Polyelectrolyte membrane coated lithium manganese iron phosphate positive electrode material and preparation method thereof

Abstract

The invention discloses a polyelectrolyte membrane coated lithium manganese iron phosphate positive electrode material and a preparation method thereof, a manganese iron phosphate precursor and lithium carbonate are added into a solvent for mixing, wet grinding, drying, sintering and crushing, and after grinding, the mixed material is activated and coated, and then the subsequent drying step is carried out. In the activating step, phosphoric acid is added into a mixed material obtained by wet grinding to adjust the pH value to be 5-6, and then standing is performed for 1-2 hours; and the coating is that polyelectrolyte is added into the activated mixed material for coating for 1-2 hours. The lithium manganese iron phosphate positive electrode material is coated with the polyelectrolyte membrane, so that Mn < 3 + > dissolution caused by shape distortion caused by a John-Teller effect is effectively avoided, meanwhile, the uniformity and the stability of an electrode interface are improved, local polarization is reduced, the resistivity is sharply reduced, the cycle stability is relatively good, rapid charging and discharging of an LFP battery at a high rate (5C) can be realized, and the lithium manganese iron phosphate positive electrode material has a good application prospect. And meanwhile, corrosion of electrolyte to the surface of the material is effectively prevented.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery cathode material technology, and in particular to a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material and its preparation method. Background Technology

[0002] With the continuous development of portable electronic devices, electric vehicles, and the large-scale energy storage market, the demand for lithium-ion batteries continues to grow. Cathode materials have a significant impact on the performance of lithium-ion batteries. Olivine-type cathode materials offer advantages such as high safety performance and low cost; among them, lithium manganese iron phosphate (LiMn) is a prime example. x Fe 1-x Both lithium iron phosphate (LiFePO4) and lithium iron phosphate (LiFePO4) belong to the olivine-type cathode materials. LiFePO4 has high safety and cycle life, but its voltage plateau is low, making it difficult to meet the demand for higher energy density. Lithium manganese iron phosphate (LMP) is a new type of phosphate-based lithium-ion battery cathode material formed by doping a certain proportion of manganese into lithium iron phosphate. Compared with lithium iron phosphate, the high voltage characteristics of manganese give LMP a higher voltage plateau, thus achieving higher energy density. Furthermore, LMP retains the stable olivine-type structure of lithium iron phosphate while taking into account its high safety, long cycle life, low cost, and environmental friendliness. LMP is currently one of the research hotspots for lithium battery cathode materials.

[0003] Lithium manganese iron phosphate (LMFP), as an upgraded version of lithium iron phosphate (LFP), is actually formed by replacing the Fe sites in lithium iron phosphate with Mn. This is because Mn has bidirectional channels and a higher ion valence (Mn... 3+ or Mn 4+ Therefore, it possesses a higher energy density (697 Wh / kg) and voltage plateau (3.8-4.1 V). However, due to the John-Teller effect, the ionic radius of Mn (0.83 Å) is slightly larger than that of Fe (0.78 Å), which easily leads to Mn... 3+ The dissolution and distortion of the material shape, as well as the occurrence of dislocations and vacancies between crystal lattices, ultimately lead to a decrease in the cycle stability of lithium manganese iron phosphate, and a rapid decline in battery life and energy density. Secondly, the bond energy of Mn-O is higher than that of Fe-O, which indicates that Li ion transport needs to overcome greater diffusion resistance, resulting in LMFP having lower intrinsic electronic conductivity and ionic conductivity.

[0004] Currently, coating is often used to improve the problem of manganese leaching in lithium manganese iron phosphate materials. Commonly used coating methods include physical vapor deposition (PVD) and chemical vapor deposition (CVD). However, existing surface coating technologies have some problems and limitations. The structure and performance of the coating layer often cannot be effectively controlled, resulting in low stability and durability of the coating layer. Some coating layers hinder the transport of lithium ions, thus affecting the specific capacity. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material to prevent Mn from entering the cathode. 3+ The dissolution and material shape distortion ensure cycle life, improve energy efficiency and energy density, and increase electronic conductivity.

[0006] The technical solution adopted by the present invention to solve its technical problem is: a method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material. The preparation method is to add the manganese iron phosphate precursor and lithium carbonate to a solvent for mixing, wet grinding, drying, sintering and pulverizing. After wet grinding, the mixture is activated, coated and then subjected to subsequent drying steps.

[0007] The activated mixture obtained by wet grinding is added with phosphoric acid to adjust the pH to 5-6, and then left to stand for 1-2 hours.

[0008] The coating step involves adding a polyelectrolyte to the activated mixture and coating it for 1-2 hours. The polyelectrolyte is a polycationic electrolyte or a polyanionic electrolyte.

[0009] The coating process is repeated 1 to 4 times. For odd-numbered coatings, a polycationic electrolyte is added; for even-numbered coatings, a polyanionic electrolyte is added. After each coating, the slurry is kept in its original state, and more polyelectrolyte is added for the next coating cycle.

[0010] When the pH is adjusted to 5-6, phosphoric acid undergoes a slight complexation reaction with Mn²⁺ / Fe²⁺ on the surface of the LMFP precursor, forming hydrophilic binding sites such as hydroxyl groups (-OH) on the particle surface, while also improving the charge on the particle surface. If the pH is below 5, it will corrode the precursor and cause lattice damage, and if the pH is above 6, effective hydroxyl sites cannot be formed.

[0011] The first layer is coated with a polycationic electrolyte, and the second layer is coated with a polyanionic electrolyte. These two layers alternate to form a polyelectrolyte film. After activation, the surface of the activated mixture carries a negative charge, which attracts the polycationic electrolyte and improves the stability of the structure.

[0012] Layer-by-layer self-assembled polyelectrolyte membranes can be tightly combined together through electrostatic interactions and bridging between anions and cations. This is due to the large number of conductive lithium-philic groups (-SO₄²⁻) between the layers. 3- -COO - -NH 2- The abundant electron cloud (such as Mn3+ and LiPF6 salt) accelerates the rapid transport of electrons and lithium ions on the outer surface of the material, approaching zero resistance. The protective coating also effectively prevents the electrolyte (especially unstable carbonate solvents and LiPF6 salts under high voltage) from eroding the surface of the cathode material, reducing side reactions such as excessive SEI film growth and gas generation. This unique and pioneering structure not only prevents the dissolution of Mn3+ and distortion of the material shape, but also accelerates the insertion and extraction transport speed of electrons and lithium ions, withstands high-rate and high-voltage discharge, reduces polarization, and improves electronic conductivity.

[0013] Furthermore, the polycationic electrolyte can be polydiallyldimethylammonium chloride, polyethyleneimine, polylysine, or chitosan oligosaccharide; the polyanionic electrolyte can be sodium polystyrene sulfonate, polyacrylic acid, sodium carboxymethyl cellulose, or sodium alginate.

[0014] Furthermore, the mass of the polycationic electrolyte or polyanionic electrolyte added in each coating step is 0.2% to 0.6% of the mass of the manganese iron phosphate precursor. The mass of the polycationic electrolyte or polyanionic electrolyte added in each coating step determines the thickness of the coating layer; less than 0.2% results in incomplete coating, while more than 0.6% results in an excessively thick film that hinders Li. + transmission.

[0015] Furthermore, after each coating, a polyelectrolyte film is formed on the surface of the activated mixture, with each polyelectrolyte film having a thickness of 10-20 nm; the film thickness corresponding to each layer of 0.2-0.6% polyelectrolyte is 10-20 nm.

[0016] Furthermore, the solvent is water. The manganese iron phosphate precursor and lithium carbonate are added to the water, and the solid content is controlled to be 10-20%. The mixture is mixed for 2-3 hours.

[0017] Furthermore, the wet milling process controls the particle size to be between 0.42 and 0.45 μm. Controlling the particle size to 0.42–0.45 μm during wet milling is to ensure the specific surface area of ​​the precursor, allowing the activation / coating effect to occur on the entire particle surface.

[0018] Furthermore, the coated material is spray-dried and granulated, and the resulting granulated powder is sintered under a protective atmosphere to obtain polyelectrolyte membrane-coated lithium iron phosphate cathode material.

[0019] The particle size of the spray-dried granules is controlled to be 24–30 μm, and the spray-drying outlet temperature is 100–110 °C.

[0020] The sintering temperature under the protective atmosphere is 700-750℃ and the sintering time is 10-12h. The protective atmosphere is N2 or Ar as the protective gas and the oxygen content is less than 5ppm.

[0021] Furthermore, the sintered material is pulverized by an air jet mill, with the grading frequency controlled at 50-60 Hz, the feeding frequency at 20-30 Hz, and the D50 particle size after pulverization required to be 1.0-1.2 μm.

[0022] A polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material, wherein the polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material is prepared by any of the above-described methods for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material.

[0023] The beneficial effects of this invention are: the method for preparing lithium manganese iron phosphate cathode material coated with a polyelectrolyte membrane of this invention effectively avoids the shape distortion caused by the John-Teller effect, thus preventing the loss of Mn. 3+ Dissolution, Mn 3+ The dissolution rate is no higher than 0.0076%, which improves the uniformity and stability of the electrode interface, reduces local polarization, and makes the resistivity decrease sharply and the cycle stability better. This is conducive to realizing the rapid charge and discharge of LFP batteries at high rates (5C). Its 5C energy efficiency can reach up to 97.1%, the resistivity is less than 8.3Ω•cm, and the capacity retention rate after 1000 cycles at 5C is no less than 81.8%. It also effectively prevents the electrolyte from corroding the material surface.

[0024] This invention application is the first to address the core defect of LMFP (Mn³) by starting from "interface stability control". + The leaching design scheme uses a dense polyelectrolyte membrane to prevent Mn³⁺ from being released. + The John-Teller effect is addressed at its root by eliminating leaching and electrolyte erosion. For the first time, the LMFP coating process is implemented in the liquid phase stage, after wet grinding and before drying and sintering. In this liquid phase, LMFP particles exhibit good dispersion and no agglomeration, enabling molecular-level uniform coating of the polyelectrolyte membrane. Furthermore, during sintering, the polyelectrolyte membrane undergoes slight bonding with the LMFP matrix, enhancing membrane adhesion and avoiding the problem of easy detachment of vapor-phase coated membranes in existing technologies. Addressing the issue of the strong hydrophobicity of the LMFP precursor surface, making it difficult to bind with organic polyelectrolytes, phosphoric acid activation forms hydrophilic binding sites such as hydroxyl groups on its surface, significantly improving the coating density and adhesion of the polyelectrolyte membrane. Detailed Implementation

[0025] The present invention will be further described below with reference to embodiments.

[0026] Example 1:

[0027] A method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material includes the following steps:

[0028] (1) Prepare materials according to the mass ratio of ferromanganese phosphate:lithium carbonate = 1:0.19. Add ferromanganese phosphate precursor and lithium carbonate to water to obtain an aqueous solution. The water bath temperature of the aqueous solution is controlled at 35℃ and the solid content is controlled at 15%. The chemical formula of the ferromanganese phosphate precursor is Fe 0.4 Mn 0.6 PO4, D50 is 0.9-1.2μm, the lithium carbonate is industrial grade ≥99.5% and the polyelectrolyte is industrial grade conventional type;

[0029] (2) The above mixture is homogenized and stirred for 2 hours, and then wet-milled using a sand mill. The particle size is controlled to be 0.42-0.45 μm. The wet milling and subsequent activation and coating are in slurry state.

[0030] (3) After grinding, add 85% phosphoric acid to the mixture and stir evenly to activate it. Adjust the pH value to 5 and then let it stand at room temperature for 1 hour.

[0031] (4) After the activation is complete, add 0.4% of polydiallyldimethylammonium chloride (PDDA) by mass of manganese iron phosphate and stir for 1 hour to complete the coating; the stirring speed is 100 rpm and the reaction temperature is 35℃.

[0032] (5) Spray dry the obtained coated material to granulate, control the spray drying granulation particle size to be 27-30 μm and the spray drying outlet temperature to be 105℃, sinter the obtained granulated powder at 730℃ for 10h under nitrogen atmosphere (oxygen content less than 5ppm), and pulverize the sintered material by air jet mill, control the classification frequency to be 60Hz, the feeding frequency to be 20Hz, and require the D50 particle size after pulverization to be 1.0-1.2μm, to obtain polyelectrolyte membrane coated lithium manganese iron phosphate cathode material.

[0033] Example 2:

[0034] A method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material includes the following steps:

[0035] (1) Prepare materials according to the mass ratio of ferromanganese phosphate:lithium carbonate = 1:0.19. Add ferromanganese phosphate precursor and lithium carbonate to water to obtain an aqueous solution. The water bath temperature of the aqueous solution is controlled at 35℃ and the solid content is controlled at 18%. The chemical formula of the ferromanganese phosphate precursor is Fe 0.4Mn 0.6 PO4, D50 is 0.9-1.2μm, the lithium carbonate is industrial grade ≥99.5% and the polyelectrolyte is industrial grade conventional type;

[0036] (2) The above mixture is homogenized and stirred for 3 hours, and then wet-milled using a sand mill. The particle size is controlled to be 0.42-0.45 μm. The wet milling and subsequent activation and coating are in slurry state.

[0037] (3) After grinding, add 85% phosphoric acid to the mixture and stir evenly to activate it. Adjust the pH value to 6 and then let it stand for 1 hour.

[0038] (4) After the activation is complete, add 0.4% of polydiallyldimethylammonium chloride (PDDA) by mass of manganese iron phosphate and stir for 1 hour to complete the first coating; the stirring rate is 100 rpm and the reaction temperature is 35℃.

[0039] Then add sodium polystyrene sulfonate (PSS) at 0.4% of the mass of ferromanganese phosphate, and stir for 1 hour to complete the second coating.

[0040] (5) Spray dry the obtained coated material to granulate, control the spray drying granulation particle size to be 27-30 μm and the spray drying outlet temperature to be 105℃, sinter the obtained granulated powder at 730℃ for 10h under nitrogen atmosphere (oxygen content less than 5ppm), and pulverize the sintered material by air jet mill, control the classification frequency to be 60Hz, the feeding frequency to be 20Hz, and require the D50 particle size after pulverization to be 1.0-1.2μm, to obtain polyelectrolyte membrane coated lithium manganese iron phosphate cathode material.

[0041] Example 3:

[0042] A method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material includes the following steps:

[0043] (1) Prepare materials according to the mass ratio of ferromanganese phosphate:lithium carbonate = 1:0.19. Add ferromanganese phosphate precursor and lithium carbonate to water to obtain an aqueous solution. The water bath temperature of the aqueous solution is controlled at 35℃ and the solid content is controlled at 15%. The chemical formula of the ferromanganese phosphate precursor is Fe 0.4 Mn 0.6 PO4, D50 is 0.9-1.2μm, the lithium carbonate is industrial grade ≥99.5% and the polyelectrolyte is industrial grade conventional type;

[0044] (2) The above mixture is homogenized and stirred for 2 hours, and then wet-milled using a sand mill. The particle size is controlled to be 0.42-0.45 μm. The wet milling and subsequent activation and coating are in slurry state.

[0045] (3) After grinding, add 85% phosphoric acid to the mixture and stir evenly to activate it. Adjust the pH value to 5 and then let it stand for 2 hours.

[0046] (4) After the activation is complete, add 0.4% of polydiallyldimethylammonium chloride (PDDA) by mass of manganese iron phosphate and stir for 1 hour to complete the first coating; the stirring rate is 100 rpm and the reaction temperature is 35℃.

[0047] Then add sodium polystyrene sulfonate (PSS) at 0.4% of the mass of ferromanganese phosphate, and stir for 1 hour to complete the second coating.

[0048] Finally, add 0.4% (by weight) of polydiallyldimethylammonium chloride (PDDA) of manganese ferric phosphate and stir for 1 hour to complete the third coating.

[0049] (5) Spray dry the obtained coated material to granulate, control the spray drying granulation particle size to be 27-30 μm and the spray drying outlet temperature to be 105℃, sinter the obtained granulated powder at 730℃ for 10h under nitrogen atmosphere (oxygen content less than 5ppm), and pulverize the sintered material by air jet mill, control the classification frequency to be 60Hz, the feeding frequency to be 20Hz, and require the D50 particle size after pulverization to be 1.0-1.2μm, to obtain polyelectrolyte membrane coated lithium manganese iron phosphate cathode material.

[0050] Example 4:

[0051] A method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material includes the following steps:

[0052] (1) Prepare materials according to the mass ratio of ferromanganese phosphate:lithium carbonate = 1:0.19. Add ferromanganese phosphate precursor and lithium carbonate to water to obtain an aqueous solution. The water bath temperature of the aqueous solution is controlled at 35℃ and the solid content is controlled at 15%. The chemical formula of the ferromanganese phosphate precursor is Fe 0.4 Mn 0.6 PO4, D50 is 0.9-1.2μm, the lithium carbonate is industrial grade ≥99.5% and the polyelectrolyte is industrial grade conventional type;

[0053] (2) The above mixture is homogenized and stirred for 2 hours, and then wet-milled using a sand mill. The particle size is controlled to be 0.42-0.45 μm. The wet milling and subsequent activation and coating are in slurry state.

[0054] (3) After grinding, add 85% phosphoric acid to the mixture and stir evenly to activate it. Adjust the pH value to 5 and then let it stand for 1 hour.

[0055] (4) After the activation is complete, add 0.4% of polydiallyldimethylammonium chloride (PDDA) by mass of manganese iron phosphate and stir for 1 hour to complete the first coating; the stirring rate is 100 rpm and the reaction temperature is 35℃.

[0056] Then add sodium polystyrene sulfonate (PSS) at 0.4% of the mass of ferromanganese phosphate, and stir for 1 hour to complete the second coating.

[0057] Then add 0.4% (by weight) of polydiallyldimethylammonium chloride (PDDA) of manganese ferric phosphate and stir for 1 hour to complete the third coating.

[0058] Finally, add 0.4% sodium polystyrene sulfonate (PSS) by weight of ferromanganese phosphate and stir for 1 hour to complete the fourth coating.

[0059] (5) Spray dry the obtained coated material to granulate, control the spray drying granulation particle size to be 27-30 μm and the spray drying outlet temperature to be 105℃, sinter the obtained granulated powder at 730℃ for 10h under nitrogen atmosphere (oxygen content less than 5ppm), and pulverize the sintered material by air jet mill, control the classification frequency to be 60Hz, the feeding frequency to be 20Hz, and require the D50 particle size after pulverization to be 1.0-1.2μm, to obtain polyelectrolyte membrane coated lithium manganese iron phosphate cathode material.

[0060] Example 5:

[0061] A method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material includes the following steps:

[0062] (1) Prepare materials according to the mass ratio of ferromanganese phosphate:lithium carbonate = 1:0.19. Add ferromanganese phosphate precursor and lithium carbonate to water to obtain an aqueous solution. The water bath temperature of the aqueous solution is controlled at 35℃ and the solid content is controlled at 12%. The chemical formula of the ferromanganese phosphate precursor is Fe 0.4 Mn 0.6 PO4, D50 is 0.9-1.2μm, the lithium carbonate is industrial grade ≥99.5% and the polyelectrolyte is industrial grade conventional type;

[0063] (2) The above mixture is homogenized and stirred for 3 hours, and then wet-milled using a sand mill. The particle size is controlled to be 0.42-0.45 μm. The wet milling and subsequent activation and coating are in slurry state.

[0064] (3) After grinding, add 85% phosphoric acid to the mixture and stir evenly to activate it. Adjust the pH value to 5 and then let it stand for 1 hour.

[0065] (4) After the activation is complete, add 0.6% (by weight) of polyethyleneimine (PEI) of manganese iron phosphate and stir for 1.5 h to complete the first coating; the stirring rate is 100 rpm and the reaction temperature is 35℃.

[0066] Then, 0.6% (by weight) of polyacrylic acid (PAA) was added, and the mixture was stirred for 1.5 hours to complete the second coating. The stirring speed was 100 rpm, and the reaction temperature was 35°C.

[0067] Finally, add 0.6% (by weight) of polyethyleneimine (PEI) of manganese ferric phosphate and stir for 1.5 h to complete the third coating; the stirring speed is 100 rpm and the reaction temperature is 35℃.

[0068] (5) Spray dry the obtained coated material to granulate, control the spray drying granulation particle size to be 27-30 μm and the spray drying outlet temperature to be 105℃, sinter the obtained granulated powder at 730℃ for 10h under nitrogen atmosphere (oxygen content less than 5ppm), and pulverize the sintered material by air jet mill, control the classification frequency to be 60Hz, the feeding frequency to be 20Hz, and require the D50 particle size after pulverization to be 1.0-1.2μm, to obtain polyelectrolyte membrane coated lithium manganese iron phosphate cathode material.

[0069] Comparative Example 1: (No coating, otherwise the same as Example 1)

[0070] A method for preparing lithium manganese iron phosphate cathode material includes the following steps:

[0071] (1) Prepare materials according to the mass ratio of ferromanganese phosphate:lithium carbonate = 1:0.19. Add ferromanganese phosphate precursor and lithium carbonate to water to obtain an aqueous solution. The water bath temperature of the aqueous solution is controlled at 35℃ and the solid content is controlled at 15%. The chemical formula of the ferromanganese phosphate precursor is Fe 0.4 Mn 0.6 PO4, D50 is 0.9-1.2μm, the lithium carbonate is industrial grade ≥99.5% and the polyelectrolyte is industrial grade conventional type;

[0072] (2) The above mixture is homogenized and stirred for 2 hours, and then wet-milled using a sand mill, with the particle size controlled at 0.42-0.45 μm;

[0073] (3) Spray dry the wet-milled material to granulate it. Control the spray-drying granulation particle size to be 27-30 μm and the spray drying outlet temperature to be 105℃. Sinter the obtained granulated powder at 730℃ for 10h under a nitrogen atmosphere (oxygen content less than 5ppm). Then, pulverize the sintered material by air jet mill. Control the classification frequency to be 60Hz, the feeding frequency to be 20Hz, and the D50 particle size after pulverization to be 1.0-1.2μm to obtain lithium manganese iron phosphate cathode material.

[0074] Comparative Example 2: (No excitation, otherwise the same as Example 3)

[0075] A method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material includes the following steps:

[0076] (1) Prepare materials according to the mass ratio of ferromanganese phosphate:lithium carbonate = 1:0.19. Add ferromanganese phosphate precursor and lithium carbonate to water to obtain an aqueous solution. The water bath temperature of the aqueous solution is controlled at 35℃ and the solid content is controlled at 15%. The chemical formula of the ferromanganese phosphate precursor is Fe 0.4 Mn 0.6 PO4, D50 is 0.9-1.2μm, the lithium carbonate is industrial grade ≥99.5% and the polyelectrolyte is industrial grade conventional type;

[0077] (2) The above mixture is homogenized and stirred for 2 hours, and then wet-milled using a sand mill. The particle size is controlled to be 0.42-0.45 μm. The wet milling and subsequent coating are in slurry state.

[0078] (3) After grinding, add 0.4% of polydiallyldimethylammonium chloride (PDDA) by mass of manganese iron phosphate, and stir for 1 hour to complete the first coating; the stirring rate is 100 rpm and the reaction temperature is 35℃.

[0079] Then add sodium polystyrene sulfonate (PSS) at 0.4% of the mass of manganese ferric phosphate, and stir for 1 hour to complete the second coating; the stirring speed is 100 rpm and the reaction temperature is 35℃;

[0080] Finally, add 0.4% (by weight) of polydiallyldimethylammonium chloride (PDDA) of manganese ferric phosphate, and stir for 1 hour to complete the third coating; the stirring speed is 100 rpm and the reaction temperature is 35℃.

[0081] (4) Spray dry the obtained coated material to granulate, control the spray drying granulation particle size to be 27-30 μm and the spray drying outlet temperature to be 105℃, sinter the obtained granulated powder at 730℃ for 10h under nitrogen atmosphere (oxygen content less than 5ppm), and pulverize the sintered material by air jet mill, control the classification frequency to be 60Hz, the feeding frequency to be 20Hz, and require the D50 particle size after pulverization to be 1.0-1.2μm, to obtain polyelectrolyte membrane coated lithium manganese iron phosphate cathode material.

[0082] Comparative Example 3: (Polyelectrolyte addition was 0.1%, other details were the same as in Example 1)

[0083] A method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material includes the following steps:

[0084] (1) Prepare materials according to the mass ratio of ferromanganese phosphate:lithium carbonate = 1:0.19. Add ferromanganese phosphate precursor and lithium carbonate to water to obtain an aqueous solution. The water bath temperature of the aqueous solution is controlled at 35℃ and the solid content is controlled at 15%. The chemical formula of the ferromanganese phosphate precursor is Fe 0.4 Mn 0.6 PO4, D50 is 0.9-1.2μm, the lithium carbonate is industrial grade ≥99.5% and the polyelectrolyte is industrial grade conventional type;

[0085] (2) The above mixture is homogenized and stirred for 2 hours, and then wet-milled using a sand mill. The particle size is controlled to be 0.42-0.45 μm. The wet milling and subsequent activation and coating are in slurry state.

[0086] (3) After grinding, add 85% phosphoric acid to the mixture and stir evenly to activate it. Adjust the pH value to 5 and then let it stand for 1 hour.

[0087] (4) After the activation is complete, add 0.1% of polydiallyldimethylammonium chloride (PDDA) by mass of manganese iron phosphate and stir for 1 hour to complete the coating; the stirring rate is 100 rpm and the reaction temperature is 35℃.

[0088] (5) Spray dry the obtained coated material to granulate, control the spray drying granulation particle size to be 27-30 μm and the spray drying outlet temperature to be 105℃, sinter the obtained granulated powder at 730℃ for 10h under nitrogen atmosphere (oxygen content less than 5ppm), and pulverize the sintered material by air jet mill, control the classification frequency to be 60Hz, the feeding frequency to be 20Hz, and require the D50 particle size after pulverization to be 1.0-1.2μm, to obtain polyelectrolyte membrane coated lithium manganese iron phosphate cathode material.

[0089] Comparative Example 4: (Polyelectrolyte addition was 1.0%, other details were the same as in Example 1)

[0090] A method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material includes the following steps:

[0091] (1) Prepare materials according to the mass ratio of ferromanganese phosphate:lithium carbonate = 1:0.19. Add ferromanganese phosphate precursor and lithium carbonate to water to obtain an aqueous solution. The water bath temperature of the aqueous solution is controlled at 35℃ and the solid content is controlled at 15%. The chemical formula of the ferromanganese phosphate precursor is Fe 0.4 Mn 0.6 PO4, D50 is 0.9-1.2μm, the lithium carbonate is industrial grade ≥99.5% and the polyelectrolyte is industrial grade conventional type;

[0092] (2) The above mixture is homogenized and stirred for 2 hours, and then wet-milled using a sand mill. The particle size is controlled to be 0.42-0.45 μm. The wet milling and subsequent activation and coating are in slurry state.

[0093] (3) After grinding, add 85% phosphoric acid to the mixture and stir evenly to activate it. Adjust the pH value to 5 and then let it stand for 1 hour.

[0094] (4) After the activation is complete, add 1.0% of polydiallyldimethylammonium chloride (PDDA) by mass of manganese iron phosphate and stir for 1 hour to complete the coating; the stirring speed is 100 rpm and the reaction temperature is 35℃.

[0095] (5) Spray dry the obtained coated material to granulate, control the spray drying granulation particle size to be 27-30 μm and the spray drying outlet temperature to be 105℃, sinter the obtained granulated powder at 730℃ for 10h under nitrogen atmosphere (oxygen content less than 5ppm), and pulverize the sintered material by air jet mill, control the classification frequency to be 60Hz, the feeding frequency to be 20Hz, and require the D50 particle size after pulverization to be 1.0-1.2μm, to obtain polyelectrolyte membrane coated lithium manganese iron phosphate cathode material.

[0096] Comparative Example 5: (6 coating cycles, other details same as Example 1)

[0097] A method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material includes the following steps:

[0098] (1) Prepare materials according to the mass ratio of ferromanganese phosphate:lithium carbonate = 1:0.19. Add ferromanganese phosphate precursor and lithium carbonate to water to obtain an aqueous solution. The water bath temperature of the aqueous solution is controlled at 35℃ and the solid content is controlled at 15%. The chemical formula of the ferromanganese phosphate precursor is Fe 0.4 Mn 0.6 PO4, D50 is 0.9-1.2μm, the lithium carbonate is industrial grade ≥99.5% and the polyelectrolyte is industrial grade conventional type;

[0099] (2) The above mixture is homogenized and stirred for 2 hours, and then wet-milled using a sand mill. The particle size is controlled to be 0.42-0.45 μm. The wet milling and subsequent activation and coating are in slurry state.

[0100] (3) After grinding, add 85% phosphoric acid to the mixture and stir evenly to activate it. Adjust the pH value to 5 and then let it stand for 2 hours.

[0101] (4) After the activation is complete, add 0.4% of polydiallyldimethylammonium chloride (PDDA) by mass of manganese iron phosphate and stir for 1 hour to complete the first coating; the stirring rate is 100 rpm and the reaction temperature is 35℃.

[0102] Then add sodium polystyrene sulfonate (PSS) at 0.4% of the mass of manganese ferric phosphate, and stir for 1 hour to complete the second coating; the stirring speed is 100 rpm and the reaction temperature is 35℃;

[0103] PDDA and PSS were alternately coated using the above coating method, and six coatings were completed.

[0104] (5) Spray dry the obtained coated material to granulate, control the spray drying granulation particle size to be 27-30 μm and the spray drying outlet temperature to be 105℃, sinter the obtained granulated powder at 730℃ for 10h under nitrogen atmosphere (oxygen content less than 5ppm), and pulverize the sintered material by air jet mill, control the classification frequency to be 60Hz, the feeding frequency to be 20Hz, and require the D50 particle size after pulverization to be 1.0-1.2μm, to obtain polyelectrolyte membrane coated lithium manganese iron phosphate cathode material.

[0105] The performance of the lithium manganese iron phosphate cathode materials obtained in the examples and comparative examples was tested, and the results are shown in Tables 1 and 2.

[0106] Table 1 Cycle performance of lithium manganese iron phosphate cathode material

[0107]

[0108] The resistivity of Example 4 (4 coatings) is lower than that of Example 3 (3 coatings) because the 4-coating film is more dense and contains more Mn³. + Lower dissolution rate; Example 5 uses PEI / PAA coating, which has high 5C energy efficiency because its functional groups are more conducive to Li + Transmission; Comparative Example 1 (uncoated) showed high Mn³⁺ dissolution and high resistivity because, without a protective coating, the LMFP particles were in direct contact with the electrolyte, resulting in the John-Teller effect, which caused lattice distortion and Mn³⁺ dissolution. + Dissolution; Comparative Example 2 (unexcited) showed poor coating density because LMFP lacks hydroxyl binding sites on its surface, making the polyelectrolyte membrane prone to detachment; Comparative Example 3 (polyelectrolyte addition of 0.1%) had insufficient polyelectrolyte addition, resulting in incomplete coating and leading to Mn³ + High dissolution rate and high resistivity; Comparative Example 4 (polyelectrolyte addition of 1.0%) showed that excessive polyelectrolyte addition resulted in an excessively thick film layer, hindering Li... + Transmission; Comparative Example 5 (with 6 coating layers) has too many layers, which prevents electrons from transmitting quickly, forming a diffusion channel obstruction and resulting in a decrease in discharge capacity.

[0109] Table 2 Physicochemical properties of lithium manganese iron phosphate cathode material

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Claims

1. A method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material, wherein the preparation method comprises mixing a lithium manganese iron phosphate precursor and lithium carbonate in a solvent, wet grinding, drying, sintering, and pulverizing, characterized in that: The mixture is activated and coated after wet grinding before being dried. The activated mixture obtained by wet grinding is added with phosphoric acid to adjust the pH to 5-6, and then left to stand for 1-2 hours. The coating step involves adding a polyelectrolyte to the activated mixture and coating it for 1-2 hours. The polyelectrolyte is a polycationic electrolyte or a polyanionic electrolyte. The coating step is performed 1 to 4 times. The polyelectrolyte added in odd-numbered coating steps is a polycationic electrolyte, and the polyelectrolyte added in even-numbered coating steps is a polyanionic electrolyte.

2. The method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material according to claim 1, characterized in that: Polycationic electrolytes can be selected from polydiallyldimethylammonium chloride, polyethyleneimine, polylysine, or chitosan oligosaccharide; polyanionic electrolytes can be selected from sodium polystyrene sulfonate, polyacrylic acid, sodium carboxymethyl cellulose, or sodium alginate.

3. The method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The mass of the polycationic electrolyte or polyanionic electrolyte added in each coating step is 0.2% to 0.6% of the mass of the manganese iron phosphate precursor.

4. The method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material according to claim 3, characterized in that: After each coating, a polyelectrolyte film is formed on the surface of the excited mixture, and the thickness of each polyelectrolyte film coating is 10-20 nm.

5. The method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The solvent is water. The manganese iron phosphate precursor and lithium carbonate are added to the water, and the solid content is controlled at 10-20%. The mixture is mixed for 2-3 hours.

6. The method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The wet grinding process controls the particle size to be between 0.42 and 0.45 μm.

7. The method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The coated material is spray-dried and granulated, and the resulting granulated powder is sintered under a protective atmosphere to obtain polyelectrolyte membrane-coated lithium iron phosphate cathode material. The particle size of the spray-dried granules is controlled to be 24–30 μm, and the spray-drying outlet temperature is 100–110 °C. The sintering temperature under the protective atmosphere is 700-750℃ and the sintering time is 10-12h. The protective atmosphere is N2 or Ar as the protective gas and the oxygen content is less than 5ppm.

8. The method for preparing a polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material according to claim 1, characterized in that: The sintered material is pulverized by an air jet mill, with the grading frequency controlled at 50-60 Hz, the feeding frequency at 20-30 Hz, and the D50 particle size after pulverization required to be 1.0-1.2 μm.

9. A polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material, characterized in that: The polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material is prepared by the preparation method of the polyelectrolyte membrane-coated lithium manganese iron phosphate cathode material according to any one of claims 1 to 8.

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