iron manganese phosphate positive electrode material provided in the embodiment of the present application can prepare a lithium iron manganese phosphate positive electrode material with high electronic conductivity, high ion diffusion rate and high compaction density, and can also improve the processing performance and cycle performance of the lithium iron manganese phosphate positive electrode material, which is beneficial to the electrochemical performance of the lithium iron manganese phosphate positive electrode material.

[0010] In one embodiment, the method for preparing the polymer nanospheres comprises:

[0011] dissolving an emulsion stabilizer in the second solution to obtain an emulsion stabilizer solution;

[0012] Inert gas is introduced into the emulsion stabilizer solution, and then polymer monomer and polar substituent monomer are added, and an emulsion is obtained after heating reaction;

[0013] The initiator solution is added to the emulsion, and a polymerization reaction is carried out under an inert atmosphere to obtain the polymer nanoparticles.

[0014] In one embodiment, the step of mixing the precipitated intermediate with a lithium source and sintering in an inert atmosphere to obtain the lithium iron manganese phosphate positive electrode material comprises:

[0015] The precipitated intermediate is mixed with a lithium source, and sintered in an inert atmosphere to obtain a precursor;

[0016] After the carbon source and the precursor are uniformly mixed, they are sintered in an inert atmosphere to obtain the lithium manganese iron phosphate positive electrode material.

[0017] In one embodiment, the polymer nanospheres have three different size ranges, and the particle sizes of the polymer nanospheres in the three different size ranges are 50nm-100nm, 200nm-300nm, and 400nm-500nm, respectively.

[0018] In one embodiment, the mass ratio of the polymer nanoparticles of three different size ranges is 1:(1-9):(1-15).

[0019] In one embodiment, the molar ratio of the polymer nanospheres to the precipitant is 1:(45-50).

[0020] In one embodiment, the molar ratio of the manganese element in the manganese source, the iron element in the iron source, and the phosphorus element in the phosphorus source is (0.70-0.88):(0.20-0.22):1.

[0021] In one embodiment, the molar ratio of the iron element in the iron source to the precipitant is 1:(128-140).

[0022] In one embodiment, the molar ratio of the lithium element in the lithium source to the precipitated intermediate is (1-1.2):1.

[0023] In one embodiment, the first solution includes water and ethylene glycol, and the volume ratio of water to ethylene glycol is 1:(3-5).

[0024] In one embodiment, the conditions of the precipitation reaction include at least one of a reaction temperature of 80° C. to 90° C., a stirring speed of 200 rpm to 400 rpm, and a reaction time of 2 h to 4 h.

[0025] In one embodiment, the molecular weight of the emulsion stabilizer is 8000 g / mol to 12000 g / mol.

[0026] In one embodiment, the second solution includes water and ethanol, and the volume ratio of water to ethanol is 1:1.

[0027] In one embodiment, the added volume of the polar substituent monomer is 5% to 8% of the added volume of the polymer monomer.

[0028] In one embodiment, the concentration of the initiator solution is 0.15 g / mL to 0.25 g / mL.

[0029] In a second aspect, the present application provides a lithium iron manganese phosphate positive electrode material, wherein the lithium iron manganese phosphate positive electrode material is prepared by the preparation method of the lithium iron manganese phosphate positive electrode material as described above.

[0030] In a third aspect, the present application provides a positive electrode plate, which includes a lithium iron manganese phosphate positive electrode material prepared by the preparation method of the lithium iron manganese phosphate positive electrode material as described above, or includes the lithium iron manganese phosphate positive electrode material as described above.

[0031] In a fourth aspect, the present application provides a secondary battery, comprising a negative electrode plate, a separator, and the positive electrode plate as described above. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings required for use in the embodiments of the present application will be described below.

[0033] Figure 1 It is the first charge and discharge curve diagram of each embodiment and comparative example of the present application at a rate of 0.1C;

[0034] Figure 2 It is the first charge and discharge curve diagram of each embodiment and comparative example of the present application at a 1C rate;

[0035] Figure 3 is a primary particle size distribution diagram of Example 1 of the present application;

[0036] Figure 4 is the primary particle size distribution diagram of Example 4 of the present application;

[0037] Figure 5 This is the primary particle size distribution diagram of Comparative Example 1 of the present application;

[0038] Figure 6 is a scanning electron microscope (SEM) morphology image of Example 1 of the present application;

[0039] Figure 7 is a scanning electron microscope (SEM) morphology image of Example 4 of the present application;

[0040] Figure 8 This is a scanning electron microscope (SEM) morphology image of Comparative Example 1 of the present application. DETAILED DESCRIPTION

[0041] The technical solutions in the embodiments of the present application will be described below in conjunction with the drawings in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all of the embodiments.

[0042] In this document, the terms "first", "second", etc. are used for descriptive purposes only and should not be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features. Thus, a feature defined as "first" or "second" may explicitly or implicitly include one or more of the features. In the description of this application, unless otherwise specified, "plurality" means two or more.

[0043] In addition, in this article, directional terms such as "upper" and "lower" are defined relative to the orientation of the structure schematically placed in the drawings. It should be understood that these directional terms are relative concepts. They are used for relative description and clarification, and they can change accordingly according to the change of the orientation of the structure.

[0044] Batteries such as lithium-ion secondary batteries usually include a positive electrode, a negative electrode and a separator. The positive electrode is used to provide active lithium ions to the battery, and the negative electrode can be used as a carrier of active lithium ions and electrons during the battery charging and discharging process. The separator is arranged between the positive electrode and the negative electrode to separate the positive electrode and the negative electrode. The separator is used to allow active lithium ions to pass through and block electrons from passing through. The positive electrode includes a positive electrode material, which is used to provide active lithium ions for the lithium-ion battery. The active lithium ions migrate between the positive electrode and the negative electrode, thereby realizing the battery charging and discharging.

[0045] Lithium iron manganese phosphate cathode materials have the advantages of environmental friendliness, high energy density, and good cycle stability, and have broad application prospects in electric vehicles and energy storage systems. However, lithium iron manganese phosphate cathode materials still have the following problems: low electronic conductivity, low ion diffusion rate, low compaction density, poor processing performance and poor cycle performance, which hinder the practical application of lithium iron manganese phosphate cathode materials.

[0046] The present application provides a method for preparing a lithium iron manganese phosphate positive electrode material. The method for preparing a lithium iron manganese phosphate positive electrode material comprises step S1, step S2 and step S3. The specific steps are as follows:

[0047] Step S1, providing or preparing polymer nanoparticles of different size ranges, wherein the polymer nanoparticles include at least one polar substituent selected from the group consisting of carboxyl, hydroxyl, aldehyde, amino, and amide;

[0048] Step S2, adding polymer nanoparticles of different size ranges, a precipitant, a manganese source, an iron source, and a phosphorus source into the first solution, and causing a precipitation reaction under an inert atmosphere, and filtering to obtain a precipitated intermediate;

[0049] Step S3, mixing the precipitated intermediate with a lithium source, and sintering in an inert atmosphere to obtain a lithium iron manganese phosphate positive electrode material.

[0050] Wherein, in step S1, the polymer nanospheres are nanoscale spherical particles formed by polymers. During the nucleation and growth phases of the lithium manganese iron phosphate particles, the polymer nanospheres can be used as templates. The polymer nanospheres can control the size, morphology, structure, etc. of the lithium manganese iron phosphate particles by using their spatial confinement effect.

[0051] In addition, the polymer nanospheres are polymer nanospheres modified with polar substituents such as carboxyl, hydroxyl, aldehyde, amino or amide. The polar substituents such as carboxyl, hydroxyl, aldehyde, amino or amide in the polymer nanospheres can effectively improve the aggregation between the particles of the lithium iron manganese phosphate positive electrode material, which is beneficial to improving the processing performance of the lithium iron manganese phosphate positive electrode material.

[0052] In one embodiment, polar substituents such as carboxyl, hydroxyl, aldehyde, amino or amide are distributed on the outer surface of the polymer nanospheres.

[0053] In step S2, under the action of a precipitant, the manganese source, iron source and phosphorus source are deposited with the polymer nanospheres as templates to obtain a precipitated intermediate. In the precipitated intermediate, the manganese source, iron source and phosphorus source reacted with the precipitant can achieve good contact with the polymer nanospheres.

[0054] In step S3, the lithium source reacts with manganese, iron and phosphorus in the precipitation intermediate to generate lithium manganese iron phosphate. The polymer nanoparticles in the precipitation intermediate are carbonized and coated on the surface of the lithium manganese iron phosphate.

[0055] In one embodiment, the size ranges of the polymer nanoparticles do not overlap. The polymer nanoparticles with gradient size ranges can be used to prepare lithium manganese iron phosphate particles with different size ranges.

[0056] In the present application, polymer nanospheres of different sizes modified with polar substituents such as carboxyl, hydroxyl, aldehyde, amino or amide are introduced as templates and carbon sources, and combined with carburizing heat treatment, lithium iron manganese phosphate positive electrode materials are prepared. During the heat treatment process, the polymer nanospheres can not only optimize the electron transfer path by constructing a conductive carbon layer, but the polar substituents in the polymer nanospheres can also refine the lithium iron manganese phosphate particles and inhibit the aggregation of lithium iron manganese phosphate nanoparticles, thereby improving the processing performance of lithium iron manganese phosphate. Polymer nanospheres of different size ranges can regulate the particle distribution of lithium iron manganese phosphate, thereby achieving particle grading, and obtaining a lithium iron manganese phosphate positive electrode material with a high compaction density, which improves the problem of reduced compaction density caused by particle refinement to a certain extent. The interconnected conductive carbon coating formed after the carbonization of the polymer nanospheres significantly improves the diffusion coefficient of lithium ions in the lithium iron manganese phosphate positive electrode material, which helps to improve the discharge specific capacity and cycle performance of the lithium iron manganese phosphate positive electrode material. The method for preparing the lithium iron manganese phosphate positive electrode material provided in the embodiment of the present application can prepare a lithium iron manganese phosphate positive electrode material with high electronic conductivity, high ion diffusion rate and high compaction density, and can improve the processing performance and cycle performance of the lithium iron manganese phosphate positive electrode material, which is beneficial to the electrochemical performance of the lithium iron manganese phosphate positive electrode material.

[0057] In one embodiment, the polymer nanospheres include at least one of polystyrene nanospheres, polyvinyl chloride nanospheres, polyethylene nanospheres, polypropylene nanospheres, and polytetrafluoroethylene nanospheres. Exemplarily, the polymer nanospheres may be carboxylated polystyrene nanospheres.

[0058] In one embodiment, the polymer nanospheres have three different size ranges, and the particle sizes of the polymer nanospheres in the three different size ranges are 50 nm to 100 nm, 200 nm to 300 nm, and 400 nm to 500 nm, respectively.

[0059] In the embodiment of the present application, the particle size of the polymer nano-microspheres is set within the above-mentioned small, medium and large ranges to achieve particle grading, which is conducive to synthesizing lithium iron manganese phosphate positive electrode materials with suitable particle sizes, thereby improving the electrochemical properties and processing properties of lithium iron manganese phosphate positive electrode materials, and improving the compaction density of lithium iron manganese phosphate positive electrode materials. Specifically, polymer nano-microspheres of 50nm to 100nm can refine lithium iron manganese phosphate particles, improve processing properties, and are also conducive to accelerating the migration and diffusion of lithium ions to improve kinetic properties. Polymer nanoparticles of 400nm to 500nm guarantee a certain compaction density. As a transition between the above-mentioned small size range and the large size range, polymer nano-microspheres of 200nm to 300nm can avoid the particle size of the lithium iron manganese phosphate particles formed being too small or too large, which is conducive to improving the cycle performance.

[0060] In one embodiment, the mass ratio of polymer nanoparticles of three size ranges is 1:(1-9):(1-15).

[0061] In the embodiment of the present application, the added mass proportion of polymer nanospheres with a size range of 50nm to 100nm will affect the performance of the lithium iron manganese phosphate positive electrode material. If the added mass of polymer nanospheres in a small size range is too small, the proportion of polymer nanospheres in the medium size range and the large size range is too large, and it is difficult to improve the migration and conduction efficiency of lithium ions through polymer nanospheres in a small size range. If the added mass of polymer nanospheres in a small size range is too large, and the proportion of polymer nanospheres in the medium size range and the large size range is too small, the lithium iron manganese phosphate particles will be too small, reducing the compaction density, and causing too little active material in the lithium iron manganese phosphate positive electrode material to cause a decrease in electrochemical performance. In addition, if the added mass of polymer nanospheres in a small size range and a large size range is too large, the small particles of lithium iron manganese phosphate material are prone to side reactions, which is not conducive to improving the cycle performance of the battery, and the large particles of lithium iron manganese phosphate material are too large. Increase the charge transfer impedance and interface impedance, which will also have a negative impact on the cycle performance of the battery. Therefore, it is necessary to introduce a certain proportion of medium-sized polymer nanospheres to ensure the number of medium-sized lithium manganese iron phosphate particles.

[0062] The embodiment of the present application controls the added quality of polymer nanospheres of different size ranges within an appropriate range, combines the advantages of polymer nanospheres of different size ranges, and can take into account the particle distribution, compaction density, processing performance and electrochemical performance of lithium manganese iron phosphate.

[0063] In one embodiment, the first solution includes water and ethylene glycol.

[0064] In one embodiment, the volume ratio of water to ethylene glycol is 1:(3-5), which is conducive to the occurrence of coprecipitation reaction.

[0065] In one embodiment, the reaction temperature in the precipitation reaction is 80°C to 90°C.

[0066] In one embodiment, stirring is performed during the precipitation reaction, and the stirring speed is 200 rpm to 400 rpm. The stirring speed affects the particle size of the precipitated intermediate, and the stirring speed affects the growth of the manganese source, iron source and phosphorus source on the polymer nanospheres. In the embodiment of the present application, by controlling the stirring speed of the precipitation reaction within a suitable range, it is beneficial to prepare a lithium manganese iron phosphate positive electrode material with better electrochemical performance.

[0067] In one embodiment, the reaction time in the precipitation reaction is 2 h to 4 h.

[0068] In one embodiment, the molar ratio of the polymer nanospheres to the precipitant is 1: (45-50). In the embodiment of the present application, by regulating the relative content of the precipitant, it is beneficial for the polymer nanospheres, manganese source, iron source and phosphorus source to precipitate completely. By regulating the relative content of the polymer nanospheres, it is beneficial for the manganese source, iron source and phosphorus source to precipitate and grow better in the polymer nanospheres, so that the polymer nanospheres can be better used to improve the electronic conductivity, ion diffusion rate and compaction density of the lithium manganese iron phosphate positive electrode material.

[0069] In one embodiment, the molar ratio of manganese in the manganese source, iron in the iron source, and phosphorus in the phosphorus source is (0.70-0.88):(0.20-0.22):1.

[0070] In one embodiment, the molar ratio of the iron element in the iron source to the precipitant is 1:(128-140). In the examples of the present application, the amount of the precipitant added is regulated to facilitate complete precipitation of the iron source.

[0071] In one embodiment, the molar ratio of lithium element in the lithium source to the precipitated intermediate is (1-1.2): 1. In the examples of the present application, the amount of the precipitant added is regulated to facilitate complete precipitation of the lithium source.

[0072] In one embodiment, the precipitant includes one or more of urea, ammonia water, sodium carbonate, sodium oxalate and the like.

[0073] In one embodiment, the manganese source includes one or more of manganese acetate, manganese nitrate, manganese sulfate, manganese carbonate and manganese chloride.

[0074] In one embodiment, the iron source includes one or more of ferric acetate, ferric nitrate, ferric sulfate, ferric carbonate and ferric chloride.

[0075] In one embodiment, the phosphorus source includes one or more of sodium phosphate, calcium phosphate, sodium dihydrogen phosphate, ammonium dihydrogen phosphate, and the like.

[0076] In one embodiment, the lithium source includes one or more of lithium hydroxide, lithium carbonate, lithium nitrate, lithium sulfate, lithium chloride, lithium phosphate, lithium acetate, and the like.

[0077] In one embodiment, the inert atmosphere comprises one or more of nitrogen, helium, or argon.

[0078] In one embodiment, the method for preparing polymer nanospheres includes step S1-1, step S1-2 and step S1-3, and the specific steps are as follows:

[0079] Step S1-1, dissolving an emulsion stabilizer in a second solution to obtain an emulsion stabilizer solution;

[0080] Step S1-2, introducing an inert gas into the emulsion stabilizer solution, and then adding a polymer monomer and a polar substituent monomer, and heating the reaction to obtain an emulsion;

[0081] Step S1-3, adding the initiator solution to the emulsion, and performing polymerization reaction under an inert atmosphere to obtain polymer nanospheres.

[0082] In step S1-1, the emulsion stabilizer is used to stabilize the emulsion formed by the polymer monomer and the polar substituent monomer in step S1-2, so that the size of the prepared polymer nanoparticles is more uniform and controllable. The emulsion stabilizer includes polyvinyl pyrrolidone.

[0083] In step S1-2, the polymer monomer is used to polymerize to form a polymer, and the polar substituent monomer is used to modify the polymer. The polar substituent monomer is a monomer used to form a polar substituent such as a carboxyl group, a hydroxyl group, an aldehyde group, an amino group or an amide. The polymer monomer and the polar substituent monomer undergo an emulsification reaction to obtain a colloid particle.

[0084] In step S1-3, under the action of an initiator, the polymer monomers in the colloid particles are polymerized to form a polymer. Since the polymer monomers are polymerized in the colloid particles, the polymer formed by polymerization is in the form of microspheres. By controlling the size of the colloid particles, the particle size of the polymer microspheres can be regulated to be nanometer-level, that is, polymer nanospheres are obtained. Since the colloid particles also include polar substituent monomers, the prepared polymer nanospheres include at least one polar substituent selected from carboxyl, hydroxyl, aldehyde, amino, and amide.

[0085] In the embodiment of the present application, the prepared polymer nanospheres have uniform particle size and good morphology. The polymer nanospheres are used as templates and carbon sources to prepare lithium manganese iron phosphate positive electrode materials, thereby improving the electronic conductivity, ion diffusion rate, compaction density and processing performance of the lithium manganese iron phosphate positive electrode materials.

[0086] In one embodiment, the polymer nanospheres include carboxyl groups, that is, the polymer nanospheres are carboxylated polymer nanospheres. The polar substituent monomer is an acid functional monomer. The polar substituent monomer includes one or more of maleic anhydride, acrylic acid, methacrylic acid or methyl methacrylate.

[0087] In one embodiment, the initiator includes one or more of azobisisobutylamidine hydrochloride, azobisisobutylimidazoline hydrochloride, azobiscyanovaleric acid, and azobisisopropylimidazoline.

[0088] In one embodiment, the temperature of the emulsification reaction in step S1-2 is 70°C to 80°C, and the reaction time is 30min to 60min.

[0089] In one embodiment, the polymerization reaction temperature in step S1-3 is 70°C to 80°C, and the reaction time is 24h to 48h.

[0090] In one embodiment, the molecular weight of the emulsion stabilizer is 8000 g / mol to 12000 g / mol. In the present embodiment, the molecular weight of the emulsion stabilizer is regulated so that the emulsion stabilizer has a better effect of stabilizing the emulsion, and the size of the prepared polymer nanospheres is more uniform and controllable.

[0091] In one embodiment, the second solution comprises water and ethanol.

[0092] In one embodiment, the volume ratio of water to ethanol is 1:1.

[0093] In one embodiment, the volume of the polar substituent monomer added is 5% to 8% of the volume of the polymer monomer added, that is, the ratio of the volume of the polar substituent monomer to the volume of the polymer monomer is 5% to 8%.

[0094] In one embodiment, the concentration of the initiator solution is 0.15 g / mL to 0.25 g / mL, which is beneficial to the occurrence of the polymerization reaction and the completion of the polymerization.

[0095] In one embodiment, the precipitated intermediate is mixed with a lithium source, and sintered in an inert atmosphere to obtain a lithium iron manganese phosphate positive electrode material, including steps S3-1 and S3-2. The specific steps are as follows:

[0096] Step S3-1, mixing the precipitated intermediate with a lithium source, and sintering in an inert atmosphere to obtain a precursor;

[0097] Step S3-2, after the carbon source and the precursor are evenly mixed, they are sintered in an inert atmosphere to obtain a lithium manganese iron phosphate positive electrode material.

[0098] Among them, in step S3-1, although the polymer nano-microspheres in the precipitation intermediate can form a coating layer after carbonization to coat the outer surface of the lithium iron manganese phosphate, the coating layer is relatively thin and theoretically does not completely coat the lithium iron manganese phosphate. In the embodiment of the present application, in step S3-2, a carbon source is further added to supplement the conductive properties of the lithium iron manganese phosphate particles, and the carbonized polymer nano-microspheres are used to coat the surface of the lithium iron manganese phosphate particles, further improving the electronic conductivity and ion diffusion rate of the lithium iron manganese phosphate positive electrode material.

[0099] In one embodiment, the carbon source includes one or more of sucrose, glucose, fructose, cellulose, starch, citric acid, tannic acid, graphene, polyvinyl pyrrolidone, and the like.

[0100] In one embodiment, the mass ratio of the precursor to the carbon source is 100:(10-20). In the embodiment of the present application, by controlling the amount of the carbon source added, while improving the electronic conductivity and lithium ion diffusion coefficient of the lithium iron manganese phosphate positive electrode material, the mass of the lithium iron manganese phosphate positive electrode material will not be increased too much, so that the lithium iron manganese phosphate positive electrode material has a higher energy density.

[0101] In one embodiment, the sintering temperature in step S3-2 is 700°C to 800°C, the heating rate is 5°C / min, and the sintering time is 3h to 4h.

[0102] The present application provides a lithium iron manganese phosphate positive electrode material, and the lithium iron manganese phosphate positive electrode material is prepared by the preparation method of the lithium iron manganese phosphate positive electrode material as described above.

[0103] The present application provides a positive electrode plate, which includes the lithium iron manganese phosphate positive electrode material as described above, or includes the lithium iron manganese phosphate positive electrode material prepared by the preparation method of the lithium iron manganese phosphate positive electrode material as described above.

[0104] The present application provides a secondary battery, which includes a negative electrode plate, a separator and the positive electrode plate as described above.

[0105] In order to illustrate the beneficial effects of the method of the present application, the present application also provides the following examples and comparative examples:

[0106] Example 1

[0107] Embodiment 1 provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising:

[0108] Step 1-1: 0.2 g, 0.6 g and 1.2 g of polyvinyl pyrrolidone with a molecular weight of 8000 are respectively placed in 100 mL of a mixed solution of deionized water and ethanol, wherein the volume ratio of deionized water to ethanol is 1:1, and stirred to dissolve until completely dissolved, to obtain three polyvinyl pyrrolidone solutions of different concentrations;

[0109] Step 1-2: nitrogen was continuously introduced into the three polyvinyl pyrrolidone solutions of different concentrations for 30 min, and then 10 mL of styrene and 0.5 mL of methacrylic acid were added, and the mixture was heated in an oil bath at 70° C. for 30 min to obtain a white emulsion;

[0110] Step 1-3: Add 1.2 mL of 0.15 g / mL azobisisobutylamidine hydrochloride aqueous solution to the above white emulsion, and allow polymerization to occur in a nitrogen atmosphere at 70°C for 24 hours. Wash the precipitate obtained by the polymerization reaction with water until it is neutral, rinse with ethanol, and dry it in a vacuum at 60°C for 24 hours to obtain carboxylated polystyrene nanospheres with three different size ranges of 50 nm to 70 nm, 200 nm to 230 nm, and 400 nm to 420 nm.

[0111] Step 2: 0.01 g of carboxylated polystyrene nanospheres with a size range of 50 nm to 70 nm, 0.06 g of carboxylated polystyrene nanospheres with a size range of 200 nm to 230 nm, 0.13 g of carboxylated polystyrene nanospheres with a size range of 400 nm to 420 nm, 4.84 g of urea, 2.4 mmol MnSO4·H2O, 0.6 mmol FeSO4·7H2O and 3.0 mmol NH4H2PO4 were slowly added to a mixed solvent of 10 mL H2O and 40 mL ethylene glycol, and the mixture was reacted under continuous magnetic stirring at 80°C in a nitrogen atmosphere for 2 h to obtain a precipitated intermediate, and the stirring speed was 200 rpm.

[0112] Step 3-1: After filtering, washing and drying the precipitated intermediate, 3.0 mmol Li2CO3 was added, and after grinding, it was pre-sintered at 400°C for 4 hours in a nitrogen atmosphere. After cooling to room temperature, the sample was taken out and ground, sintered at 800°C for 8 hours, and finally annealed at 700°C for 10 hours. The black precursor powder was obtained by natural cooling, and the heating rate was 5°C / min.

[0113] Step 3-2: After sucrose and the above-mentioned black precursor powder are uniformly mixed in a mass ratio of 100:10, sintering at 700°C for 3h in a nitrogen atmosphere to obtain a gray-black lithium manganese iron phosphate positive electrode material, with a heating rate of 5°C / min.

[0114] Example 2

[0115] Embodiment 2 provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising:

[0116] Step 1-1: 0.3 g, 0.9 g and 1.4 g of polyvinyl pyrrolidone with a molecular weight of 8000 are placed in a mixed solution of 120 mL of deionized water and ethanol, wherein the volume ratio of deionized water to ethanol is 1:1.5, and stirred until completely dissolved to obtain three polyvinyl pyrrolidone solutions of different concentrations;

[0117] Step 1-2: nitrogen was continuously introduced into the three polyvinyl pyrrolidone solutions of different concentrations for 40 min, and then 12 mL of styrene and 0.72 mL of methacrylic acid were added, and the mixture was heated in an oil bath at 80° C. for 40 min to obtain a white emulsion;

[0118] Step 1-3: 1.4 mL of 0.20 g / mL azobisisobutylamidine hydrochloride aqueous solution was added to the above white emulsion, and a polymerization reaction occurred in a nitrogen atmosphere at 75°C for 24 hours. The precipitate obtained by the polymerization reaction was washed with water until neutral, then rinsed with ethanol and dried in a vacuum at 60°C for 24 hours to obtain carboxylated polystyrene nanospheres with three different size ranges of 60 nm to 80 nm, 210 nm to 230 nm, and 410 nm to 440 nm.

[0119] Step 2: 0.02 g of carboxylated polystyrene nanospheres with a size range of 60 nm to 80 nm, 0.08 g of carboxylated polystyrene nanospheres with a size range of 210 nm to 230 nm, 0.13 g of carboxylated polystyrene nanospheres with a size range of 410 nm to 440 nm, 4.67 g of urea, 2.3 mmol MnSO4·H2O, 0.6 mmol FeSO4·7H2O and 2.9 mmol NH4H2PO4 were slowly added to a mixed solvent of 10 mL H2O and 30 mL ethylene glycol, and the reaction was carried out under continuous magnetic stirring at 80°C in a nitrogen atmosphere for 3 h to obtain a precipitated intermediate, and the stirring speed was 300 rpm.

[0120] Step 3-1: After filtering, washing and drying the precipitated intermediate, 3.0 mmol Li2CO3 was added, and after grinding, it was pre-sintered at 450°C for 4 hours in a nitrogen atmosphere. After cooling to room temperature, the sample was taken out and ground, and sintered at 800°C for 9 hours. Finally, it was annealed at 700°C for 10 hours, and naturally cooled to obtain a black precursor powder. The heating rate was 5°C / min.

[0121] Step 3-2: After sucrose and the above-mentioned black precursor powder are uniformly mixed in a mass ratio of 100:12, sintering at 750°C in a nitrogen atmosphere for 3 hours to obtain a gray-black lithium manganese iron phosphate positive electrode material, with a heating rate of 5°C / min.

[0122] Example 3

[0123] Embodiment 3 provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising:

[0124] Step 1-1: 0.2 g, 0.6 g and 1.2 g of polyvinyl pyrrolidone with a molecular weight of 10,000 are placed in a mixed solution of 130 mL of deionized water and ethanol, wherein the volume ratio of deionized water to ethanol is 1:1.8, and stirred until completely dissolved to obtain three polyvinyl pyrrolidone solutions of different concentrations;

[0125] Step 1-2: nitrogen was continuously introduced into the three polyvinyl pyrrolidone solutions of different concentrations for 50 min, and then 15 mL of styrene and 0.9 mL of methacrylic acid were added, and the mixture was heated in an oil bath at 80° C. for 60 min to obtain a white emulsion;

[0126] Step 1-3: 1.5 mL of azobisisobutyramidine hydrochloride aqueous solution with a concentration of 0.20 g / mL was added to the above white emulsion, and a polymerization reaction occurred in a nitrogen atmosphere at 80°C for 36 hours. The precipitate obtained by the polymerization reaction was washed with water until neutral, then rinsed with ethanol and dried in a vacuum at 60°C for 24 hours to obtain carboxylated polystyrene nanospheres with a size distribution in three different size ranges of 50-70 nm, 200-230 nm, and 400-420 nm.

[0127] Step 2: 0.05 g of carboxylated polystyrene nanospheres with a size range of 50 nm to 70 nm, 0.1 g of carboxylated polystyrene nanospheres with a size range of 200 nm to 230 nm, 0.1 g of carboxylated polystyrene nanospheres with a size range of 400 nm to 420 nm, 5.0 g of urea, 2.4 mmol MnSO4·H2O, 0.6 mmol FeSO4·7H2O and 3.0 mmol NH4H2PO4 were slowly added to a mixed solvent of 10 mL H2O and 50 mL ethylene glycol, and the mixture was reacted under continuous magnetic stirring at 90° C. in a nitrogen atmosphere for 4 h to obtain a precipitated intermediate, and the stirring speed was 400 rpm.

[0128] Step 3-1: After filtering, washing and drying the precipitated intermediate, 3.1 mmol Li2CO3 was added, and after grinding, it was pre-sintered at 400°C for 4 hours in a nitrogen atmosphere. After cooling to room temperature, the sample was taken out and ground, sintered at 800°C for 10 hours, and finally annealed at 700°C for 10 hours. The black precursor powder was obtained by natural cooling, and the heating rate was 10°C / min.

[0129] Step 3-2: After sucrose and the above-mentioned black precursor powder are uniformly mixed in a mass ratio of 100:15, sintering at 800°C in a nitrogen atmosphere for 4 hours to obtain a gray-black lithium manganese iron phosphate positive electrode material, with a heating rate of 10°C / min.

[0130] Example 4

[0131] Embodiment 4 provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising:

[0132] Step 1-1: 0.2 g, 0.6 g and 1.4 g of polyvinyl pyrrolidone with a molecular weight of 10,000 are placed in a 150 mL mixed solution of deionized water and ethanol, wherein the volume ratio of deionized water to ethanol is 1:2, and stirred to dissolve until completely dissolved, to obtain three polyvinyl pyrrolidone solutions of different concentrations;

[0133] Step 1-2: nitrogen was continuously introduced into the three polyvinyl pyrrolidone solutions of different concentrations for 60 min, and then 15 mL of styrene and 1.05 mL of methacrylic acid were added, and the mixture was heated in an oil bath at 80° C. for 60 min to obtain a white emulsion;

[0134] Step 1-3: 1.6 mL of azobisisobutylamidine hydrochloride aqueous solution with a concentration of 0.20 g / mL was added to the above white emulsion, and a polymerization reaction occurred in a nitrogen atmosphere at 80°C for 48 hours. The precipitate obtained by the polymerization reaction was washed with water until neutral, then rinsed with ethanol and dried in a vacuum at 60°C for 24 hours to obtain carboxylated polystyrene nanospheres with three different size ranges of 60 nm to 80 nm, 210 nm to 230 nm, and 410 nm to 440 nm.

[0135] Step 2: 0.07 g of carboxylated polystyrene nanospheres with a size range of 60 nm to 80 nm, 0.07 g of carboxylated polystyrene nanospheres with a size range of 210 nm to 230 nm, 0.07 g of carboxylated polystyrene nanospheres with a size range of 410 nm to 440 nm, 4.84 g of urea, 2.2 mmol MnSO4·H2O, 0.6 mmol FeSO4·7H2O and 2.8 mmol NH4H2PO4 were slowly added to a mixed solvent of 10 mL H2O and 40 mL ethylene glycol, and the reaction was carried out under continuous magnetic stirring at 80°C in a nitrogen atmosphere for 4 h to obtain a precipitated intermediate, and the stirring speed was 400 rpm.

[0136] Step 3-1: After filtering, washing and drying the precipitated intermediate, 2.0 mmol Li2CO3 was added, and after grinding, it was pre-sintered at 400°C for 5 h in a nitrogen atmosphere. After cooling to room temperature, the sample was taken out and ground, sintered at 800°C for 8 h, and finally annealed at 700°C for 10 h. The black precursor powder was obtained by natural cooling, and the heating rate was 10°C / min.

[0137] Step 3-2: After sucrose and the above-mentioned black precursor powder are uniformly mixed in a mass ratio of 100:15, sintering at 800°C in a nitrogen atmosphere for 4 hours to obtain a gray-black lithium manganese iron phosphate positive electrode material, with a heating rate of 10°C / min.

[0138] Example 5

[0139] Example 5 provides a method for preparing a lithium iron manganese phosphate positive electrode material. The difference from Example 1 is that Example 5 does not include step 3-2, and the black precursor powder obtained in step 3-1 is the lithium iron manganese phosphate positive electrode material.

[0140] Example 6

[0141] Example 6 provides a method for preparing a lithium iron manganese phosphate positive electrode material. The difference from Example 1 is that in step 2 of Example 6, the added carboxylated polystyrene nanospheres are as follows: 0.1g of carboxylated polystyrene nanospheres with a size range of 50nm to 70nm, 0.05g of carboxylated polystyrene nanospheres with a size range of 200nm to 230nm, and 0.05g of carboxylated polystyrene nanospheres with a size range of 400nm to 420nm.

[0142] Example 7

[0143] Example 7 provides a method for preparing a lithium manganese iron phosphate positive electrode material. The difference from Example 1 is that in step 2 of Example 7, the added carboxylated polystyrene nanospheres are as follows: 0.06 g of carboxylated polystyrene nanospheres with a size range of 200 nm to 230 nm, and 0.13 g of carboxylated polystyrene nanospheres with a size range of 400 nm to 420 nm.

[0144] Example 8

[0145] Example 8 provides a method for preparing a lithium manganese iron phosphate positive electrode material. The difference from Example 1 is that in step 2 of Example 8, the carboxylated polystyrene nanospheres added are as follows: 0.01 g of carboxylated polystyrene nanospheres with a size range of 50 nm to 70 nm and 0.13 g of carboxylated polystyrene nanospheres with a size range of 400 nm to 420 nm.

[0146] Example 9

[0147] Example 9 provides a method for preparing a lithium manganese iron phosphate positive electrode material, which is different from Example 1 in that in step 1-2 of Example 8, methacrylic acid is replaced by 3-aminopropyltriethoxysilane, and amino polystyrene nanospheres are obtained in step 1-3.

[0148] Comparative Example 1

[0149] Comparative Example 1 provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising:

[0150] (1) 4.84 g of urea, 2.2 mmol of MnSO4·H2O, 0.6 mmol of FeSO4·7H2O and 2.8 mmol of NH4H2PO4 were slowly added to a mixed solvent of 10 mL of H2O and 40 mL of ethylene glycol, and the mixture was reacted at 80°C for 4 h under a nitrogen atmosphere with continuous magnetic stirring at a speed of 400 rpm to obtain a precipitated intermediate.

[0151] (2) After filtering, washing and drying the precipitated intermediate, 2.9 mmol Li2CO3 was added, and after grinding, it was pre-sintered at 400°C for 5 h in a nitrogen atmosphere. After cooling to room temperature, the sample was taken out and ground, sintered at 800°C for 8 h, and finally annealed at 700°C for 10 h. The black precursor powder was obtained by natural cooling at a heating rate of 10°C / min.

[0152] (3) After sucrose and the above-mentioned black precursor powder were uniformly mixed in a mass ratio of 100:20, the mixture was sintered at 700° C. for 3 h in a nitrogen atmosphere to obtain a gray-black lithium manganese iron phosphate positive electrode material at a heating rate of 10° C. / min.

[0153] Comparative Example 2

[0154] Comparative Example 2 provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising:

[0155] (1) 0.2 g of polyvinyl pyrrolidone with a molecular weight of 8000 was placed in a 100 mL mixed solution of deionized water and ethanol, wherein the volume ratio of deionized water to ethanol was 1:1, and stirred until completely dissolved, and then nitrogen was continuously introduced into the polyvinyl pyrrolidone solution for 30 minutes. Subsequently, 10 mL of styrene was added, and the mixture was heated in an oil bath at 70°C for 30 minutes to obtain a white emulsion;

[0156] (2) adding 1.2 mL of a 0.15 g / mL aqueous solution of azobisisobutylamidine hydrochloride to the white emulsion, and causing a polymerization reaction in a nitrogen atmosphere at 70° C. for 24 h, washing the precipitate obtained by the polymerization reaction with water until it is neutral, then rinsing with ethanol and drying in a vacuum at 60° C. for 24 h to obtain polystyrene nanospheres, wherein the polystyrene nanospheres are non-carboxylated polystyrene nanospheres;

[0157] (3) 0.2 g of polystyrene nanospheres, 4.84 g of urea, 2.4 mmol of MnSO4·H2O, 0.6 mmol of FeSO4·7H2O and 3.0 mmol of NH4H2PO4 were slowly added to a mixed solvent of 10 mL of H2O and 40 mL of ethylene glycol, and the mixture was reacted at 80°C for 2 h under a nitrogen atmosphere with continuous magnetic stirring at a speed of 200 rpm to obtain a precipitated intermediate.

[0158] (4) After filtering, washing and drying the precipitated intermediate, 3.0 mmol Li2CO3 was added, and after grinding, it was pre-sintered at 400°C for 4 h in a nitrogen atmosphere. After cooling to room temperature, the sample was taken out and ground, and then sintered at 800°C for 8 h. Finally, it was annealed at 700°C for 10 h, and naturally cooled to obtain a black powder. The heating rate was 5°C / min.

[0159] (5) After sucrose and the above black powder were uniformly mixed in a mass ratio of 100:10, the mixture was sintered at 700° C. for 3 h in a nitrogen atmosphere to obtain a gray-black lithium manganese iron phosphate positive electrode material at a heating rate of 10° C. / min.

[0160] Comparative Example 3

[0161] Comparative Example 3 provides a method for preparing a lithium manganese iron phosphate positive electrode material, comprising:

[0162] Step 1-1: placing 0.2 g of polyvinyl pyrrolidone with a molecular weight of 8000 in a mixed solution of 100 mL of deionized water and ethanol, wherein the volume ratio of deionized water to ethanol is 1:1, and stirring and dissolving until the polyvinyl pyrrolidone is completely dissolved to obtain a polyvinyl pyrrolidone solution;

[0163] Step 1-2: nitrogen was continuously introduced into the polyvinyl pyrrolidone solution for 30 min, and then 10 mL of styrene and 0.5 mL of methacrylic acid were added, and the mixture was heated in an oil bath at 70° C. for 30 min to obtain a white emulsion;

[0164] Step 1-3: Add 1.2 mL of 0.15 g / mL azobisisobutyramidine hydrochloride aqueous solution to the above white emulsion, and allow polymerization to occur in a nitrogen atmosphere at 70°C for 24 hours. Wash the precipitate obtained by the polymerization reaction with water until it is neutral, rinse with ethanol, and dry it in a vacuum at 60°C for 24 hours to obtain carboxylated polystyrene nanospheres with a size distribution of 50 nm to 70 nm.

[0165] Step 2: 0.2 g of carboxylated polystyrene nanospheres with a size range of 50 nm to 70 nm, 4.84 g of urea, 2.4 mmol of MnSO4·H2O, 0.6 mmol of FeSO4·7H2O and 3.0 mmol of NH4H2PO4 were slowly added to a mixed solvent of 10 mL of H2O and 40 mL of ethylene glycol, and the mixture was reacted under continuous magnetic stirring at 80°C in a nitrogen atmosphere for 2 h to obtain a precipitated intermediate, with a stirring speed of 200 rpm.

[0166] Step 3-1: After filtering, washing and drying the precipitated intermediate, 3.0 mmol Li2CO3 was added, and after grinding, it was pre-sintered at 400°C for 4 hours in a nitrogen atmosphere. After cooling to room temperature, the sample was taken out and ground, sintered at 800°C for 8 hours, and finally annealed at 700°C for 10 hours. The black precursor powder was obtained by natural cooling, and the heating rate was 5°C / min.

[0167] Step 3-2: After sucrose and the above-mentioned black precursor powder are uniformly mixed in a mass ratio of 100:10, sintering at 700°C for 3h in a nitrogen atmosphere to obtain a gray-black lithium manganese iron phosphate positive electrode material, with a heating rate of 5°C / min.

[0168] The lithium manganese iron phosphate positive electrode materials provided in the above examples and comparative examples are respectively assembled into positive electrode sheets and lithium ion batteries according to the following methods:

[0169] Positive electrode: lithium iron manganese phosphate positive electrode material is prepared into a uniform slurry in a mass ratio of lithium iron manganese phosphate positive electrode material: carbon black: polyvinylidene fluoride = 80:10:10 with N-methylpyrrolidone as solvent, the positive electrode slurry is coated on the surface of aluminum foil, vacuum dried at 110°C overnight, and rolled to obtain a positive electrode sheet;

[0170] Negative electrode: lithium metal sheet;

[0171] Electrolyte: Commercial lithium-ion electrolyte, LiPF6 electrolyte in a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) was used;

[0172] Diaphragm: Polypropylene microporous separator;

[0173] Lithium-ion battery assembly: Assemble button-type lithium-ion full batteries in an inert atmosphere glove box in the order of lithium metal sheet-diaphragm-electrolyte-positive electrode sheet.

[0174] Related performance tests

[0175] Electrochemical performance test:

[0176] The electrochemical performance of each lithium-ion battery assembled above was tested as shown in Table 1, and the test conditions were as follows:

[0177] Constant current and constant voltage charging, the first cycle charge and discharge voltage is 2.0V~4.3V, the current is 0.1C / 1C, and the cut-off current is 0.01C.

[0178] The first charge and discharge curves of each embodiment and comparative example at 0.1C and 1C rates are shown in FIG. Figure 1 and Figure 2 shown.

[0179] The relevant performance test results are shown in Tables 1 to 3 below:

[0180] Table 1 Electrochemical test results

[0181]

[0182] Table 2 Conventional property test results

[0183]

[0184]

[0185] Table 3 Processing performance test results

[0186]

[0187] Table 1 above and Figure 1 and Figure 2 It shows that with the increase of the amount of carboxylated polystyrene nanospheres added, the discharge performance and cycle performance of the material first increase and then decrease, so the appropriate introduction of carboxylated polystyrene nanospheres is helpful to improve the electrochemical performance of lithium manganese iron phosphate. When the addition ratio of 50nm-100nm, 200nm-300nm, and 400nm-500nm carboxylated polystyrene nanospheres is 1:2:2, the electrochemical performance is the best, indicating that appropriately increasing the mass proportion of polymer nanospheres in a small size range is beneficial to accelerate the migration and conduction of lithium ions. By comparing Example 1 and Example 6, it can be seen that too much small size range will lead to too little active material and cause a decrease in electrochemical performance.

[0188] No polystyrene nanospheres were introduced in Comparative Example 1, and no additional carbon source coating was introduced in Example 5. Compared with Example 1, Comparative Example 1 and Example 5 show that a single carburizing heat treatment or a single introduction of polar substituent-modified polystyrene nanospheres is not sufficient to construct a good conductive carbon network, verifying the necessity of combining multi-stage carburizing heat treatment with carboxylated polystyrene nanosphere template method.

[0189] Comparative Example 2 shows that the electrochemical performance of the polystyrene nanospheres without carboxyl functionalization is slightly reduced.

[0190] Comparative Example 3 introduced carboxyl-functionalized polystyrene nanospheres of a single size range without grading, and the electrochemical performance was inferior to that of the graded lithium manganese iron phosphate material.

[0191] Example 9 introduces amino-functionalized polystyrene nanospheres. Compared with Example 1, the electrochemical performance is slightly reduced, indicating that the carboxyl group is more conducive to improving the electrochemical performance than the amino group. This may be because the carboxyl group also contains carbon elements that can enhance the conductivity.

[0192] Table 2 above shows that as the mass proportion of polymer nanospheres in a small size range increases, the compaction density of the lithium iron manganese phosphate positive electrode material first increases and then decreases. When the mass ratio of 50nm to 100nm, 200nm to 300nm, and 400nm to 500nm carboxylated polystyrene nanospheres is 1:4:6.5, the compaction density is the largest, which shows that the particle grading is conducive to the improvement of the compaction density and solves the problem of reduced compaction density caused by particle refinement. As the mass proportion of polymer nanospheres in a small size range increases, the size of lithium iron manganese phosphate particles decreases, but the specific surface area decreases, indicating that the introduction of carboxylated polystyrene nanospheres is conducive to improving the agglomeration of lithium iron manganese phosphate particles, and further explains the effect of improving processing performance. The mass proportion and specific surface area of ​​the polymer nanospheres in the small size range of Example 6 are greater than those of Examples 1 to 4, indicating that from the perspective of controlling the specific surface area, the mass proportion of polymer nanospheres in a small size range cannot be too large, otherwise it may exceed the ability range of carboxylated polymer nanospheres to improve processing performance. The carbon content of Example 1 is significantly higher than that of all comparative examples, which is beneficial to improving the energy density of the lithium manganese iron phosphate positive electrode material. It should be noted that the D50 particle size in Table 2 is the particle size of the secondary particles after agglomeration.

[0193] Compared with Example 1, Examples 7 and 8 only introduce polymer nanospheres of two size ranges, simplifying the size distribution of polymer nanospheres, making the regulation effect of the particle distribution of lithium manganese iron phosphate worse, resulting in a decrease in compaction density and poor electrical properties. Among them, the size range of the polymer nanospheres of Example 7 does not include 50nm to 70nm, and it is impossible to refine the particles of lithium manganese iron phosphate by polymer nanospheres of a small size range, and the electrical performance is significantly reduced. The size range of the polymer nanospheres of Example 8 does not include 200nm to 230nm, making the remaining two size ranges span too large, without 200nm to 230nm polymer nanospheres as a transition, and the cycle performance is poor.

[0194] In Comparative Example 1 and Example 5, the polystyrene nanospheres modified by single carburizing heat treatment or single introduction of polar substituents will result in the particle size being too large or being easily agglomerated.

[0195] Comparative Example 2 introduces non-carboxyl functionalized polystyrene nanospheres. Compared with the conventional properties of the examples, the specific surface area is significantly increased, indicating that carboxyl functionalization of polystyrene nanospheres can reduce the specific surface area, thereby improving the processing performance.

[0196] Comparative Example 3 introduced carboxyl-functionalized polystyrene nanospheres of a single size range without grading. Its compaction density increased with the increase of the introduced size, but was lower than that of the graded lithium manganese iron phosphate material, and its processing performance was slightly reduced compared to that after grading.

[0197] As can be seen from Table 3, the viscosity of the embodiments and the comparative examples is basically within the range of 9000mPa·s to 12000mPa·s. On this basis, the solid content values ​​of embodiments 1 to 4 are greater than those of the remaining embodiments and all comparative examples, wherein the solid content increase values ​​of embodiments 1 to 4 compared to the comparative examples can reach up to 5%, indicating that the present application uses polystyrene nanoparticles modified by polar substituents in a variety of different size ranges, and reasonably controls the mass proportion of a variety of size ranges, which can improve the processing performance of lithium manganese iron phosphate positive electrode materials and the energy density of the battery without significantly increasing the viscosity. In addition, the peeling force of embodiment 1 is greater than that of other embodiments and other comparative examples except comparative example 1, and the elongation of embodiment 4 is greater than that of other embodiments and other comparative examples except comparative example 3, further indicating that the present application uses polymer nanoparticles modified by polar substituents as a carbon source and improves particle agglomeration, and combines multi-stage carburizing heat treatment, which can take into account both electrochemical performance and processing performance.

[0198] Figures 3 to 5 They are respectively the primary particle size distribution diagrams of Example 1, Example 4 and Comparative Example 1, Figures 6 to 8 The SEM morphology images of Example 1, Example 4 and Comparative Example 1 are shown respectively. Figure 3 and Figure 6 As shown, in Example 1, the mass proportion of polymer nanoparticles in the small size range is the smallest in Examples 1 to 4, so that the final large-sized lithium manganese iron phosphate particles account for the largest proportion, and their particle size is more concentrated in 400nm to 500nm, and the appearance size in the SEM morphology is larger. Figure 4 and Figure 7 As shown, in Example 4, the particle size of lithium manganese iron phosphate is regulated by polymer nanoparticles of different size ranges, so that the lithium manganese iron phosphate particles of small size range account for the largest proportion in Examples 1 to 4, and their particle size is more concentrated in 200nm to 250nm, and the appearance size in the SEM morphology is relatively small. Figure 5 and Figure 8 As shown, the particle size distribution of the polystyrene nanoparticles without carboxyl functionalization in Comparative Example 1 is more dispersed, but relatively concentrated in 250nm-300nm, which may be due to the effect of sucrose as a carbon source carbon coating on its size. However, since the polystyrene nanoparticles without carboxyl functionalization in Comparative Example 1 have a lower sphericity than those in Examples 1 and 4, they are mostly long strips and have obvious agglomeration. Figures 3 to 8 It can be seen that the introduction of carboxyl-functionalized polystyrene nanospheres can regulate the particle size D10, particle size D50, particle size D90 and particle size D99 of lithium manganese iron phosphate material within an appropriate range, and reduce its agglomeration degree to improve its performance.

[0199] Among them, the particle size D10 of the lithium iron manganese phosphate positive electrode material refers to the particle size corresponding to the cumulative particle size distribution percentage of the particles in the lithium iron manganese phosphate positive electrode material reaching 10%. The particle size D50 refers to the particle size corresponding to the cumulative particle size distribution percentage of the particles in the lithium iron manganese phosphate positive electrode material reaching 50%. The particle size D90 refers to the particle size corresponding to the cumulative particle size distribution percentage of the particles in the lithium iron manganese phosphate positive electrode material reaching 90%. The particle size D99 refers to the particle size corresponding to the cumulative particle size distribution percentage of the particles in the lithium iron manganese phosphate positive electrode material reaching 99%.

[0200] The above is a detailed introduction to the lithium manganese iron phosphate positive electrode material, preparation method and application thereof provided in the embodiments of the present application. Specific examples are used in this article to illustrate the principles and embodiments of the present application. The description of the above embodiments is only used to help understand the method of the present application and its core idea; at the same time, for general technical personnel in this field, according to the idea of ​​the present application, there will be changes in the specific embodiments and application scope. In summary, the content of this specification should not be understood as a limitation on the present application.

Claims

1. A method for preparing a lithium manganese iron phosphate positive electrode material, characterized in that: The preparation method of the lithium manganese iron phosphate positive electrode material comprises: Providing or preparing polymer nanoparticles of different size ranges, wherein the polymer nanoparticles include at least one polar substituent selected from the group consisting of carboxyl, hydroxyl, aldehyde, amino, and amide; Adding the polymer nanoparticles of different size ranges, a precipitant, a manganese source, an iron source, and a phosphorus source into the first solution, and causing a precipitation reaction under an inert atmosphere, and filtering to obtain a precipitated intermediate; The precipitation intermediate is mixed with a lithium source and sintered in an inert atmosphere to obtain the lithium manganese iron phosphate positive electrode material.

2. The method for preparing the lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The preparation method of the polymer nanospheres comprises: dissolving an emulsion stabilizer in the second solution to obtain an emulsion stabilizer solution; Inert gas is introduced into the emulsion stabilizer solution, and then polymer monomer and polar substituent monomer are added, and an emulsion is obtained after heating reaction; The initiator solution is added to the emulsion, and a polymerization reaction is carried out under an inert atmosphere to obtain the polymer nanoparticles.

3. The method for preparing the lithium iron manganese phosphate positive electrode material according to claim 1 or 2, characterized in that: The step of mixing the precipitated intermediate with a lithium source and sintering in an inert atmosphere to obtain the lithium manganese iron phosphate positive electrode material comprises: The precipitated intermediate is mixed with a lithium source, and sintered in an inert atmosphere to obtain a precursor; After the carbon source and the precursor are uniformly mixed, they are sintered in an inert atmosphere to obtain the lithium manganese iron phosphate positive electrode material.

4. The method for preparing the lithium manganese iron phosphate positive electrode material according to any one of claims 1 to 3, characterized in that: The polymer nanospheres have three different size ranges; and / or, The particle sizes of the polymer nanoparticles in three different size ranges are 50nm-100nm, 200nm-300nm, and 400nm-500nm respectively; and / or, The mass ratio of the polymer nanoparticles of three different size ranges is 1:(1-9):(1-15); and / or, The molar ratio of the polymer nanospheres to the precipitant is 1:(45-50).

5. The method for preparing the lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The molar ratio of manganese in the manganese source, iron in the iron source and phosphorus in the phosphorus source is (0.70-0.88): (0.20-0.22): 1; and / or, The molar ratio of the iron element in the iron source to the precipitant is 1:(128-140); and / or, The molar ratio of the lithium element in the lithium source to the precipitated intermediate is (1-1.2):

1.

6. The method for preparing the lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The first solution comprises water and ethylene glycol, and the volume ratio of water to ethylene glycol is 1:(3-5); and / or, The conditions of the precipitation reaction include at least one of a reaction temperature of 80° C. to 90° C., a stirring speed of 200 rpm to 400 rpm, and a reaction time of 2 h to 4 h.

7. The method for preparing lithium iron manganese phosphate positive electrode material according to claim 2, characterized in that: The molecular weight of the emulsion stabilizer is 8000 g / mol to 12000 g / mol; and / or, The second solution comprises water and ethanol, and the volume ratio of water to ethanol is 1:1; and / or, The added volume of the polar substituent monomer is 5% to 8% of the added volume of the polymer monomer; and / or, The concentration of the initiator solution is 0.15 g / mL to 0.25 g / mL.

8. A lithium manganese iron phosphate positive electrode material, characterized in that: The lithium iron manganese phosphate positive electrode material is prepared by the preparation method of the lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 7.

9. A positive electrode sheet, characterized in that: The positive electrode plate includes the lithium iron manganese phosphate positive electrode material prepared by the preparation method of the lithium iron manganese phosphate positive electrode material as described in any one of claims 1 to 7, or includes the lithium iron manganese phosphate positive electrode material as described in claim 8.

10. A secondary battery, characterized in that: The secondary battery comprises a negative electrode sheet, a separator and the positive electrode sheet as claimed in claim 9.

Images