A method for preparing lithium manganese iron phosphate positive electrode material

Abstract

The invention discloses a preparation method of a lithium manganese iron phosphate cathode material, comprising the following steps: 1) dissolving manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate in water to obtain a first solution, adding a surfactant to the first solution to obtain a second solution; 2) placing the second solution in a constant temperature water bath and heating it, stirring continuously until the water is completely evaporated to obtain a mixture; 3) adding a carbon source to the mixture and mixing it with anhydrous ethanol as a medium for wet ball milling to a particle size of 100-200 μm, drying, calcining, and naturally cooling to room temperature to obtain a precursor powder; 4) dissolving the precursor powder and polypropylene in anhydrous ethanol, stirring continuously under constant temperature water bath conditions until the anhydrous ethanol solution evaporates and then drying to constant weight, calcining, and obtaining the lithium manganese iron phosphate cathode material. The present invention can significantly improve the electrochemical performance of the prepared high lithium manganese iron phosphate cathode material and the storage performance of the lithium battery by adding a surfactant and polypropylene, thereby improving the service life of the lithium battery.

Description

Technical Field

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

[0002] As a new energy source, lithium-ion batteries have become a hot topic of research in both industry and academia. Cathode materials are a crucial component of lithium-ion batteries, playing a crucial role in their electrochemical and safety performance. Commonly used cathode materials for lithium-ion batteries include lithium manganese phosphate, lithium iron phosphate, and lithium iron manganese phosphate. Of these, lithium iron manganese phosphate has been widely used due to its high discharge voltage and thermal stability.

[0003] At present, the commonly used preparation method of lithium manganese iron phosphate adopts the high-temperature solid-phase method. The high-temperature solid-phase method refers to mixing the precursor reactants in a certain proportion through stirring and grinding, and then sintering them at a certain temperature for a period of time under a certain atmosphere (air or inert gas). The final product is obtained through cooling, crushing, grinding and other processes.

[0004] The specific operating steps of the high-temperature solid-phase method are as shown in patent application number CN201811504451.3, which discloses a method for preparing lithium manganese iron phosphate, comprising the following steps: (1) mixing elemental iron, manganese dioxide and a phosphoric acid aqueous solution to obtain a mixture A, and ball-milling the mixture A to obtain hydrogen manganese iron phosphate; (2) mixing the hydrogen manganese iron phosphate, lithium carbonate and glucose, sand-milling the mixture to a product particle size D50 of 0.2 μm-1 μm, drying, and calcining to obtain lithium manganese iron phosphate.

[0005] The lithium iron phosphate material prepared by this method has a small particle size, which makes the gaps between the particles of the material too small, making it difficult for the electrolyte to completely enter between the material particles, resulting in lithium ions in the electrolyte having difficulty reaching between the internal particles of the lithium iron phosphate material, causing the discharge specific capacity of the lithium iron phosphate material to decrease and the cycle performance to be poor; and because the material has a small particle size and a large specific surface area, it is easy to absorb water after long-term storage, causing the battery to be corroded by water and damaged, thereby reducing the battery life. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art and provide a method for preparing a lithium manganese iron phosphate positive electrode material with a simple process flow, which can effectively avoid water absorption and improve electrochemical performance.

[0007] The technical solution adopted by the present invention to solve the technical problem is: a method for preparing a lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0008] 1) dissolving manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate in water to obtain a first solution, and adding a surfactant to the first solution to obtain a second solution;

[0009] 2) heating the second solution in a constant temperature water bath while stirring continuously until the water is completely evaporated to obtain a mixture;

[0010] 3) Adding a carbon source to the mixture, mixing with anhydrous ethanol as a medium, wet ball milling, drying, calcining, and naturally cooling to room temperature to obtain a precursor powder;

[0011] 4) Dissolve the precursor powder and polypropylene in anhydrous ethanol, stir continuously in a constant temperature water bath until the anhydrous ethanol solution evaporates, then dry to constant weight and calcine to obtain the lithium manganese iron phosphate positive electrode material.

[0012] Furthermore, the molar ratio of manganese sulfate, ferrous sulfate, lithium hydroxide and ammonium dihydrogen phosphate is (0.6-0.7): (0.3-0.4): (0.5-0.6): 1.

[0013] Furthermore, the surfactant is Tween, and the solid-liquid ratio of the surfactant to the first solution is 1:10-15.

[0014] Furthermore, the temperature of the water bath heating in step 2) is 120-160°C.

[0015] Furthermore, the calcination treatment in step 3) is performed as follows: pre-calcination at 300° C. for 3 hours under a nitrogen atmosphere, and then calcination at 700° C. for 10 hours.

[0016] Furthermore, the carbon source is one of carbon black and glucose or a mixture of the two, and the molar ratio of the carbon source to the mixture is 0.3-0.6:1.

[0017] Furthermore, the temperature of the wet ball milling is 60-80°C, the wet ball milling time is 3-5 hours, and the drying temperature is 80-90°C.

[0018] Furthermore, the mass ratio of the polypropylene to the precursor powder is 0.3-0.6:1.

[0019] Furthermore, the temperature of the constant temperature water bath in step 4) is 90-95°C.

[0020] Furthermore, the calcination operation in step 4) is as follows: calcining at 600-700° C. for 6-8 hours under a nitrogen atmosphere.

[0021] The beneficial effects of the preparation method of the lithium manganese iron phosphate positive electrode material of the present invention are as follows:

[0022] (1) The preparation process of the present invention is simple and easy to operate. By adding a surfactant to the solution after the raw materials are dissolved, the particle size of the prepared material particles can be appropriately reduced, which can effectively shorten the time of subsequent wet ball milling and improve the preparation efficiency of the lithium manganese iron phosphate positive electrode material. The addition of the surfactant can also effectively improve the discharge capacity and cycle performance of the positive electrode material, thereby improving the electrochemical performance of the prepared high-manganese iron lithium phosphate positive electrode material.

[0023] (2) The present invention adds polypropylene to the precursor powder, and the polypropylene can be coated on the surface of the lithium iron manganese phosphate, so that the lithium iron manganese phosphate is separated from the outside world, thereby avoiding the water absorption phenomenon of the lithium iron manganese phosphate positive electrode material, extending the storage performance of the lithium battery composed of the lithium iron manganese phosphate positive electrode material, and improving the service life of the lithium battery. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 — is a process flow chart of a method for preparing a lithium manganese iron phosphate positive electrode material of the present invention. DETAILED DESCRIPTION

[0025] The present invention will be further described below with reference to the accompanying drawings and examples, but these specific embodiments do not limit the scope of protection of the present invention in any way.

[0026] Example 1

[0027] A method for preparing a lithium manganese iron phosphate positive electrode material comprises the following steps:

[0028] 1) dissolving manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate in water to obtain a first solution, wherein the molar ratio of manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate is 0.65:0.35:0.55:1; and adding Tween to the first solution at a material-liquid ratio of 1:12 to obtain a second solution;

[0029] 2) heating the second solution in a constant temperature water bath at 150°C with continuous stirring until the water is completely evaporated to obtain a mixture;

[0030] 3) adding carbon black to the mixture in a molar ratio of carbon black to the mixture of 0.5:1, and wet-milling the mixture using anhydrous ethanol as a medium at a temperature of 75° C. for 4 hours, followed by drying at 85° C.; then pre-calcining the mixture at 300° C. for 3 hours under a nitrogen atmosphere, calcining the mixture at 700° C. for 10 hours, and then naturally cooling the mixture to room temperature to obtain a precursor powder;

[0031] 4) The precursor powder and polypropylene were dissolved in anhydrous ethanol, with a mass ratio of polypropylene to precursor powder of 0.5:1. The mixture was stirred continuously in a constant temperature water bath at 90°C until the anhydrous ethanol solution evaporated and then dried to a constant weight. The mixture was then calcined at 650°C for 7 hours under a nitrogen atmosphere to obtain a lithium manganese iron phosphate positive electrode material.

[0032] Example 2

[0033] A method for preparing a lithium manganese iron phosphate positive electrode material comprises the following steps:

[0034] 1) dissolving manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate in water to obtain a first solution, wherein the molar ratio of manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate is 0.6:0.3:0.5:1; and adding Tween to the first solution at a material-liquid ratio of 1:10 to obtain a second solution;

[0035] 2) heating the second solution in a constant temperature water bath at 120°C with continuous stirring until the water is completely evaporated to obtain a mixture;

[0036] 3) Glucose was added to the mixture in a molar ratio of glucose to the mixture of 0.3:1, and wet ball milling was performed using anhydrous ethanol as a medium. The wet ball milling temperature was 60° C. and the wet ball milling time was 3 hours. The mixture was then dried at 80° C.; the mixture was pre-calcined at 300° C. for 3 hours under a nitrogen atmosphere, and then calcined at 700° C. for 10 hours, and then naturally cooled to room temperature to obtain a precursor powder;

[0037] 4) The precursor powder and polypropylene were dissolved in anhydrous ethanol, with a mass ratio of polypropylene to precursor powder of 0.3:1. The mixture was stirred continuously in a constant temperature water bath at 90°C until the anhydrous ethanol solution evaporated and then dried to a constant weight. The mixture was then calcined at 600°C for 6 hours under a nitrogen atmosphere to obtain a lithium manganese iron phosphate positive electrode material.

[0038] Example 3

[0039] A method for preparing a lithium manganese iron phosphate positive electrode material comprises the following steps:

[0040] 1) dissolving manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate in water to obtain a first solution, wherein the molar ratio of manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate is 0.7:0.4:0.6:1; and adding Tween to the first solution at a material-liquid ratio of 1:15 to obtain a second solution;

[0041] 2) heating the second solution in a constant temperature water bath at 160°C with continuous stirring until the water is completely evaporated to obtain a mixture;

[0042] 3) adding carbon black to the mixture at a molar ratio of carbon black to the mixture of 0.6:1, and wet-milling the mixture using anhydrous ethanol at a temperature of 80° C. for 5 hours, followed by drying at 90° C.; pre-calcining the mixture at 300° C. for 3 hours under a nitrogen atmosphere, calcining the mixture at 700° C. for 10 hours, and then naturally cooling the mixture to room temperature to obtain a precursor powder;

[0043] 4) The precursor powder and polypropylene were dissolved in anhydrous ethanol at a mass ratio of polypropylene to precursor powder of 0.6:1. The mixture was stirred continuously in a constant temperature water bath at 95°C until the anhydrous ethanol solution evaporated and then dried to a constant weight. The mixture was then calcined at 700°C for 8 hours under a nitrogen atmosphere to obtain a lithium manganese iron phosphate positive electrode material.

[0044] Comparative Example 1

[0045] A method for preparing a lithium manganese iron phosphate positive electrode material comprises the following steps:

[0046] 1) dissolving manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate in water to obtain a first solution, wherein the molar ratio of manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate is 0.65:0.35:0.55:1;

[0047] 2) heating the first solution in a constant temperature water bath at 150°C with continuous stirring until the water is completely evaporated to obtain a mixture;

[0048] 3) adding carbon black to the mixture in a molar ratio of carbon black to the mixture of 0.5:1, and wet-milling the mixture using anhydrous ethanol as a medium at a temperature of 75° C. for 4 hours, followed by drying at 85° C.; then pre-calcining the mixture at 300° C. for 3 hours under a nitrogen atmosphere, calcining the mixture at 700° C. for 10 hours, and then naturally cooling the mixture to room temperature to obtain a precursor powder;

[0049] 4) Dissolve the precursor powder in anhydrous ethanol and stir continuously in a constant-temperature water bath at 90°C until the anhydrous ethanol solution evaporates. Dry to a constant weight and then calcine at 650°C for 7 hours under a nitrogen atmosphere to obtain the lithium manganese iron phosphate cathode material.

[0050] Comparative Example 2

[0051] A method for preparing a lithium manganese iron phosphate positive electrode material comprises the following steps:

[0052] 1) dissolving manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate in water to obtain a first solution, wherein the molar ratio of manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate is 0.65:0.35:0.55:1;

[0053] 2) heating the first solution in a constant temperature water bath at 150°C with continuous stirring until the water is completely evaporated to obtain a mixture;

[0054] 3) adding carbon black to the mixture in a molar ratio of carbon black to the mixture of 0.5:1, and wet-milling the mixture using anhydrous ethanol as a medium at a temperature of 75° C. for 4 hours, followed by drying at 85° C.; then pre-calcining the mixture at 300° C. for 3 hours under a nitrogen atmosphere, calcining the mixture at 700° C. for 10 hours, and then naturally cooling the mixture to room temperature to obtain a precursor powder;

[0055] 4) The precursor powder and polypropylene were dissolved in anhydrous ethanol, with a mass ratio of polypropylene to precursor powder of 0.5:1. The mixture was stirred continuously in a constant temperature water bath at 90°C until the anhydrous ethanol solution evaporated and then dried to a constant weight. The mixture was then calcined at 650°C for 7 hours under a nitrogen atmosphere to obtain a lithium manganese iron phosphate positive electrode material.

[0056] Comparative Example 3

[0057] A method for preparing a lithium manganese iron phosphate positive electrode material comprises the following steps:

[0058] 1) dissolving manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate in water to obtain a first solution, wherein the molar ratio of manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate is 0.65:0.35:0.55:1; and adding Tween to the first solution at a material-liquid ratio of 1:12 to obtain a second solution;

[0059] 2) heating the second solution in a constant temperature water bath at 150°C with continuous stirring until the water is completely evaporated to obtain a mixture;

[0060] 3) adding carbon black to the mixture in a molar ratio of carbon black to the mixture of 0.5:1, and wet-milling the mixture using anhydrous ethanol as a medium at a temperature of 75° C. for 4 hours, followed by drying at 85° C.; then pre-calcining the mixture at 300° C. for 3 hours under a nitrogen atmosphere, calcining the mixture at 700° C. for 10 hours, and then naturally cooling the mixture to room temperature to obtain a precursor powder;

[0061] 4) The precursor powder was dissolved in anhydrous ethanol, stirred continuously in a constant temperature water bath at 90°C until the anhydrous ethanol solution evaporated and then dried to constant weight, and then calcined at 650°C for 7h under a nitrogen atmosphere to obtain lithium manganese iron phosphate positive electrode material.

[0062] The present invention measured the electrochemical properties (discharge specific capacity) of the lithium manganese iron phosphate cathode materials prepared in Examples 1-3 and Comparative Examples 1-3 using button cells. The cathode active materials of these button cells were the lithium manganese iron phosphate cathode materials prepared in Examples 1-3 and Comparative Examples 1-3, respectively. A lithium sheet was used as the negative electrode. Acetylene black was used as the conductive agent. Polyvinylidene fluoride was used as the binder. A Celgard 2325 composite membrane was used as the separator. The electrolyte was a 1 mol / L lithium phosphate hexafluorophosphate solution of dimethyl carbonate (DMC) and ethylene carbonate (EC) in an equal volume ratio. The mass ratio of the positive electrode active material, conductive agent, and binder was 92:2:3. The button cells were assembled in a glove box with an argon protective atmosphere. Constant current charge and discharge tests were performed using a blue electric test cabinet, with the test voltage range set to 2.5V to 4.5V. The results are shown in Tables 1 and 2.

[0063]

[0064]

[0065] As can be seen from Table 1, the lithium manganese iron phosphate positive electrode materials prepared by the method of the present invention in Examples 1-3 have a discharge specific capacity at a discharge rate of 0.2C and 0.5C that is higher than 150 mAh·g-1, and a discharge specific capacity at a discharge rate of 1C that is higher than 140 mAh·g-1. In addition, the discharge specific capacity of the electrode materials in Examples 1-3 does not show obvious attenuation after 200 and 300 cycles at a discharge rate of 1C.

[0066] By comparing the data in Table 2, it can be seen that the discharge specific capacity and storage time of the lithium iron manganese phosphate positive electrode materials prepared in Comparative Examples 1-3 are significantly smaller than those in Example 1, and their cycle performance is also significantly lower than that in Example 1; among Comparative Examples 1-3, the discharge specific capacity and cycle performance of the lithium iron manganese phosphate positive electrode material prepared in Comparative Example 3 without adding polypropylene for treatment are the largest, and the best, the discharge specific capacity and cycle performance of the lithium iron manganese phosphate positive electrode material prepared in Comparative Example 2 without adding surfactant for treatment are second, and the discharge specific capacity and cycle performance of the lithium iron manganese phosphate positive electrode material prepared in Comparative Example 1 without adding polypropylene and surfactant for treatment indicate that the discharge specific capacity and cycle performance of the lithium iron manganese phosphate positive electrode material can be significantly increased by adding surfactant treatment, thereby improving the electrochemical properties of the lithium iron manganese phosphate positive electrode material; but the storage time corresponding to Comparative Example 2 is longer than that of Comparative Example 3, indicating that the addition of polypropylene can reduce the water absorption phenomenon of the battery and improve the storage performance of the lithium iron manganese phosphate positive electrode material.

[0067] The above description is merely a preferred embodiment of the present invention and does not constitute any form of limitation to the present invention. Although the present invention has been disclosed as a preferred embodiment, it is not intended to limit the present invention. Any technician familiar with the present profession can make some changes or modifications to equivalent embodiments of equivalent changes using the technical contents disclosed above without departing from the scope of the technical solution of the present invention. However, any simple modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention without departing from the content of the technical solution of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate positive electrode material, characterized in that: The following steps are involved: 1) dissolving manganese sulfate, ferrous sulfate, lithium hydroxide, and ammonium dihydrogen phosphate in water to obtain a first solution, adding Tween to the first solution at a material-to-liquid ratio of Tween to the first solution of 1:10-15 to obtain a second solution; 2) heating the second solution in a constant temperature water bath while stirring continuously until the water is completely evaporated to obtain a mixture; 3) adding a carbon source to the mixture, mixing, and wet ball milling with anhydrous ethanol as a medium to a particle size of 100-200 μm, drying, calcining, and naturally cooling to room temperature to obtain a precursor powder; 4) dissolving the precursor powder and polypropylene in anhydrous ethanol, stirring continuously in a constant temperature water bath until the anhydrous ethanol solution evaporates, drying to a constant weight, and calcining to obtain a lithium manganese iron phosphate positive electrode material.

2. The method for preparing a lithium iron manganese phosphate positive electrode material according to claim 1, wherein: The molar ratio of manganese sulfate, ferrous sulfate, lithium hydroxide and ammonium dihydrogen phosphate is (0.6-0.7): (0.3-0.4): (0.5-0.6):

1.

3. The method for preparing a lithium manganese iron phosphate positive electrode material according to claim 1, wherein: The temperature of the water bath heating in step 2) is 120-160°C.

4. The method for preparing a lithium manganese iron phosphate positive electrode material according to claim 1, wherein: The calcination treatment in step 3) is as follows: pre-calcination at 300°C for 3 hours under a nitrogen atmosphere, and then calcination at 700°C for 10 hours.

5. The method for preparing a lithium manganese iron phosphate positive electrode material according to claim 1, wherein: The carbon source is one of carbon black and glucose or a mixture of the two, and the molar ratio of the carbon source to the mixture is 0.3-0.6:

1.

6. A method for preparing a lithium iron manganese phosphate positive electrode material according to claim 5, characterized in that: The temperature of the wet ball milling is 60-80°C, the wet ball milling time is 3-5h, and the drying temperature is 80-90°C.

7. The method for preparing a lithium iron manganese phosphate positive electrode material according to claim 1, wherein: The mass ratio of the polypropylene to the precursor powder is 0.3-0.6:

1.

8. The method for preparing a lithium iron manganese phosphate positive electrode material according to claim 1, wherein: The temperature of the constant temperature water bath in step 4) is 90-95°C.

9. A method for preparing a lithium manganese iron phosphate positive electrode material according to claim 8, characterized in that: The calcination process in step 4) is as follows: calcining at 600-700°C for 6-8h under a nitrogen atmosphere.

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