A high-voltage, dense and long-cycle lithium manganese iron phosphate positive electrode material and a preparation method thereof

Abstract

The present invention discloses a high-voltage, dense, long-cycle lithium iron manganese phosphate positive electrode material and a preparation method thereof, comprising the following steps: S1, preparing metal-doped pyrophosphate manganese iron; S2, preparing metal-doped ferromanganese phosphate; S3, preparing lithium iron manganese phosphate material; S4, performing LiPAA modification on the lithium iron manganese phosphate material. The preparation method disclosed in the present invention synthesizes manganese iron compounds containing different doping elements by liquid phase method, improves the tap density of manganese iron compounds, grows particles into different size morphologies during sintering, improves the powder compaction density of the lithium iron manganese phosphate material, and coats a layer of solid electrolyte phase LiPAA material on its surface, avoids direct contact between the lithium iron manganese phosphate material and liquid electrolyte, enhances the strength of Mn-O bonds, reduces the Jan-Taylor effect caused by Mn dissolution, makes the structure of the lithium iron manganese phosphate material more stable, and improves the diffusion coefficient of lithium ions, thereby improving the long-cycle performance of the material.

Description

Technical Field

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

[0002] Lithium-ion battery is a kind of energy storage device widely used in energy storage field, power battery and portable electronic devices. Lithium-ion battery is mainly composed of positive electrode material, negative electrode material, electrolyte, diaphragm and shell. The positive electrode materials mainly include lithium cobalt oxide, lithium manganese oxide, nickel manganese binary system, nickel cobalt manganese ternary system, nickel cobalt aluminum ternary system, lithium iron phosphate, lithium manganese iron phosphate, etc. Lithium manganese iron phosphate (LMFP, LiMn 1-x Fe x PO4) is an olive-structured solid solution formed by doping a certain proportion of manganese on the basis of lithium iron phosphate. It has good thermal stability and chemical stability. Compared with lithium iron phosphate, lithium iron manganese phosphate has a higher voltage platform. The voltage of lithium iron manganese phosphate can reach about 4.1V, while that of lithium iron phosphate is about 3.4-3.5V. The two have the same theoretical gram capacity. Because the voltage is higher, the theoretical energy density of lithium iron manganese phosphate is 15-20% higher than that of lithium iron phosphate under the same conditions. At the same time, the electrical conductivity and lithium ion conductivity are improved compared with lithium manganese phosphate, and it is a positive electrode material that deserves great attention. However, the conductivity of a single lithium iron manganese phosphate material is very poor, and pure lithium iron manganese phosphate is almost an insulator, making it difficult to give full play to the electrochemical properties of the material.

[0003] At present, the methods for modifying lithium iron manganese phosphate include nano-particle size, cation doping, surface coating with carbon, etc. The combination of lithium iron manganese phosphate and highly conductive carbonaceous materials is considered to be one of the most commonly used and effective methods to improve the capacity and rate performance of lithium iron manganese phosphate. The surface conductive coating (such as carbon coating) is used to increase the conductivity of lithium iron manganese phosphate and prevent lithium iron manganese phosphate from directly contacting the electrolyte, thereby improving the high rate capacity and cycle stability. However, there is still a serious problem of manganese dissolution in lithium iron manganese phosphate, which leads to poor battery cycle and poor material compaction density compared to lithium iron phosphate, which limits its application. Summary of the invention

[0004] Based on the problem that manganese element is easily dissolved in the structure of lithium manganese iron phosphate, resulting in poor circulation and low compaction density, the present invention proposes a method for preparing high-pressure, dense and long-circulation lithium manganese iron phosphate.

[0005] A method for preparing a high-voltage, densely-cycled lithium manganese iron phosphate positive electrode material comprises the following steps:

[0006] S1. Preparation of metal-doped pyrophosphate manganese iron: Manganese dioxide, a first iron source, and a first phosphorus source are mixed evenly, wherein the molar ratio of manganese to iron is 1-4:1, and the molar ratio of the sum of the molar amounts of manganese and iron to the molar amount of phosphorus is 1:1-3; the first phosphorus source is an aqueous solution of phosphate and phosphoric acid, and the molar ratio of phosphate to phosphoric acid in the first phosphorus source is 2-4:1, the mixture is heated to 100-130°C, stirred until all the materials are dissolved to form a solution, and a doped metal salt is added to the solution and mixed evenly, wherein the ratio of the molar amount of the doped metal element to the sum of the molar amounts of manganese and iron is 0.5-2:100; protective gas is introduced, an alkaline solution is added to adjust the pH value to 2.5-9, a precipitate is precipitated, the obtained precipitate is filtered, washed, and dried; heat treatment is performed at 200-400°C for 2-4h to obtain a first metal-doped precursor manganese iron pyrophosphate;

[0007] S2. Preparation of metal-doped ferromanganese phosphate: a first surfactant and a complexing agent are mixed with water at a molar ratio of 0.05-0.2:1 to prepare a base solution in which the concentration of the sum of the moles of the first surfactant and the complex is 0.02-0.05 mol / L; a second iron source, a manganese source, and a doped metal salt are added with water at a molar ratio of 0.5-1:0.5-1:0.06 to prepare a solution in which the total metal ion concentration is 0.8-1.2 mol / L, and the solution is added to the base solution and mixed evenly, and the volume of the added solution is the same as the volume of the base solution; Ammonia water and the second phosphorus source are added with water in a molar ratio of 0.1-0.5:1 to prepare a concentration of 0.8-1.2 mol / L of the sum of ammonia water and the first phosphorus source, and slowly dripped into the evenly mixed base liquid, the volume of the added liquid being the same as the volume of the evenly mixed base liquid; the molar ratio of the added second phosphorus source to the doped metal element is 1-4:0.06, slowly heated to 60-90°C, stirred for reaction for 3-6h, a precipitate is generated, the precipitate is filtered and washed, and heat treated at 400-600°C for 2-6h to obtain metal-doped ferromanganese phosphate;

[0008] S3. Preparation of lithium iron manganese phosphate material: mix the prepared doped precursor pyrophosphate manganese iron, metal-doped manganese iron, lithium source, carbon source and water in a molar ratio of 1:2-6:4-6:0.03-0.07:65-75, add a second surfactant and mix evenly, the mass of the second surfactant added: the mass of water is 0.005-0.01:1, grind for 2-6h, dry the slurry after D50 is less than 0.45μm, and then place it in an inert gas atmosphere, heat to 650-780℃ at a heating rate of 2-10℃ per minute, calcine for 4-20h, and crush after cooling to obtain the lithium iron manganese phosphate material.

[0009] S4. Perform LiPAA modification on the lithium iron manganese phosphate material: mix the obtained lithium iron manganese phosphate material and water in a mass ratio of 0.8-2:100 to evenly wet them, add acrylic acid, lithium hydroxide and ammonium persulfate, and stir to react for 0.2-1h, wherein the mass ratio of lithium iron manganese phosphate material to acrylic acid, lithium hydroxide and ammonium persulfate is 1:0.2-0.7:0.1-0.2:0.04-0.08, treat at the saturated vapor pressure of water at 130-150°C for 5-14h, cool and dry to obtain the high-pressure dense long-cycle lithium iron manganese phosphate positive electrode material.

[0010] The first iron source is selected from one or more of ferrous sulfate, ferrous chloride, ferrous oxalate, and ferrous acetate; the phosphate is selected from one or more of diammonium hydrogen phosphate, ammonium dihydrogen phosphate, and ammonium phosphate; and the alkaline solution is preferably an aqueous solution of NaOH or aqueous ammonia.

[0011] The second iron source is selected from one or more of ferric chloride, ferric sulfate or ferric nitrate; the manganese source is selected from one or more of manganous chloride, manganous sulfate, manganous nitrate or manganous acetate. The second phosphorus source is selected from one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate and phosphoric acid.

[0012] The lithium source is selected from one or more of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, and lithium phosphate. The second surfactant is selected from one or more of hexadecyltrimethylammonium bromide, hexadecyltrimethylammonium chloride, sodium dodecyl sulfate, sodium dodecylbenzene sulfonate, polyethylene glycol, polyethylene oxide, and polyacrylamide.

[0013] Optionally, the doped metal salt in step S1 and step S2 is selected from one or two of lanthanum nitrate, aluminum nitrate, and magnesium chloride.

[0014] Optionally, the doped metal salt in step S1 and the doped metal salt in step S2 are not the same.

[0015] Optionally, in step S1, the ratio of the molar amount of the doped metal element to the sum of the molar amounts of the manganese element and the iron element is 1:100.

[0016] Optionally, in step S2, the ratio of the molar amount of the doped metal element to the sum of the molar amounts of the manganese element and the iron element is 0.5-2:100, preferably 1:100.

[0017] Optionally, the ratio of the molar ratio of the manganese element to the iron element in step S1 to the molar ratio of the manganese element to the iron element in step S1 is 1:0.8-1.2.

[0018] Optionally, in step S3, the heating rate is 5°C / min, and the temperature is raised to 720°C and calcined at a constant temperature for 14 hours.

[0019] Optionally, the carbon source is selected from one or more of sucrose, glucose, polyvinyl alcohol, and polyethylene glycol.

[0020] Optionally, in S1, the first surfactant is selected from one or more of ammonium stearate, dodecylbenzenesulfonic acid, and hexadecyltrimethylammonium bromide; and the complexing agent is selected from one or more of citric acid, stearic acid, or tartaric acid.

[0021] The present invention also provides a high-voltage, dense and long-cycle lithium manganese iron phosphate positive electrode material, which is prepared by the above-disclosed preparation method.

[0022] The preparation method disclosed in the present invention adopts a liquid phase method to synthesize a manganese iron compound containing different doping elements, thereby improving the tap density of the manganese iron compound. During sintering, particles grow into different sizes and morphologies, thereby improving the powder compaction density of the lithium iron manganese phosphate material, and coating a layer of solid electrolyte phase LiPAA material on its surface to avoid direct contact between the lithium iron manganese phosphate material and the liquid electrolyte, thereby enhancing the strength of the Mn-O bond and reducing the Jan-Taylor effect caused by Mn dissolution, making the structure of the lithium iron manganese phosphate material more stable while improving the diffusion coefficient of lithium ions and improving the long cycle performance of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a flow chart for preparing the high-voltage, dense-cycle lithium iron manganese phosphate positive electrode material in Example 1;

[0024] Figure 2 This is a particle size distribution diagram of the high-pressure, dense, long-cycle lithium manganese iron phosphate positive electrode material obtained in Example 1;

[0025] Figure 3 is a scanning electron microscope image of the metal-doped ferromanganese pyrophosphate obtained in Example 1;

[0026] Figure 4 is a scanning electron microscope image of the metal-doped ferromanganese phosphate obtained in Example 1;

[0027] Figure 5 This is a scanning electron microscope image of the high-pressure, dense-cycle lithium manganese iron phosphate positive electrode material obtained in Example 1. DETAILED DESCRIPTION

[0028] The following will describe the implementation methods of the present application in detail with the accompanying drawings and examples, so that the implementation process of how the present application uses technical means to solve technical problems and achieve technical effects can be fully understood and implemented accordingly. Based on the examples in the present application, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present application.

[0029] The reagents, methods and equipment used in this application are conventional reagents, methods and equipment in the art. Unless otherwise specified, the reagents and materials used in this application are commercially available.

[0030] Example 1

[0031] like Figure 1 As shown, a high-voltage, dense and long-cycle lithium manganese iron phosphate positive electrode material is prepared, in which the doped cations are metal ions, specifically lanthanum ions or aluminum ions.

[0032] Step S1: prepare lanthanum-doped ferromanganese pyrophosphate: add 0.6 mol manganese dioxide, 0.4 mol ferrous sulfate, and 1.8 mol diammonium hydrogen phosphate to 0.6 mol phosphoric acid in an aqueous solution with a concentration of 85 wt%, mix well, heat to 110 ° C, stir and react for 30 minutes to ensure that all substances are completely dissolved, then add 0.01 mol lanthanum nitrate, introduce nitrogen to disperse the gas generated by the reaction, add ammonia water to adjust the pH value of the reaction system to 7.8, precipitate, wash the precipitate four times with deionized water, place the precipitate at 110 ° C and vacuum dry for 4 hours, and after cooling, place the precipitate in a muffle furnace, heat to 350 ° C and heat for 3 hours to obtain lanthanum-doped flaky ferromanganese pyrophosphate (NH4)Mn 0.6 Fe 0.38 La 0.01 P2O7, named A.

[0033] Step S2 prepares aluminum-doped ferromanganese phosphate: dissolve ammonium stearate and citric acid in 1L of deionized water at a molar ratio of 0.1:1 to prepare a base solution with a total concentration of ammonium stearate and citric acid of 0.03 mol / L; dissolve ferric chloride, manganese chloride, and aluminum chloride in deionized water at a molar ratio of 0.39:0.6:0.01 to form a 1L solution with a total metal ion concentration of 1 mol / L, and add the solution dropwise to the base solution and mix well; add ammonia water and Ammonium dihydrogen phosphate was mixed in a molar ratio of 0.3:1 to form a solution with a concentration of 1 mol / L of the sum of 2 L of ammonia water and ammonium dihydrogen phosphate, and was slowly added dropwise to the evenly mixed base liquid, and the dropping time was controlled to 85±10 min. At the same time, the reaction container was heated to 65°C and stirred for 4.5 hours to generate a precipitate. After washing and filtering with deionized water, an aluminum-doped manganese iron phosphate dihydrate slurry was obtained, which was heat-treated at 550°C for 6 hours in a muffle furnace to obtain metal aluminum-doped manganese iron phosphate Mn 0.6 Fe 0.39 Al 0.01 PO4, named B.

[0034] Step S3 prepares lithium iron manganese phosphate material: 1.25 mol of prepared A, 3.75 mol of prepared B, 5.15 mol of lithium carbonate, 0.05 mol of glucose, 11.2 g of polyacrylamide, and 1245 g of deionized water are mixed evenly and placed in a ball mill for grinding for 5 hours. After D50 is less than 0.45 μm, the slurry is spray-dried, and the dried material is placed in a nitrogen atmosphere furnace, heated to 720°C at a heating rate of 5°C / min, and calcined at a constant temperature for 12 hours, and then cooled to room temperature to obtain a sintered material, and the sintered material is crushed to obtain the lithium iron manganese phosphate material.

[0035] Step S4: LiPAA modification of the lithium iron manganese phosphate material: 0.948 g of the obtained lithium iron manganese phosphate material was added to 80 g of deionized water and stirred for 1 hour, and then 0.432 g of acrylic acid, 0.144 g of LiOH and 70 mg of ammonium persulfate (APS) were added to the above solution and stirred for 0.5 hours. The above mixed solution was then moved to a 200 mL autoclave, then heated at 140 ° C for 10 hours, and then naturally cooled to room temperature. The mixture taken out of the autoclave was dialyzed and vacuum dried at 80 ° C for 10 hours to obtain a high-pressure dense long-cycle lithium iron manganese phosphate positive electrode material.

[0036] Bulk doping with metal elements can extend the Li-O and form vacancies to facilitate Li + Migration and stabilization of material structure improve electrochemical performance.

[0037] The high-pressure, dense, long-cycle lithium manganese iron phosphate positive electrode material obtained in Example 1 was tested for particle size by a particle size tester. The particle size distribution diagram is shown in FIG. Figure 1 As shown, the particle size distribution presents a bimodal distribution, the particle size distribution ranges from 200nm-20um, the particle size distribution is mostly small particles, and the average particle size is below 600nm.

[0038] The particles of A (metal lanthanum-doped manganese iron pyrophosphate), B (metal aluminum-doped manganese iron phosphate) and high-pressure dense long-cycle manganese iron phosphate lithium positive electrode material were observed by scanning electron microscopy to observe the morphological characteristics of the particles. Figure 3 As shown in FIG. 1 , the particle morphology of A (lanthanum-doped ferromanganese pyrophosphate) is irregularly scaly, and the primary particle size distribution is about 200-500 nm; Figure 4 As shown in the figure, the particle morphology of B (metal aluminum doped manganese iron phosphate) is a rounded small particle with holes, and the primary particle size distribution is about 100-300nm. Figure 5 As shown, the particles of the high-pressure, long-cycle lithium manganese iron phosphate positive electrode material are dense, and the primary particle size distribution is around 100-500nm.

[0039] Comparative Example 1

[0040] Repeat step S1, step S2 and step S3 of Example 1 to obtain the positive electrode material prepared in Comparative Example 1.

[0041] Comparative Example 2

[0042] Step S1, step S3 and step S4 of the embodiment are repeated. In step S3, part B is replaced by part A obtained in step S1 to obtain the positive electrode material prepared in comparative example 2.

[0043] Assembly test:

[0044] According to the mass ratio, the prepared positive electrode material (Example 1, Comparative Example 1, Comparative Example 2): carbon nanotube (CNT): Pvdf = 92:4:4 was used for the experiment, and NMP was used as the solvent. The slurry was homogenized on a centrifugal dispersion device, and the coating surface density was 80m 2 / g, and then baked to assemble into 2025 type button cells for electrical performance testing. The electrolyte used was a conventional lithium manganese iron phosphate electrolyte system. Electrical performance testing was performed on the Chengde test cabinet cte-1000 equipment at a temperature of 25±2℃ and a voltage range of 2.0-4.3V.

[0045] Compare the difference between the first charge and discharge and the 1C rate, use the buckle test high temperature cycle, 0.2C charge and 0.5C discharge, voltage range 2-4.2V. At the same time, use Yuanneng powder compaction equipment to test powder pressure and powder conductivity, and calculate the lithium diffusion coefficient through the PITT test method.

[0046] The relevant test result data are shown in Table 1.

[0047] Table 1 Test results data

[0048]

[0049] It can be seen from Table 1 that the electrical properties and powder compaction density of Example 1 are better than those of Comparative Example 1 and Comparative Example 2; the powder compaction density of Comparative Example 1 is close to that of Example 1, but the capacity of Comparative Example 1 is slightly worse than that of Example 1, and the cycle level is poor; the powder compaction density of Comparative Example 2 is much different from that of Example 1, and the capacity ratio is slightly worse. The resistivity of Example 1 is much lower than that of Comparative Example 1 and Comparative Example 2, and Example 1 has excellent power performance.

Claims

1. A method for preparing a high-voltage, dense and long-cycle lithium manganese iron phosphate positive electrode material, characterized in that: The following steps are involved: S1. Preparation of metal-doped pyrophosphate manganese iron: Manganese dioxide, a first iron source, and a first phosphorus source are mixed evenly, wherein the molar ratio of manganese to iron is 1-4:1, and the molar ratio of the sum of the molar amounts of manganese and iron to the molar amount of phosphorus is 1:1-3; the first phosphorus source is an aqueous solution of phosphate and phosphoric acid, and the molar ratio of phosphate to phosphoric acid in the first phosphorus source is 2-4:1, the mixture is heated to 100-130°C, stirred until all the materials are dissolved to form a solution, and a doped metal salt is added to the solution and mixed evenly, wherein the ratio of the molar amount of the doped metal element to the sum of the molar amounts of manganese and iron is 0.5-2:100; protective gas is introduced, an alkaline solution is added to adjust the pH value to 2.5-9, a precipitate is precipitated, the obtained precipitate is filtered, washed, and dried; heat treatment is performed at 200-400°C for 2-4h to obtain a first metal-doped precursor manganese iron pyrophosphate; S2. Preparation of metal-doped ferromanganese phosphate: a first surfactant and a complexing agent are mixed with water at a molar ratio of 0.05-0.2:1 to prepare a base solution in which the concentration of the sum of the first surfactant and the complex is 0.02-0.05 mol / L; a second iron source, a manganese source, and a doped metal salt are added with water at a molar ratio of 0.5-1:0.5-1:0.06 to prepare a solution in which the total metal ion concentration is 0.8-1.2 mol / L, and the solution is added to the base solution and mixed evenly, and the volume of the added solution is the same as the volume of the base solution; Ammonia water and the second phosphorus source are added with water in a molar ratio of 0.1-0.5:1 to prepare a concentration of 0.8-1.2 mol / L of the sum of ammonia water and the first phosphorus source, and slowly dripped into the evenly mixed base liquid, and the volume of the added liquid is the same as the volume of the evenly mixed base liquid; the molar ratio of the added second phosphorus source to the doped metal element is 1-4:0.06, slowly heated to 60-90°C, stirred for reaction for 3-6h, a precipitate is generated, the precipitate is filtered and washed, and placed at 400-600°C for heat treatment for 2-6h to obtain metal-doped ferromanganese phosphate; S3, preparation of lithium iron manganese phosphate material: the prepared doped precursor pyrophosphate manganese iron, metal-doped manganese iron, lithium source, carbon source and water are mixed in a molar ratio of 1: 2-6: 4-6: 0.03-0.07: 65-75, a second surfactant is added and mixed evenly, the mass of the second surfactant added is 0.005-0.01: 1 of water, the slurry is ground for 2-6 hours, and after D50 is less than 0.45 μm, the slurry is dried, and then placed in an inert gas atmosphere, heated to 650-780°C at a heating rate of 2-10°C per minute, calcined for 4-20 hours, and pulverized after cooling to obtain the lithium iron manganese phosphate material; S4. Perform LiPAA modification on the lithium iron manganese phosphate material: mix the obtained lithium iron manganese phosphate material and water in a mass ratio of 0.8-2:100 to evenly wet them, add acrylic acid, lithium hydroxide and ammonium persulfate, and stir to react for 0.2-1h, wherein the mass ratio of lithium iron manganese phosphate material to acrylic acid, lithium hydroxide and ammonium persulfate is 1:0.2-0.7:0.1-0.2:0.04-0.08, treat at the saturated vapor pressure of water at 130-150°C for 5-14h, cool and dry to obtain the high-pressure dense long-cycle lithium iron manganese phosphate positive electrode material.

2. The method for preparing a high-pressure, densely-cycled lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The doped metal salt in step S1 and step S2 is selected from one or two of lanthanum nitrate, aluminum nitrate and magnesium chloride.

3. The method for preparing a high-pressure, dense and long-cycle lithium manganese iron phosphate positive electrode material according to claim 2, characterized in that: The doping metal salt in step S1 and the doping metal salt in step S2 are not the same.

4. The method for preparing a high-pressure, dense and long-cycle lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: In step S1, the ratio of the molar amount of the doped metal element to the sum of the molar amounts of the manganese element and the iron element is 1:

100.

5. The method for preparing a high-voltage, densely-cycled lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step S2, the ratio of the molar amount of the doped metal element to the sum of the molar amounts of the manganese element and the iron element is 0.5-2:

100.

6. The method for preparing a high-voltage, densely-cycled lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The ratio of the molar amount ratio of the manganese element to the iron element in step S1 to the molar amount ratio of the manganese element to the iron element in step S1 is 1:0.8-1.

2.

7. The method for preparing a high-voltage, densely-cycled lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In step S3, the heating rate is 5°C / min, and the temperature is raised to 720°C and calcined at a constant temperature for 14 hours.

8. The method for preparing a high-voltage, densely-cycled lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The carbon source is selected from one or more of sucrose, glucose, polyvinyl alcohol and polyethylene glycol.

9. The method for preparing a high-voltage, densely-cycled lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: In S1, the first surfactant is selected from one or more of ammonium stearate, dodecylbenzenesulfonic acid, and hexadecyltrimethylammonium bromide; and the complexing agent is selected from one or more of citric acid, stearic acid, or tartaric acid.

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

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