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

Abstract

The invention discloses a modified lithium manganese iron phosphate positive electrode material and a preparation method and application thereof, and the preparation method comprises the following steps: S1, carrying out wet grinding on 100 parts by mass of a precursor, 2-5 parts by mass of a carbon source and 0.5-1.5 parts by mass of TiCN to obtain slurry; s2, sintering and annealing the slurry to obtain a modified lithium manganese iron phosphate positive electrode material; the preparation method of the precursor comprises the following steps: mixing a lithium hydroxide solution, a modified concentrated phosphoric acid solution and a salt solution to obtain a precursor solution, and carrying out hydrothermal reaction and drying on the precursor solution to obtain the precursor. The raw materials of the modified lithium iron manganese phosphate positive electrode material are doped with nickel and cobalt, so that the capacity of the positive electrode material is improved, and the positive electrode material can be stabilized; the carbon source and TiCN are used as coating materials, so that agglomeration in the sintering process of the positive electrode material can be effectively controlled.

Description

Technical Field

[0001] The invention belongs to the technical field of lithium manganese iron phosphate positive electrode materials, and specifically relates to a modified lithium manganese iron phosphate positive electrode material and a preparation method and application thereof. Background Art

[0002] Lithium iron manganese phosphate is a very popular cathode material with good safety and stability. The low rate performance caused by poor Li+ diffusion and conductivity can be improved by carbon coating, hypervalent doping, nanocrystallization and other methods. However, lithium iron manganese phosphate cathode materials are gradually unable to meet the high energy density requirements of lithium-ion batteries for electric vehicles.

[0003] Lithium iron manganese phosphate cathode material has good safety and thermal stability, and higher energy storage density and longer cycle life will become advantages in future market competition. However, in practical applications, the problem of metal element dissolution destroying the crystal structure leads to a serious decrease in its storage or discharge capacity. Therefore, it is necessary to modify the lithium iron manganese phosphate cathode material. Summary of the invention

[0004] The present invention overcomes the above technical problems and provides a modified lithium manganese iron phosphate positive electrode material and a preparation method and application thereof. The modified lithium manganese iron phosphate positive electrode material of the present invention has excellent discharge performance and cycle performance.

[0005] The present invention solves the above technical problems through the following technical solutions.

[0006] The present invention discloses a method for preparing a modified lithium manganese iron phosphate positive electrode material, comprising the following steps:

[0007] S1. By mass, 100 parts of the precursor, 2 to 5 parts of the carbon source and 0.5 to 1.5 parts of TiCN were wet-milled to obtain a slurry;

[0008] S2. The slurry is sintered and annealed to obtain a modified lithium manganese iron phosphate positive electrode material;

[0009] The preparation method of the precursor comprises the following steps: mixing a lithium hydroxide solution, a modified concentrated phosphoric acid solution and a salt solution to obtain a precursor solution, and subjecting the precursor solution to a hydrothermal reaction and drying to obtain the precursor.

[0010] In some embodiments, the salt solution includes a manganese sulfate solution, a ferrous sulfate solution, a nickel acetate solution, and a cobalt acetate solution.

[0011] In some embodiments, the carbon source is sucrose or glucose.

[0012] In some embodiments, the modified concentrated phosphoric acid solution is a polyethylene glycol solution of phosphoric acid and RuBP (1,5-diphospho-D-ribulose, CAS No.: 14689-84-0).

[0013] In some embodiments, the concentration of lithium hydroxide in the lithium hydroxide solution is 0.8-1.0 mol / L, for example 1.0 mol / L.

[0014] In some embodiments, the molar ratio of phosphoric acid to RuBP in the modified concentrated phosphoric acid solution is 10:1 to 2.5.

[0015] In some embodiments, the concentration of phosphoric acid in the modified concentrated phosphoric acid solution is 3-5 V / V%; for example, the concentration of concentrated phosphoric acid in the modified concentrated phosphoric acid solution is 3 V / V%.

[0016] In some embodiments, the co-solvent in the modified concentrated phosphoric acid is PEG400.

[0017] In some embodiments, the concentration of manganese sulfate in the salt solution is 0.4-0.5 mol / L, for example, the concentration of manganese sulfate in the salt solution is 0.5 mol / L.

[0018] In some embodiments, the molar ratio of Mn:Fe:Ni:Co in the salt solution is 0.6:0.4-xy:x:y, wherein x=0.015-0.035, y=0.015-0.035; preferably, x=0.020-0.030, y=0.020-0.030.

[0019] In some embodiments, ascorbic acid is further added to the salt solution, and the molar ratio of manganese sulfate to ascorbic acid is 10:0.5-1.5.

[0020] In some embodiments, the hydrothermal reaction step is to heat the precursor solution at 150-170° C. for 7-10 hours, and after heating, solid-liquid separation, centrifugation and drying are performed to obtain the precursor.

[0021] In some embodiments, the wet grinding speed is 450-500 rpm, for example 450 rpm.

[0022] In some embodiments, the wet grinding time is 4.5 to 8 hours, such as 6 hours.

[0023] In some embodiments, the ball-to-material ratio of the wet grinding is 4 to 5:1, for example 4:1.

[0024] In some embodiments, the solid content of the slurry is 60-68%.

[0025] In some embodiments, the heating rate before sintering is 1-5° C. / min.

[0026] In some embodiments, the sintering temperature is 680-720° C., and the sintering is performed at this temperature for 8-12 hours.

[0027] In some embodiments, the annealing temperature is 300-350°C, and the annealing is maintained at this temperature for 1.5-2.5 hours, and then cooled to room temperature. In the present invention, the room temperature is 20°C.

[0028] In some embodiments, the sintering and the annealing are both performed in a nitrogen atmosphere, and the nitrogen flow rate is 10-25 L / min.

[0029] In some embodiments, the mixing process is to add a modified phosphoric acid solution to a lithium hydroxide solution to adjust the pH to 10.0-11.5, and then add a salt solution to adjust the pH to 6.5-7.5.

[0030] The present invention discloses a modified lithium manganese iron phosphate positive electrode material prepared by the aforementioned preparation method of the modified lithium manganese iron phosphate positive electrode material.

[0031] In the present invention, the modified lithium manganese iron phosphate positive electrode material has a discharge capacity of 158 to 163 mAh / g at 0.1C, a discharge capacity of 152 to 154 mAh / g at 1C, and a discharge capacity of 138 to 141 mAh / g at 2.0C at a temperature of 45°C and a voltage range of 2.0 to 4.35 V.

[0032] The present invention also discloses the application of the modified lithium iron manganese phosphate positive electrode material in the field of electric vehicles or energy storage systems.

[0033] On the basis of being in accordance with the common sense in the art, the above-mentioned preferred conditions can be arbitrarily combined to obtain the preferred embodiments of the present invention.

[0034] Compared with the prior art, the present invention has the following beneficial effects:

[0035] 1. The present invention relates to a method for preparing a nickel-cobalt doped lithium manganese iron phosphate (LiMnFePO4) positive electrode material. First, the capacity of the material is increased by doping nickel in the precursor, and cobalt is doped to stabilize the positive electrode material to prevent capacity decay caused by cation mixing. During the preparation process, a small amount of RuBP is added to the selected phosphoric acid solution, which serves as both a phosphorus source and a structure directing agent, and helps to initially form a nickel-cobalt doped lithium manganese iron phosphate precursor with good dispersibility and small particle size through a hydrothermal reaction.

[0036] 2. The nickel-cobalt doped lithium manganese iron phosphate precursor is then subjected to high-temperature solid-phase sintering, and the carbon source and TiCN are used as coating materials to coat the surface of the nickel-cobalt doped lithium manganese iron phosphate precursor. Carbon coating can improve the conductivity of the positive electrode material and inhibit the growth of grains and agglomeration of particles during the sintering process. TiCN, as a steric inhibitor, can avoid grain agglomeration during the sintering process, thereby controlling the particle size and preventing excessive growth of crystals; this makes the surface of the lithium manganese iron phosphate positive electrode material smooth, round and uniform, further optimizing the performance of the material.

[0037] 3. A button cell assembled from the modified lithium iron manganese phosphate positive electrode material was subjected to electrochemical performance testing. The test results show that the modified lithium iron manganese phosphate positive electrode material of the present invention has a significantly improved discharge capacity at high temperatures, showing excellent high temperature stability. Therefore, the preparation method of the present invention can not only increase the capacity of the material, but also significantly improve the performance stability of the lithium iron manganese phosphate positive electrode material at high temperatures. DETAILED DESCRIPTION

[0038] In order to facilitate the understanding of the present invention, the present invention will be described more comprehensively and carefully in combination with preferred embodiments below, but the protection scope of the present invention is not limited to the following specific embodiments.

[0039] Unless otherwise defined, all professional terms used below have the same meanings as those generally understood by those skilled in the art. The professional terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention.

[0040] The "range" disclosed in the present invention is defined in the form of a lower limit and an upper limit, and a given range is defined by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundaries of a particular range. The range defined in this way can be inclusive or exclusive of end values, and can be combined arbitrarily, that is, any lower limit can be combined with any upper limit to form a range. For example, if a range of 60-120 and 80-110 is listed for a specific parameter, it is understood that the range of 60-110 and 80-120 is also expected. In addition, if the minimum range values ​​1 and 2 are listed, and if the maximum range values ​​3, 4 and 5 are listed, the following ranges can all be expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present invention, unless otherwise specified, the numerical range "ab" represents an abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" represents that all real numbers between "0-5" have been fully listed herein, and "0-5" is just an abbreviation of these numerical combinations. In addition, when a parameter is expressed as an integer ≥ 2, it is equivalent to disclosing that the parameter is, for example, an integer of 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0041] If not otherwise specified, all embodiments and optional embodiments of the present invention can be combined with each other to form a new technical solution.

[0042] Unless otherwise specified, all technical features and optional technical features of the present invention can be combined with each other to form a new technical solution.

[0043] If not otherwise specified, all steps of the present invention may be performed sequentially or randomly, preferably sequentially. For example, the method includes steps (a) and (b), which means that the method may include steps (a) and (b) performed sequentially, or may include steps (b) and (a) performed sequentially. For example, the method may further include step (c), which means that step (c) may be added to the method in any order, for example, the method may include steps (a), (b) and (c), or may include steps (a), (c) and (b), or may include steps (c), (a) and (b), etc.

[0044] If there is no special explanation, the "include" and "comprising" mentioned in the present invention represent open-ended or closed-ended expressions. For example, the "include" and "comprising" may represent that other components not listed may also be included or only the listed components may be included or only the listed components may be included.

[0045] If not specifically stated, in the present invention, the term "or" is inclusive. For example, the phrase "A or B" means "A, B, or both A and B". More specifically, any of the following conditions satisfies the condition "A or B": A is true (or exists) and B is false (or does not exist); A is false (or does not exist) and B is true (or exists); or both A and B are true (or exist).

[0046] Example 1

[0047] Step 1: Precursor preparation: Add modified concentrated phosphoric acid solution to 1.0 mol / L lithium hydroxide solution to adjust the pH to 10.0, and then add salt solution to adjust the pH to 7.0 to obtain a precursor solution;

[0048] In this embodiment, the modified concentrated phosphoric acid solution is a PEG400 solution of concentrated phosphoric acid and RuBP, wherein the concentration of concentrated phosphoric acid is 3 V / V%, and the molar ratio of phosphoric acid:RuBP is 10:1;

[0049] In this embodiment, the salt solution is an aqueous solution of manganese sulfate, ferrous sulfate, nickel acetate and cobalt acetate, wherein the concentration of manganese sulfate is 0.5 mol / L, and the molar ratio of Mn:Fe:Ni:Co in the salt solution is 0.6:0.34:0.035:0.025; in addition, ascorbic acid needs to be added to the salt solution for antioxidant, and the molar ratio of manganese sulfate to ascorbic acid is 10:1.

[0050] Step 2: The precursor solution is heated at 160° C. for 8 h in a hydrothermal reactor, followed by solid-liquid separation and centrifugation, and then dried in a vacuum drying oven at 150° C. for 6 h to obtain a precursor.

[0051] Step 3: By weight, 100 parts of the precursor, 2.5 parts of the carbon source (glucose) and 0.8 parts of TiCN were wet-milled with water to obtain a slurry with a solid content of 62%;

[0052] The ball-to-material ratio during wet grinding was 5:1, the rotation speed was 450 rpm, and the wet grinding lasted for 6 hours.

[0053] Step 4: In a nitrogen atmosphere with a nitrogen flow rate of 15 L / min, the slurry is heated to 700°C at a heating rate of 5°C / min and sintered at this temperature for 10 hours; the annealing temperature is set to 350°C, and after dropping to this temperature, the slurry is annealed for 2 hours to obtain a modified lithium manganese iron phosphate positive electrode material.

[0054] Example 2

[0055] The difference between this embodiment and embodiment 1 is that:

[0056] Step 3: By mass, 100 parts of the precursor, 3.0 parts of the carbon source (glucose) and 0.8 parts of TiCN were wet-milled with water.

[0057] Example 3

[0058] The difference between this embodiment and embodiment 1 is that:

[0059] Step 3: By mass, 100 parts of the precursor, 2.5 parts of the carbon source (glucose) and 1.0 part of TiCN were wet-milled with water.

[0060] Example 4

[0061] The difference between this embodiment and embodiment 1 is that:

[0062] Step 3: By mass, 100 parts of the precursor, 2.5 parts of the carbon source (glucose) and 1.2 parts of TiCN were wet-milled with water.

[0063] Example 5

[0064] The difference between this embodiment and embodiment 1 is:

[0065] In step 1, the molar ratio of phosphoric acid to RuBP is 10:1.5.

[0066] Example 6

[0067] The difference between this embodiment and embodiment 1 is:

[0068] In step 1, the molar ratio of phosphoric acid to RuBP is 10:2.

[0069] Example 7

[0070] The difference between this embodiment and embodiment 1 is:

[0071] In step 1, the molar ratio of phosphoric acid to RuBP is 10:2.5.

[0072] Comparative Example 1

[0073] The difference between this comparative example and Example 1 is:

[0074] Step 3: By mass, 100 parts of the precursor, 2.5 parts of the carbon source (glucose) and 0.5 parts of TiCN were wet-milled with water.

[0075] Comparative Example 2

[0076] The difference between this comparative example and Example 1 is:

[0077] In step 1, a concentrated phosphoric acid solution is directly used instead of a modified concentrated phosphoric acid solution. The concentrated phosphoric acid solution is a PEG400 solution having a concentration of 5 V / V% of concentrated phosphoric acid and does not contain RuBP.

[0078] Comparative Example 3

[0079] The difference between this comparative example and Example 1 is:

[0080] In step 1, the molar ratio of phosphoric acid to RuBP is 10:0.5.

[0081] Comparative Example 4

[0082] The difference between this comparative example and Example 1 is:

[0083] In step 1, the molar ratio of phosphoric acid to RuBP is 10:3.

[0084] Application Examples

[0085] (1) adding a solvent of N-methylpyrrolidone (NMP) to a low-temperature lithium iron phosphate positive electrode material: acetylene black: polyvinylidene fluoride in a mass ratio of 8:1:1 to adjust the viscosity to 4000-8000 mPa·s to prepare a positive electrode slurry;

[0086] (2) The positive electrode slurry is evenly coated on one surface (perpendicular to the thickness direction) of the aluminum foil (positive electrode current collector) and dried, and then rolled and sheared to obtain a positive electrode sheet containing a positive electrode film layer.

[0087] (3) Using lithium hexafluorophosphate-ethyl methyl carbonate as the electrolyte and polypropylene as the isolation membrane, the prepared positive electrode sheet and negative lithium sheet are combined to make a button half-cell.

[0088] The button half-cell was tested for discharge capacity (measured at 45° C.) at 0.1C, 1.0C and 2.0C in a voltage range of 2.0 V to 4.35 V using a blue-power device. The test results of the embodiment and comparative example are shown in Table 1 below.

[0089] Table 1

[0090] - 0.1C 1.0C 2.0C Example 1 161.10 153.48 140.55 Example 2 158.56 152.11 139.21 Example 3 160.76 153.27 138.27 Example 4 161.42 152.36 139.10 Example 5 162.32 153.86 140.57 Example 6 161.86 153.67 140.82 Example 7 158.34 153.14 138.62 Comparative Example 1 155.27 142.17 131.61 Comparative Example 2 150.46 139.94 128.78 Comparative Example 3 153.31 143.58 133.58 Comparative Example 4 152.48 144.77 135.44

[0091] As shown in Table 1, the amount of TiCN added in Comparative Example 1 is relatively small. Therefore, when the amount of steric inhibitor added is relatively small, the positive electrode material obtained after sintering is easy to agglomerate, which further leads to a low discharge specific capacity.

[0092] Comparative Examples 2 to 4 explore the effects of the addition of RuBP to the modified concentrated phosphoric acid solution and the amount of addition on the modified lithium manganese iron phosphate positive electrode material. Adding too much RuBP to the concentrated phosphoric acid solution will result in insufficient phosphoric acid content in the modified concentrated phosphoric acid solution (phosphorus source), while adding too little RuBP or no RuBP will result in a low content of the structure directing agent or no structure directing agent, so the nickel-cobalt doped lithium manganese iron phosphate precursor obtained after the hydrothermal reaction has poor dispersibility.

[0093] Unless otherwise specified, various raw materials, reagents, instruments and equipment used in the present invention can be purchased from the market or prepared by existing methods. The specific embodiments described above further describe the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the protection scope of the present invention.

Claims

1. A method for preparing a modified lithium manganese iron phosphate positive electrode material, characterized in that: The following steps are involved: S1. By mass, 100 parts of the precursor, 2 to 5 parts of the carbon source and 0.5 to 1.5 parts of TiCN were wet-milled to obtain a slurry; S2. The slurry is sintered and annealed to obtain a modified lithium manganese iron phosphate positive electrode material; The preparation method of the precursor comprises the following steps: mixing a lithium hydroxide solution, a modified concentrated phosphoric acid solution and a salt solution to obtain a precursor solution, and subjecting the precursor solution to a hydrothermal reaction and drying to obtain the precursor.

2. The method for preparing the modified lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The salt solution includes manganese sulfate solution, ferrous sulfate solution, nickel acetate solution and cobalt acetate solution; And / or, the carbon source is sucrose or glucose; And / or, the modified concentrated phosphoric acid solution is a polyethylene glycol solution of concentrated phosphoric acid and RuBP.

3. The method for preparing the modified lithium iron manganese phosphate positive electrode material according to claim 2, characterized in that: The concentration of lithium hydroxide in the lithium hydroxide solution is 0.8-1.0 mol / L; and / or, the molar ratio of phosphoric acid to RuBP in the modified concentrated phosphoric acid solution is 10:1 to 2.5; And / or, the concentration of concentrated phosphoric acid in the modified concentrated phosphoric acid solution is 3-5 V / V%.

4. The method for preparing the modified lithium iron manganese phosphate positive electrode material according to claim 2, characterized in that: The concentration of manganese sulfate in the salt solution is 0.4-0.5 mol / L; and / or, the molar ratio of Mn:Fe:Ni:Co in the salt solution is 0.6:0.4-xy:x:y, wherein x=0.015-0.035, y=0.015-0.035; And / or, ascorbic acid is further added to the salt solution, and the molar ratio of manganese sulfate to ascorbic acid is 10:0.5-1.

5.

5. The method for preparing the modified lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The hydrothermal reaction step is to heat the precursor solution at 150-170° C. for 7-10 hours, and after heating, solid-liquid separation, centrifugation and drying are required to obtain the precursor.

6. The method for preparing the modified lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The wet grinding speed is 450-500 rpm; And / or, the wet grinding time is 4.5 to 8 hours; And / or, the ball-to-material ratio of the wet grinding is 4 to 5:1; And / or, the solid content of the slurry is 60-68%.

7. The method for preparing the modified lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The heating rate before sintering is 1-5°C / min; And / or, the sintering temperature is 680-720° C. and the sintering is carried out at this temperature for 8-12 hours; And / or, the annealing temperature is 300-350° C., and the annealing is maintained at this temperature for 1.5-2.5 hours, and then cooled to room temperature; And / or, the sintering and the annealing are both performed in a nitrogen atmosphere, and the nitrogen flow rate is 10 to 25 L / min.

8. The method for preparing the modified lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The mixing process is to add the modified phosphoric acid solution to the lithium hydroxide solution to adjust the pH to 10.0-11.5, and then add the salt solution to adjust the pH to 6.5-7.

5.

9. The modified lithium iron manganese phosphate positive electrode material obtained by the method for preparing the modified lithium iron manganese phosphate positive electrode material according to any one of claims 1 to 8.

10. The use of the modified lithium iron manganese phosphate positive electrode material according to claim 9, characterized in that: Application of the modified lithium manganese iron phosphate positive electrode material in the field of electric vehicles or energy storage systems.