A lithium manganese iron phosphate positive electrode material, a preparation method and application thereof

Abstract

The application provides a lithium manganese iron phosphate positive electrode material and a preparation method and application thereof. The preparation method of the lithium manganese iron phosphate positive electrode material comprises the following steps: (1) stirring and mixing a manganese source, a phosphorus source and a KCl aqueous solution, and then performing first sand milling to obtain a first material, wherein the molar ratio of phosphorus elements to potassium elements in the phosphorus source is 1:(1-1.7); (2) stirring and mixing the first material, a lithium source, an iron source and a carbon source, and then performing second sand milling and freezing treatment to obtain a second material, wherein the molar ratio of manganese elements in the manganese source to iron elements in the iron source is (1.2-3):1; and (3) sintering the second material, washing, drying and crushing the sintered second material into powder to obtain the lithium manganese iron phosphate positive electrode material. The [001]-oriented nanorod primary particles are used as a growth template, and KCl is used to occupy space and improve the porosity of the material, so that the capacity and rate performance of the LMFP material can be effectively improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of lithium batteries, in particular to a lithium manganese iron phosphate positive electrode material and a preparation method and application thereof. BACKGROUND

[0002] In recent years, lithium iron phosphate (LFP) has accounted for an increasingly large proportion in the power battery market, close to 60% in 2022. LFP has excellent cycle performance and safety, and is therefore widely used in power batteries. With the competition in the electric vehicle market, one of the future development directions of electric vehicles is high cruising range. The theoretical specific capacity of LFP battery is 170 mAh / g, and the working voltage is 3.4-3.5 V. The mainstream LFP battery has already approached its theoretical capacity, and it is difficult to continue to improve. In terms of high cruising range of electric vehicles, LFP does not have competitiveness with traditional ternary lithium batteries. After doping Mn elements and replacing part of Fe, lithium manganese iron phosphate (LMFP) is formed. The theoretical specific capacity of LMFP is consistent with that of LFP, while the working voltage of LMFP increases with the increase of Mn content. The current mainstream research shows that when LM x F （1-x） P (0.8 >= x >= 0.6), the cycle, capacity, and weighted working voltage can be well combined, the theoretical capacity can be increased by 10-20%, and it is the development direction of future high cruising range and high safety energy storage power batteries.

[0003] Although LMFP has higher energy density than LFP and better safety performance than ternary, there are still problems that the capacity and rate performance of LMFP battery cannot be fully developed. SUMMARY

[0004] The present application aims to at least partially solve one of the problems in the prior art. To this end, one object of the present application is to provide a lithium manganese iron phosphate positive electrode material and a preparation method and application thereof.

[0005] In a first aspect, the present application provides a preparation method of a lithium manganese iron phosphate positive electrode material, which comprises:

[0006] (1) stirring and mixing a manganese source, a phosphorus source and a KCl aqueous solution, and then performing first sand milling to obtain a first material, wherein the molar ratio of phosphorus elements to potassium elements in the phosphorus source is 1:(1-1.7);

[0007] (2) stirring and mixing the first material, a lithium source, an iron source and a carbon source, and then performing second sand milling and freezing treatment to obtain a second material, wherein the molar ratio of manganese elements in the manganese source to iron elements in the iron source is (1.2-3):1;

[0008] (3) sintering the second material, and then washing, drying and crushing into powder to obtain a lithium manganese iron phosphate positive electrode material.

[0009] According to the method for preparing the lithium manganese iron phosphate positive material provided by the application, the manganese source, the phosphorus source and the KCl aqueous solution are stirred and mixed first, and then first sand milling is performed. The traditional LMFP material has an olivine structure, and the lithium ion conduction direction is the

[010] direction one-dimensional conduction, which limits the diffusion rate of lithium ions in the LMFP material. The Mn source and the phosphoric acid and KCl are mixed first, and then sand milling is performed to form

[001] -oriented nanorod primary particles as a growth template. For reference, Figure 1 The length of the

[001] direction can be 0.3 μm to 10 μm, and the length of the

[010] direction can be 20 nm to 800 nm. By reducing the length of the

[010] direction, the deintercalation distance of lithium ions in the LMFP material is reduced, so that the capacity of the LMFP material can be better exerted.

[0010] Then the first material, the lithium source, the iron source and the carbon source are stirred and mixed, and then second sand milling is performed, and then freezing treatment is performed. Because KCl is used as a three-dimensional high-porosity template in step (1), and then reacts with the lithium source, the iron source and the carbon source, and then freezing drying forms a three-dimensional network, and then KCl is dissolved to form a cubic system, and then occupies a certain space point, and then KCl is washed away, and then the space point originally occupied by KCl is called a void, so that the problem of small particle size of the nanosized particles and difficult impregnation of the electrolyte is effectively solved, and the capacity and rate performance of the battery are improved.

[0011] In the application, KCl is added when the Mn source is sand milled with phosphoric acid, and then the lithium source, the iron source and the carbon source are sand milled and frozen treated, KCl occupies the space point and is then washed away, which not only limits the growth of the

[010] -oriented nanoparticles to a certain extent, but also can prepare high-porosity LMFP material by using KCl as a template. By forming

[001] -oriented nanorod primary particles as a growth template and cooperating with KCl to occupy the space to improve the porosity of the material, the capacity and rate performance of the LMFP material can be effectively improved.

[0012] According to the method for preparing the lithium manganese iron phosphate positive material provided by the application, in step (1), the molar ratio of phosphorus elements to potassium elements in the phosphorus source is 1: (1-1.7). By controlling the amount of potassium elements added in the above range, the material can form moderate porosity, and the compaction density can be ensured.

[0013] In some embodiments of the application, the concentration of the KCl aqueous solution is not less than 3.1 M, and preferably the KCl aqueous solution is a KCl saturated solution. By controlling the concentration of the KCl aqueous solution to be not less than 3.1 M, a pore network can be better formed.

[0014] In some embodiments of the present application, the first sand mill has a rotation speed of 1600 rpm to 2700 rpm, a temperature of 10℃ to 27℃, and a time of 30 min to 90 min.

[0015] In some embodiments of the present application, in step (2), the carbon source is selected from a mixture of glucose and PEG, and the mass ratio of the glucose to the PEG is (2-4):1. For example, the mass ratio is 2:1, 3:1, 4:1, or a range between any two of the above values. Glucose tends to be fluffy after sintering, and PEG needs to be added as an adjustment. Controlling the mass ratio of glucose to PEG in the above range is beneficial to the uniformity and compactness of the carbon coating layer after sintering, thereby improving the capacity and rate performance of the positive electrode material.

[0016] According to the preparation method of the lithium iron manganese phosphate positive electrode material provided by the present application, the molar ratio of manganese in the manganese source to iron in the iron source is (1.2-3):1. For example, the molar ratio of manganese to iron is 1.2:1, 1.5:1, 2:1, 2.5:1, 3:1, or a range between any two of the above values. Controlling the molar ratio of manganese to iron in the above range can avoid the problem of Mn dissolution leading to a decrease in the capacity of the positive electrode, thereby improving the capacity and rate performance of the positive electrode material.

[0017] In some embodiments of the present application, in step (2), the mixture of lithium source, iron source and carbon source is added to the first material.

[0018] In some embodiments of the present application, in step (2), the particle size of the second material is 200 nm to 800 nm. It should be noted that the particle size of the second material is the same as the particle size of the material after the second sand mill is completed.

[0019] In some embodiments of the present application, the second sand mill has a rotation speed of 1600 rpm to 2700 rpm and a temperature of 10℃ to 27℃. Those skilled in the art can understand that the particle size of the target material of the second sand mill is in the range of 200 nm to 800 nm, and the sand mill can be stopped.

[0020] In some embodiments of the present application, in step (2), the freezing treatment includes freezing forming and freeze drying performed in sequence.

[0021] In some embodiments of the present application, the freezing forming has a temperature of -25℃ to -30℃ and a time of 24 h to 48 h.

[0022] In some embodiments of the present application, the freeze drying has a temperature of -55℃ to -50℃ and a pressure of 10 Pa to 20 Pa.

[0023] In some embodiments of the present application, in step (3), the sintering temperature is 600-800 DEG C, and the sintering time is 6-8 h.

[0024] In some embodiments of the present application, the drying temperature is 70-90 DEG C, and the drying time is 5-7 h.

[0025] In some embodiments of the present application, in step (3), the particle size of the lithium manganese iron phosphate positive electrode material is 300-800 nm.

[0026] In some embodiments of the present application, the crushing is air flow crushing, the air flow crushing pressure is 0.2-0.6 MPa, and the crushing time is 15-25 min.

[0027] In a second aspect of the present application, a lithium manganese iron phosphate positive electrode material is provided, which is prepared by the above method. The lithium manganese iron phosphate positive electrode material has excellent capacity and rate performance.

[0028] In a third aspect of the present application, a battery is provided, which comprises the above lithium manganese iron phosphate positive electrode material. The battery has high capacity and excellent rate performance.

[0029] Further, the battery comprises a positive electrode sheet, the positive electrode sheet comprises a current collector and a positive electrode active material layer, and the positive electrode active material layer comprises the above lithium manganese iron phosphate positive electrode material, a conductive agent and a binder.

[0030] The present application has at least the following beneficial effects:

[0031] The present application adds KCl when the Mn source is sand-milled with phosphoric acid, then adds a lithium source, an iron source and a carbon source for sand-milling and freezing treatment, and then washes away the KCl occupying the space points. This not only limits the growth of

[010] -oriented nanoparticles to a certain extent, but also can prepare a high-porosity LMFP material by using KCl as a template. The present application can effectively improve the capacity and rate performance of the LMFP material by forming

[001] -oriented nanorod primary particles as a growth template and improving the material porosity by occupying space with KCl. BRIEF DESCRIPTION OF DRAWINGS

[0032] In order to more clearly illustrate the technical solutions in the present application or prior art, the following will briefly introduce the drawings needed to be used in the embodiments or prior art description. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can be obtained by those skilled in the art without any creative effort on the basis of these drawings.

[0033] Figure 1is an electron microscope image of the LMFP cathode material of the embodiment of the present application;

[0034] Figure 2 is an electron microscope image of the LMFP cathode material prepared in Embodiment 1 of the present application. DETAILED DESCRIPTION

[0035] All other embodiments obtained by a person of ordinary skill in the art based on the embodiments of the present application without creative labor fall within the scope of protection of the present application. The present application is described below with reference to specific embodiments, and it should be noted that these embodiments are merely descriptive and do not limit the present application in any way.

[0036] Embodiment 1

[0037] The present embodiment provides a LMFP cathode material, and the preparation process is as follows:

[0038] (1) 72.11 g of MnO and 112.8 g of phosphoric acid are melted into a saturated KCl solution, the molar ratio of phosphorus in the phosphorus source to potassium in the KCl is 1:1.5, and after stirring, sand grinding is performed at a speed of 2000 rpm, the temperature is controlled at 20℃, and sand grinding is performed for 60 min to obtain a first material.

[0039] (2) A colloidal solution of 60.4 g of Li2CO3, 100 g of FePO4, 27.4 g of glucose, and 9.1 g of PEG (according to a mass ratio of 3:1) is added to the first material and sand grinding is continued, the sand grinding speed is 2000 rpm, the temperature is controlled at 20℃, and sand grinding is stopped after the particle size reaches 425±50 nm, then freezing is performed at -25℃ for 35 h, and the frozen material is dried at -52℃ and 15 Pa to obtain a second material.

[0040] (3) The obtained second material is sintered in a tube furnace at 700℃ and an oxygen content of 1.5% or less for 7 h to obtain a LMFP / C@KCl material, and then the LMFP / C@KCl material is washed with deionized water, the KCl in the filtered material is removed, and the material is broken into powder by air flow crushing at a crushing pressure of 0.4 MPa for 20 min, and then dried in a vacuum oven at 80℃ for 6 h to obtain a LMFP / C nanomaterial with a particle size of 400 nm~700 nm. The specific electron microscope image of the cathode material can be referred to Figure 2 .

[0041] The present embodiment provides a battery, and the specific preparation process is as follows:

[0042] The LMFP / C material prepared above was mixed with PVDF (PVDF was dissolved in NMP with a concentration of 5wt%), conductive carbon black (SP) in a mass ratio of 90:5:5, and stirred for 20 min until the surface of the slurry was reflective and had no particle feeling. The slurry was uniformly coated on an aluminum foil, rolled, cut, and the negative electrode was a lithium sheet. The battery was assembled into a button cell, and the voltage test was 2.0-4.5v, and the cutoff current was 0.05C for detection.

[0043] Example 2

[0044] This example provides a LMFP positive electrode material, and the difference between its preparation method and that of Example 1 is:

[0045] The total added mass of glucose and PEG in Example 2 is the same as that in Example 1, and the mass ratio of glucose to PEG in Example 2 is 4:1 (the mass ratio of glucose to PEG in Example 1 is 3:1).

[0046] This example provides a battery, and the specific preparation process is the same as that of Example 1.

[0047] Example 3

[0048] This example provides a LMFP positive electrode material, and the difference between its preparation method and that of Example 1 is:

[0049] The total added moles of manganese source and iron source in Example 3 are the same as those in Example 1, and the molar ratio of manganese source to iron source in Example 3 is 7:3 (the molar ratio of manganese source to iron source in Example 1 is 6:4).

[0050] This example provides a battery, and the specific preparation process is the same as that of Example 1.

[0051] Comparative Example 1

[0052] This comparative example provides a LMFP positive electrode material, and the preparation process is as follows:

[0053] (1) 72.11g of MnO and 112.8g of phosphoric acid were dissolved in a deionized water solution (the volume of deionized water was the same as that of the KCl saturated aqueous solution in Example 1), and after stirring, sand milling was performed at a speed of 2000rpm and a temperature of 20℃ for 60min to obtain a first material.

[0054] (2) A colloidal solution of 60.4g of Li2CO3, 100g of FePO4, 27.4g of glucose, and 9.1g of PEG (mixed in a mass ratio of 3:1) was added to the above first material and sand milling was continued at a speed of 2000rpm and a temperature of 20℃. Sand milling was stopped when the particle size reached 425±50nm, and the slurry was spray dried at 110℃ to obtain a second material.

[0055] (3) The obtained second material is sintered in a tube furnace at 700°C, below 1.5% oxygen content for 7h, and then crushed into powder to obtain the LMFP / C material with a particle size of 400nm-700nm.

[0056] The comparative example provides a battery, and the specific preparation process and battery performance test method are the same as those of Example 1.

[0057] Comparative Example 2

[0058] The comparative example provides a LMFP positive electrode material, and the preparation process is as follows:

[0059] (1) 83.32g Mn3O4, 100g FePO4, 60.4g Li2CO3, 112.8g phosphoric acid, 27.4g glucose and 9.1g PEG are added into a saturated KCl solution (the volume of the KCl saturated aqueous solution is the same as that of Comparative Example 2 and Example 1), stirred and sand-milled, the sand-milling speed is 2000rpm, the temperature is controlled at 20°C, and the sand-milling is stopped after the particle size reaches 425±50nm, then frozen at-25°C for 35h, freeze-formed, and then freeze-dried at-52°C, 15Pa to obtain the material.

[0060] (2) The obtained material is sintered in a tube furnace at 700°C, below 1.5% oxygen content for 7h to obtain the LMFP / C@KCl material, and then the LMFP / C@KCl material is washed with deionized water, dried in a vacuum oven at 80°C for 6h, and then the KCl in the filtered material is crushed into powder to obtain the LMFP / C nanomaterial with a particle size of 400nm-700nm.

[0061] The comparative example provides a battery, and the specific preparation process and battery performance test method are the same as those of Example 1.

[0062] Comparative Example 3

[0063] The comparative example provides a LMFP positive electrode material, and the preparation process is as follows:

[0064] (1) 72.11g MnO, 100g FePO4, 60.4g Li2CO3, 112.8g phosphoric acid, 27.4g glucose and 9.1g PEG are added into a deionized water solution (the volume of the deionized water is the same as that of the KCl saturated aqueous solution in Example 1), stirred and sand-milled, the sand-milling speed is 2000rpm, the temperature is controlled at 20°C, and the sand-milling is stopped after the particle size reaches 425±50nm to obtain the material.

[0065] (2) The obtained material is sintered in a tube furnace at 700°C, below 1.5% oxygen content for 7h, and then crushed into powder to obtain the LMFP / C material with a particle size of 400nm-700nm.

[0066] The comparative example provides a battery, and the specific preparation process and battery performance test method are the same as those of example 1.

[0067] The test results of the electrical performance of the batteries prepared in the examples and the comparative examples are shown in Table 1.

[0068] Table 1

[0069]

[0070] From Table 1, it can be seen from example 1 and example 2 that the ratio of different carbon sources has a certain influence on the capacity and rate, and the sintered glucose is prone to be fluffy, and PEG needs to be added for adjustment, which is beneficial to the uniformity and compactness of the carbon coating layer after sintering, and the ratio of glucose:PEG is 3:1, which is better than 4:1. It can be seen from example 1 and example 3 that when the ratio of Mn / Fe is increased, the capacity and rate performance decrease obviously, which is mainly because the dissolution of Mn leads to the continuous decrease of the positive electrode capacity, resulting in poor cycle performance. It can be seen from example 1 and comparative example 1 that comparative example 1 does not prepare a high porosity template, so the wetting degree of the electrolyte is not enough, which leads to that the capacity cannot be fully released, and at the same time, the rate performance decreases obviously. It can be seen from example 1 and comparative example 2 that the example 1 is formed by sand milling the reaction of Mn source and phosphoric acid to form

[001] oriented nanoparticles, and since its

[001] direction is the dominant surface and grows less in the

[010] direction, the Li deintercalation rate is greatly improved, and the battery performance of example 1 is significantly better than that of comparative example 2.

[0071] It can be seen from example 1 and comparative example 3 that comparative example 3 does not prepare a high porosity and nano-sized LMFP, and its capacity and rate performance are poor.

[0072] Finally, it should be noted that: the above examples are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing examples, or make equivalent replacement for part of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing a lithium manganese iron phosphate positive electrode material, characterized in that: include: (1) A manganese source, a phosphorus source, and a KCl aqueous solution are stirred and mixed, and then subjected to a first sand milling to obtain a first material, wherein the molar ratio of phosphorus element to potassium element in the phosphorus source is 1:(1-1.7); and the concentration of the KCl aqueous solution is not less than 3.1M; (2) The first material, lithium source, iron source and carbon source are stirred and mixed, and then sand-milled for a second time, and then frozen to obtain a second material, wherein the molar ratio of manganese element in the manganese source to iron element in the iron source is (1.2-3):1; the carbon source is selected from a mixture of glucose and PEG, and the mass ratio of the glucose to the PEG is (2-4):1; (3) After sintering the second material, washing, drying, and crushing it into powder to obtain lithium manganese iron phosphate positive electrode material.

2. The method according to claim 1, characterized in that In step (1), the rotation speed of the first sanding is 1600 rpm to 2700 rpm, the temperature of the first sanding is 10° C. to 27° C., and the time of the first sanding is 30 min to 90 min.

3. The method according to claim 1 or 2, characterized in that In step (2), the particle size of the second material is 200 nm to 800 nm; And / or, the rotation speed of the second sand mill is 1600 rpm~2700 rpm, and the temperature is 10°C~27°C.

4. The method according to claim 1 or 2, characterized in that In step (2), the freezing treatment includes freeze-molding and freeze-drying performed in sequence.

5. The method according to claim 4, characterized in that: The freezing molding temperature is -25°C to -30°C, and the freezing molding time is 24h to 48h; And / or, the freeze-drying temperature is -55°C to -50°C, and the freeze-drying pressure is 10Pa to 20Pa.

6. The method according to claim 1 or 2, characterized in that: In step (3), the sintering temperature is 600°C to 800°C, and the sintering time is 6h to 8h; And / or, the drying temperature is 70° C. to 90° C., and the drying time is 5 h to 7 h.

7. The method according to claim 1 or 2, characterized in that: In step (3), the particle size of the lithium manganese iron phosphate positive electrode material is 300nm~800nm; And / or, the crushing is air flow crushing, the air flow crushing pressure is 0.2MPa~0.6MPa, and the crushing time is 15min~25min.

8. A lithium manganese iron phosphate positive electrode material, characterized in that: The method according to any one of claims 1 to 7 is used to prepare the present invention.

9. A battery, characterized in that: Including the lithium manganese iron phosphate positive electrode material as described in claim 8.

Images