Method for preparing lithium manganese iron phosphate composite cathode material based on sol-gel method

Abstract

The application provides a method for preparing a lithium manganese iron phosphate composite positive electrode material based on a sol-gel method, and belongs to the technical field of lithium ion battery positive electrode materials.The method is as follows: raw material solutions are respectively preheated to preset temperatures;first, manganese salt solution and complexing agent solution are uniformly mixed, then ferrous salt solution, phosphorus-containing compound solution and lithium salt solution are added, and concentrated ammonia water is added dropwise to obtain a reaction solution; the reaction solution is placed in an ultrasonic microwave combined reaction system, and ultrasonic wave treatment, ultrasonic wave-microwave synergistic treatment and microwave treatment are carried out to obtain a dry gel; the product is cooled, a carbon source is added, and grinding and magnetic removal treatment are carried out to obtain a precursor powder; in a microwave reaction system, the precursor powder is subjected to microwave gradient high-temperature sintering under the condition of a protective atmosphere, and the lithium manganese iron phosphate composite positive electrode material is obtained after cooling.The application shortens the preparation period, effectively improves the uniformity of the material, and the prepared positive electrode material exhibits excellent dispersity, electronic conductivity and stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of lithium ion battery positive electrode materials, and in particular relates to a method for preparing a lithium manganese iron phosphate composite positive electrode material based on a sol-gel method. Background Art

[0002] As a core component of new energy industries such as electric vehicles (EVs), hybrid electric vehicles (HEVs) and large energy storage devices, lithium-ion batteries (LIBs) are key to improving their performance, including power density, energy density and cycle life. Among the raw materials of lithium-ion batteries, cathode materials are still the key to improving electrochemical performance and reducing total costs. x Fe 1-x PO4 is studied as the most promising cathode material for lithium-ion batteries due to its high energy density, good cycle stability and environmental friendliness.

[0003] Currently, the predominant technical approach for preparing lithium manganese iron phosphate materials is the solid-phase method. While mature and simple, this method suffers from uneven raw material mixing, high reaction temperatures, poor batch consistency, susceptibility to impurities, and difficulty controlling the material's microstructure. These shortcomings directly impact the material's charge-discharge capacity, rate cycling, and high- and low-temperature performance. While the traditional sol-gel method improves the raw material mixing issue to some extent, it still suffers from issues such as a long and difficult-to-control gelation process, prone to agglomeration during the drying process, and difficulty in precisely controlling the crystal structure during high-temperature sintering, leading to unstable electrochemical properties. Summary of the Invention

[0004] In view of this, the object of the present invention is to provide a method for preparing lithium manganese iron phosphate composite positive electrode material based on the sol-gel method, aiming to solve at least one technical problem in the background technology.

[0005] The present invention is achieved in that:

[0006] The present invention provides a method for preparing a lithium manganese iron phosphate composite positive electrode material based on a sol-gel method, the method comprising the following steps:

[0007] Preheating the lithium salt solution, manganese salt solution, ferrous salt solution, phosphorus compound solution, and complexing agent solution to preset temperatures respectively;

[0008] Under continuous stirring and heat preservation, the manganese salt solution and the complexing agent solution are first mixed uniformly, and then the ferrous salt solution, the phosphorus compound solution and the lithium salt solution are added in sequence, and concentrated ammonia water is added dropwise to obtain a reaction solution with a pH of 2 to 5;

[0009] The reaction solution is placed in an ultrasonic-microwave combined reaction system, and subjected to ultrasonic treatment alone, ultrasonic-microwave synergistic treatment, and microwave treatment alone in sequence to obtain a fluffy honeycomb xerogel;

[0010] The xerogel is thermally decomposed, and the obtained product is mixed with a carbon source in a predetermined ratio, and then ground and demagnetized in sequence to obtain a precursor powder;

[0011] In the microwave reaction system, the precursor powder was subjected to microwave gradient sintering at 200W to 450W under protective atmosphere conditions, and the powder product LiMn was obtained after cooling. x Fe 1-x PO4 / C is a lithium manganese iron phosphate composite positive electrode material.

[0012] Furthermore, the reaction solution is placed in an ultrasonic-microwave combined reaction system, and ultrasonic treatment alone, ultrasonic-microwave synergistic treatment, and microwave treatment alone are successively performed to obtain a fluffy honeycomb xerogel. The specific operations are as follows:

[0013] Place the reaction vessel containing the reaction solution in the ultrasonic microwave combined reaction system, first turn on only the ultrasonic dispersion device, and perform ultrasonic treatment on the reaction solution with an ultrasonic power of 200W to 400W and a treatment time of 10min to 30min;

[0014] Then, the microwave heating device is turned on for ultrasonic-microwave synergistic treatment, the microwave power is set to 250W-500W, and the reaction time is 30min-60min;

[0015] When the reaction solution turns into a yellow sol, the ultrasonic dispersion device is turned off and the microwave heating treatment is continued until a fluffy honeycomb xerogel is formed.

[0016] Furthermore, the ultrasonic-microwave combined reaction system is also provided with a container containing deionized water.

[0017] Furthermore, the dry gel is thermally decomposed, and the obtained product is mixed with a carbon source in a preset ratio and then ground and demagnetized in sequence to obtain a precursor powder. The specific operations are as follows:

[0018] The dry gel was placed in a muffle furnace and heated to 300°C to 400°C for thermal decomposition for 3h to 6h;

[0019] After cooling to room temperature in the furnace, the pyrolysis product is taken out, and the pyrolysis product is mixed with the carbon source in a mass ratio of 8 to 12:1 and ground to obtain a uniform mixed powder;

[0020] The mixed powder is placed in a rotary demagnetizer and subjected to a demagnetization treatment in a magnetic field of 4000GS to 6000GS for 8 minutes to 12 minutes to obtain a precursor powder.

[0021] Furthermore, the carbon source is selected from glucose or sucrose.

[0022] Furthermore, in the microwave reaction system, the precursor powder is subjected to a microwave gradient sintering at 200W to 450W under protective atmosphere conditions, and the specific operation is as follows:

[0023] First, sinter at 300°C to 450°C for 15 minutes under a microwave power of 200W to 250W;

[0024] Sintering at 800°C to 1000°C for 20min to 30min under a microwave power of 350W to 450W;

[0025] After cooling in the furnace, the final powder product LiMn x Fe 1-x PO4 / C;

[0026] The pressure during the sintering process is controlled to be 0.01MPa to 0.5MPa.

[0027] Furthermore, the lithium salt is selected from lithium hydroxide monohydrate, lithium nitrate, lithium acetate, lithium chloride or lithium oxalate;

[0028] The manganese salt is selected from manganese acetate, manganese carbonate, manganese sulfate, manganese nitrate or manganese chloride;

[0029] The ferrous salt is selected from ferrous nitrate, ferrous sulfate or ferrous chloride;

[0030] The phosphorus-containing compound is selected from phosphoric acid or phosphates, and the phosphates are selected from ammonium dihydrogen phosphate or triammonium phosphate;

[0031] According to the molar ratio, lithium salt: phosphorus-containing compound: manganese salt: ferrous salt = 1: (1.1-1.3): x: (1-x), 0.6≦x≦0.8.

[0032] Furthermore, the complexing agent is selected from citric acid; the molar ratio of the lithium salt to the complexing agent is 1:1-2.

[0033] Furthermore, the preset temperature for preheating is 70°C to 90°C.

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

[0035] 1. The present invention prepares lithium manganese iron phosphate composite positive electrode materials through ultrasonic / microwave-assisted sol-gel combined with microwave gradient high-temperature sintering, which significantly shortens the preparation cycle of lithium manganese iron phosphate materials, improves the accuracy of temperature control, and effectively improves the uniformity of the material. The lithium manganese iron phosphate composite positive electrode materials prepared by the present invention exhibit excellent dispersibility, electronic conductivity and stability.

[0036] 2. The process threshold of the present invention is low, energy consumption is low, and the preparation process is green, environmentally friendly and pollution-free, providing a new method for the preparation of high-performance lithium manganese iron phosphate materials that is efficient, controllable and environmentally friendly.

[0037] 3. The present invention uses microwaves to assist in the gelation and sintering processes. Microwave heating can directly act on the molecules of the material, allowing the reaction system to achieve a uniform temperature distribution in a short period of time, thereby effectively and accurately controlling the temperature during the gelation process and sintering, shortening the gelation and sintering time, and improving production efficiency. In addition, a lower microwave power is used in the initial stage of microwave sintering to gradually heat the material, avoiding sudden temperature changes that may cause structural damage to the material. Subsequently, the microwave power is gradually increased, which can more effectively promote the reaction degree of the material, shorten the time required for the overall sintering reaction, and maintain the structure and performance of the material. At the same time, ultrasonic dispersion technology is used in the gelation process to break up the agglomerates in the gel through its cavitation effect and mechanical vibration, so that the particles are evenly dispersed. This helps to reduce the interaction force between particles during the drying process, prevent the occurrence of agglomeration, and thus produce a more uniform particle size distribution and a more dense and orderly microstructure, greatly improving the electronic conductivity and stability of the material. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a process flow chart of the present invention. DETAILED DESCRIPTION

[0039] In order to make the purpose, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific implementation cases described herein are only used to explain the present invention and are not intended to limit the present invention.

[0040] See Figure 1 A method for preparing a lithium manganese iron phosphate composite positive electrode material based on a sol-gel method comprises the following steps S11 to S15:

[0041] S11, preheating the lithium salt solution, manganese salt solution, ferrous salt solution, phosphorus compound solution, and complexing agent solution to preset temperatures respectively;

[0042] Weigh raw materials such as lithium salt (purity ≥99.9%), phosphorus-containing compound (analytical grade), manganese salt (purity ≥99.9%), ferrous salt (purity ≥99.9%), and an appropriate amount of complexing agent raw materials according to a certain molar ratio; the molar ratio of lithium salt: phosphorus-containing compound: manganese salt: ferrous salt is 1:(1.1-1.3):x:(1-x), 0.6<x≦0.8, for example, it can be 1:1.1:0.6:0.4, 1:1.2:0.7:0.3, or 1:1.3:0.8:0.2; but the values ​​listed are not limited to those listed, and other values ​​not listed within the numerical range are also applicable. In this step, the phosphorus-containing compound needs to be appropriately excessive to increase the conversion rate of the lithium salt; the molar ratio of lithium salt to complexing agent is 1:1-2; and the complexing agent can be appropriately excessive.

[0043] The above raw materials are added with water at a liquid-to-solid ratio of 3 to 4:1 to prepare a lithium salt solution, a manganese salt solution, a ferrous salt solution, a phosphorus compound solution, and a complexing agent solution, respectively. The mixture is stirred thoroughly with a mechanical stirrer until all the raw materials are completely dissolved and a clear solution is formed. The respective solutions are preheated in a constant temperature water bath to 70° C. to 90° C.; the preheated temperature can be, for example, 70° C., 80° C., or 90° C.; but the values ​​listed are not limited thereto, and other values ​​not listed within the numerical range are also applicable;

[0044] The above raw materials use lithium salts, phosphorus-containing compounds, manganese salts, ferrous salts and complexing agents permitted in the art. For example, lithium salts include but are not limited to lithium hydroxide monohydrate, lithium nitrate, lithium acetate, lithium chloride or lithium oxalate; manganese salts include but are not limited to manganese acetate, manganese carbonate, manganese sulfate or manganese chloride; the ferrous salts include but are not limited to ferrous nitrate, ferrous sulfate or ferrous chloride; phosphorus-containing compounds include but are not limited to phosphoric acid, ammonium dihydrogen phosphate or triammonium phosphate; and complexing agents include but are not limited to citric acid.

[0045] S12. While continuously stirring and keeping warm, first mix the manganese salt solution and the complexing agent solution, then add the ferrous salt solution, the phosphorus compound solution, and the lithium salt solution in sequence, and dropwise add concentrated ammonia water to obtain a reaction solution with a pH of 2 to 5;

[0046] S13, placing the reaction solution in an ultrasonic-microwave combined reaction system, and sequentially performing ultrasonic treatment alone, ultrasonic-microwave synergistic treatment, and microwave treatment alone to obtain a fluffy honeycomb xerogel;

[0047] The specific operations are as follows:

[0048] (1) placing a reaction vessel containing a reaction solution in an ultrasonic-microwave combined reaction system, first turning on only the ultrasonic dispersion device, and subjecting the reaction solution to ultrasonic treatment, with an ultrasonic power of 200W to 400W and a treatment time of 10min to 30min, so that the reaction solution is fully mixed and uniform; during the reaction process, a container containing a certain amount of deionized water needs to be placed in the ultrasonic-microwave combined reaction system to prevent the wet gel from causing the system to idle in the later stage of drying; the ultrasonic power can be 200W, 300W, or 400W, and the treatment time can be 10min, 20min, or 30min; but the values ​​listed are not limited thereto, and other values ​​not listed within the numerical range are also applicable;

[0049] (2) On the basis of continuous ultrasonic treatment, the microwave heating device is turned on to perform ultrasonic-microwave synergistic treatment, the microwave power is set to 250W to 500W, and the reaction time is 30min to 60min; the microwave power can be 250W, 350W, 500W, and the ultrasonic-microwave synergistic treatment time can be 30min, 45min, 60min; but it is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable;

[0050] (3) When the reaction solution turns into a yellow sol, the ultrasonic dispersion device is turned off and the microwave heating treatment is continued until a fluffy honeycomb dry gel is formed.

[0051] The present invention utilizes the rapid and uniform heating characteristics of microwaves to achieve a uniform temperature distribution in the reaction system in a short time, thereby effectively and accurately controlling the temperature of the gelation process and shortening the gelation time, replacing the long hydrothermal evaporation and aging steps in the sol-gel process, and improving production efficiency; at the same time, it is supplemented by ultrasonic dispersion technology, making full use of its cavitation effect and mechanical vibration. The strong fragmentation effect of ultrasound can break the agglomerates in the gel, making the particles evenly dispersed, ensuring that the solute is fully mixed, and achieving uniform dispersion of the substance in the gel state, further promoting the reaction. This helps to reduce the interaction force between particles in the sol-gel process, prevent the occurrence of agglomeration, and thus prepare a more uniform particle size distribution, a more compact and orderly microstructure, and greatly improve the electronic conductivity and stability of the material.

[0052] S14, thermally decomposing the xerogel, mixing the obtained product with a carbon source in a preset ratio, and then grinding and demagnetizing the mixture in sequence to obtain a precursor powder;

[0053] The specific operations are as follows:

[0054] (1) The xerogel is placed in a muffle furnace and heated to 300° C. to 400° C. for thermal decomposition treatment for 3 h to 6 h. The thermal decomposition temperature can be 300° C., 350° C., or 400° C., and the treatment time can be 3 h, 4.5 h, or 6 h, but is not limited to the values ​​listed. Other values ​​not listed within the numerical range are also applicable.

[0055] (2) After cooling to room temperature in the furnace, the pyrolysis product is taken out, and the pyrolysis product is mixed with a carbon source (preferably glucose or sucrose) in a mass ratio of 8 to 12:1 and ground to obtain a uniform mixed powder; the mass ratio can be 8:1, 10:1, or 12:1, but is not limited to the listed values, and other values ​​not listed within the numerical range are also applicable;

[0056] (3) The mixed powder is placed in a rotating demagnetizer and demagnetized in a magnetic field of 4000 GS to 6000 GS for 8 to 12 minutes to obtain a precursor powder. The magnetic field strength can be 4000 GS, 5000 GS, or 6000 GS, and the treatment time can be 8 minutes, 10 minutes, or 12 minutes, but the values ​​listed are not limited to these values. Other values ​​within the numerical range that are not listed are also applicable.

[0057] S15. In a microwave reaction system, the precursor powder is subjected to a microwave gradient high temperature sintering at 200W to 450W under protective atmosphere conditions, and the powder product LiMn is obtained after cooling. x Fe 1-x PO4 / C is a lithium manganese iron phosphate composite positive electrode material.

[0058] The specific operations are as follows:

[0059] (1) First, sintering for 15 minutes at a microwave power of 200W to 250W; the microwave power can be 200W, 230W, or 250W, but is not limited to the values ​​listed above. Other values ​​not listed within the numerical range are also applicable; during this stage, the sintering temperature is controlled between 300°C and 450°C;

[0060] (2) Then sintering at a microwave power of 350W to 450W for 20min to 30min; the microwave power can be 350W, 400W, or 450W, and the sintering time can be 20min, 25min, or 30min; but the values ​​listed are not limited thereto, and other values ​​not listed within the numerical range are also applicable; in this stage, the sintering temperature is controlled between 800°C and 1000°C;

[0061] (3) Finally, the final powder product LiMn is obtained after cooling in the furnace x Fe 1-x PO4 / C;

[0062] During the high-temperature sintering process, the pressure is controlled at 0.01 MPa to 0.5 MPa. A flowing inert atmosphere (such as argon, nitrogen, or helium) is also required to prevent product oxidation. The pressure can be 0.1 MPa, 0.25 MPa, or 0.5 MPa, but is not limited to the values ​​listed. Other values ​​within the numerical range are also applicable.

[0063] The sintering process of the present invention utilizes microwave gradient high-temperature sintering. Microwave heating directly acts on the material molecules, achieving a uniform temperature distribution in the reaction system within a short period of time. This effectively and accurately controls the temperature during sintering, shortens the sintering time, and improves production efficiency. Furthermore, the initial microwave sintering phase uses a relatively low microwave power to gradually heat the material, preventing structural damage caused by sudden temperature changes. Subsequently, the microwave power is gradually increased, which more effectively promotes the reaction of the material, shortens the overall sintering reaction time, and maintains the material's structure and properties.

[0064] Example 1

[0065] A method for preparing a lithium manganese iron phosphate composite positive electrode material based on a sol-gel method, the method comprising the following steps:

[0066] S11. Weigh lithium hydroxide monohydrate (41.96 g, 1.0 eq, purity ≥99.9%), ammonium dihydrogen phosphate (126.53 g, 1.1 eq, analytical grade), manganese acetate (103.81 g, 0.6 eq, purity ≥99.9%), ferrous chloride (50.7 g, 0.4 eq, purity ≥99.9%), and citric acid (384.28 g, 2 eq); add deionized water at a liquid-to-solid ratio of 3:1 and dissolve in a beaker; stir mechanically in a 70°C water bath until the solution is clear.

[0067] S12. Under continuous stirring and insulation at 70° C., the manganese acetate solution and the citric acid solution were preferentially mixed and mechanically stirred at a speed of 300 r / min for 20 min, and then the ferrous chloride solution, the ammonium dihydrogen phosphate solution and the lithium hydroxide solution were poured in sequence, and finally concentrated ammonia was slowly added dropwise to adjust the pH of the solution to 5 to obtain a reaction solution;

[0068] S13, transferring the reaction solution to a beaker, and then placing it in an ultrasonic-microwave combined reaction system, and placing a beaker filled with a certain amount of deionized water at the same time; first, only turning on the ultrasonic dispersion device, and performing ultrasonic treatment at an ultrasonic power of 200 W for 10 minutes to ensure that the reaction solution is fully mixed; on the basis of continuous ultrasonic treatment, turning on the microwave heating device, and performing ultrasonic-microwave synergistic treatment at a microwave power of 250 W for 60 minutes, when the reaction solution is converted into a yellow sol, turning off the ultrasonic dispersion device and continuing the microwave heating treatment until a fluffy honeycomb xerogel is formed;

[0069] S14, placing the honeycomb xerogel in a muffle furnace and thermally decomposing it at 300°C for 6 hours, cooling the product to room temperature in the furnace and then taking it out, mixing the thermal decomposition product with glucose in a mass ratio of 8:1, placing it in an agate mortar and grinding it, placing the mixed powder in a rotary demagnetizer and demagnetizing it in a magnetic field of 5000GS for 10 minutes to obtain a precursor powder;

[0070] S15. Place the precursor powder in a microwave reaction system, control the pressure to 0.1 MPa, and sinter it at a microwave power of 200 W and a temperature of 300°C to 450°C for 15 minutes, then sinter it at a microwave power of 350 W and a temperature of 800°C to 1000°C for 30 minutes; maintain an argon atmosphere during the sintering process; and then cool it in the furnace to obtain the final powder product LiMn 0.6 Fe 0.4 PO4 / C.

[0071] The LiMn prepared in this example 0.6 Fe 0.4 The charge and discharge specific capacity of PO4 / C material was tested at discharge rates of 1C and 2C, and the results are shown in Table 1.

[0072] Example 2

[0073] A method for preparing a lithium manganese iron phosphate composite positive electrode material based on a sol-gel method, the method comprising the following steps:

[0074] S11. Weigh lithium nitrate (68.95 g, 1.0 eq, purity ≥99.9%), phosphoric acid (127.4 g, 1.3 eq, analytical grade), manganese nitrate (143.16 g, 0.8 eq, purity ≥99.9%), ferrous nitrate (35.97 g, 0.2 eq, purity ≥99.9%), and citric acid (384.28 g, 2 eq); dissolve each in deionized water at a liquid-to-solid ratio of 4:1 in a beaker; and continuously stir mechanically in a 90°C water bath until the solution is clear.

[0075] S12. Under continuous stirring and insulation at 90° C., the manganese acetate solution and the citric acid solution were preferentially mixed and mechanically stirred at a speed of 300 r / min for 20 min, and then the ferrous chloride solution, the ammonium dihydrogen phosphate solution and the lithium hydroxide solution were poured in sequence, and finally concentrated ammonia was slowly added dropwise to adjust the pH of the solution to 2 to obtain a reaction solution;

[0076] S13, transferring the reaction solution to a beaker, and then placing it in an ultrasonic-microwave combined reaction system, and placing a beaker filled with a certain amount of deionized water at the same time; first, only turning on the ultrasonic dispersion device, and performing ultrasonic treatment at an ultrasonic power of 400 W for 30 minutes to ensure that the reaction solution is fully mixed; on the basis of continuous ultrasonic treatment, turning on the microwave heating device, and performing ultrasonic-microwave synergistic treatment at a microwave power of 500 W for 30 minutes, when the reaction solution is converted into a yellow sol, turning off the ultrasonic dispersion device and continuing the microwave heating treatment until a fluffy honeycomb xerogel is formed;

[0077] S14, placing the honeycomb xerogel in a muffle furnace and thermally decomposing it at 400°C for 3 hours, cooling the product to room temperature in the furnace and then taking it out, mixing the thermal decomposition product with glucose in a mass ratio of 12:1, placing it in an agate mortar and grinding it, placing the mixed powder in a rotary demagnetizer and demagnetizing it in a magnetic field of 5000GS for 10 minutes to obtain a precursor powder;

[0078] S15. Place the precursor powder in a microwave reaction system, control the pressure to 0.5 MPa, and sinter it at a microwave power of 250 W and a temperature of 300°C to 450°C for 15 minutes, then sinter it at a microwave power of 450 W and a temperature of 800°C to 1000°C for 20 minutes; maintain an argon atmosphere during the sintering process; and then cool it in the furnace to obtain the final powder product LiMn 0.8 Fe 02 PO4 / C.

[0079] The LiMn prepared in this example 0.8 The charge and discharge specific capacity of FeO2PO4 / C material was tested at discharge rates of 1C and 2C, and the results are shown in Table 1.

[0080] Example 3

[0081] A method for preparing a lithium manganese iron phosphate composite positive electrode material based on a sol-gel method, the method comprising the following steps:

[0082] S11. Weigh lithium chloride (42.39 g, 1.0 eq, purity ≥99.9%), triammonium phosphate (178.87 g, 1.2 eq, analytical grade), manganese chloride (113.31 g, 0.7 eq, purity ≥99.9%), ferrous sulfate (45.57 g, 0.3 eq, purity ≥99.9%), and citric acid (384.28 g, 2 eq); dissolve each in deionized water at a liquid-to-solid ratio of 4:1 in a beaker; and continuously stir mechanically in an 80°C water bath until the solution becomes clear.

[0083] S12. Under continuous stirring and insulation at 80° C., the manganese acetate solution and the citric acid solution were preferentially mixed and mechanically stirred at a speed of 300 r / min for 20 min, and then the ferrous chloride solution, the ammonium dihydrogen phosphate solution and the lithium hydroxide solution were poured in sequence, and finally concentrated ammonia was slowly added dropwise to adjust the pH of the solution to 4 to obtain a reaction solution;

[0084] S13, transferring the reaction solution to a beaker, and then placing it in an ultrasonic-microwave combined reaction system, and placing a beaker filled with a certain amount of deionized water at the same time; first, only turning on the ultrasonic dispersion device, and performing ultrasonic treatment at an ultrasonic power of 300 W for 20 minutes to ensure that the reaction solution is fully mixed; on the basis of continuous ultrasonic treatment, turning on the microwave heating device, and performing ultrasonic-microwave synergistic treatment at a microwave power of 350 W for 50 minutes, when the reaction solution is converted into a yellow sol, turning off the ultrasonic dispersion device and continuing the microwave heating treatment until a fluffy honeycomb xerogel is formed;

[0085] S14, placing the honeycomb xerogel in a muffle furnace and thermally decomposing it at 350°C for 5 hours, cooling the product to room temperature in the furnace and then taking it out, mixing the thermal decomposition product with sucrose at a mass ratio of 10:1, placing it in an agate mortar and grinding it, placing the mixed powder in a rotary demagnetizer and demagnetizing it in a magnetic field of 6000GS for 8 minutes to obtain a precursor powder;

[0086] S15. Place the precursor powder in a microwave reaction system, control the pressure to 0.25 MPa, and sinter it at a microwave power of 230 W and a temperature of 300°C to 450°C for 15 minutes, then sinter it at a microwave power of 400 W and a temperature of 800°C to 1000°C for 25 minutes; maintain a nitrogen atmosphere during the sintering process; and then cool it in the furnace to obtain the final powder product LiMn 0.7 Fe 0.3 PO4 / C.

[0087] The LiMn prepared in this example 0.7 Fe 0.3 The charge and discharge specific capacity of PO4 / C material was tested at discharge rates of 1C and 2C, and the results are shown in Table 1.

[0088] Comparative Example 1

[0089] Comparative Example 1 is a method for preparing lithium manganese iron phosphate composite positive electrode materials based on the sol-gel method. The difference between this method and Example 1 is only in step S13, which uses ultrasonic treatment alone throughout the process. The other steps and reaction conditions are the same as those in Example 1.

[0090] In this comparative example 1, step S13 is specifically as follows: placing the reaction solution in an ultrasonic reaction system and performing ultrasonic treatment at an ultrasonic power of 200 W until a dry gel is formed.

[0091] The LiMn prepared in Comparative Example 1 0.6 Fe 0.4 The charge and discharge specific capacity of PO4 / C material was tested at discharge rates of 1C and 2C, and the results are shown in Table 1.

[0092] Comparative Example 2

[0093] Comparative Example 2 is a method for preparing lithium manganese iron phosphate composite positive electrode materials based on the sol-gel method. The difference between this method and Example 1 is only in step S13, which uses microwave treatment alone throughout the process. The other steps and reaction conditions are the same as those in Example 1.

[0094] In this comparative example 2, step S13 is specifically as follows: placing the reaction solution in a microwave reaction system, placing a beaker filled with a certain amount of deionized water, and performing microwave treatment at a microwave power of 250 W until a dry gel is formed.

[0095] The LiMn prepared in this comparative example 2 0.6 Fe 0.4 The charge and discharge specific capacity of PO4 / C material was tested at discharge rates of 1C and 2C, and the results are shown in Table 1.

[0096] Comparative Example 3

[0097] Comparative Example 3 is a method for preparing lithium manganese iron phosphate composite positive electrode materials based on the sol-gel method. The difference between this method and Example 1 is only in step S13, which uses ultrasonic and microwave synergistic treatment throughout the process. The other steps and reaction conditions are the same as those in Example 1.

[0098] Step S13 of this comparative example 3 is specifically as follows: the reaction solution is transferred to a beaker, which is then placed in an ultrasonic-microwave combined reaction system, and a beaker containing a certain amount of deionized water is placed at the same time; ultrasonic-microwave synergistic treatment is performed under the conditions of a microwave power of 250 W and an ultrasonic power of 200 W until a dry gel is formed.

[0099] The LiMn prepared in this comparative example 3 0.6 Fe 0.4 The charge and discharge specific capacity of PO4 / C material was tested at discharge rates of 1C and 2C, and the results are shown in Table 1.

[0100] Comparative Example 4

[0101] Comparative Example 4 is a method for preparing lithium manganese iron phosphate composite positive electrode materials based on the sol-gel method. The difference between this method and Example 1 is only in step S15, in which constant microwave low power is used for sintering throughout the process. The other steps and reaction conditions are the same as those in Example 1.

[0102] The specific steps of step S15 of comparative example 4 are as follows: placing the precursor powder in a microwave reaction system, controlling the pressure to 0.1 MPa, sintering the precursor powder in a microwave reaction system at a power of 200 W for 60 min, and maintaining an argon atmosphere during the sintering process; and then cooling the precursor powder in the furnace to obtain the final powder product LiMn 0.6 Fe 0.4 PO4 / C.

[0103] The LiMn prepared in this comparative example 4 0.6 Fe 0.4 The charge and discharge specific capacity of PO4 / C material was tested at discharge rates of 1C and 2C, and the results are shown in Table 1.

[0104] Comparative Example 5

[0105] Comparative Example 5 is a method for preparing lithium manganese iron phosphate composite positive electrode material based on the sol-gel method. The difference between this method and Example 1 is only in step S15, in which constant microwave high power is used for sintering treatment throughout the process. The other steps and reaction conditions are the same as those in Example 1.

[0106] The specific steps of step S15 of this comparative example 5 are as follows: placing the precursor powder in a microwave reaction system, controlling the pressure to 0.1 MPa, sintering at a microwave power of 350 W for 40 min, and maintaining an argon atmosphere during the sintering process; then cooling the system to obtain the final powder product LiMn 0.6 Fe 0.4 PO4 / C.

[0107] The LiMn prepared in Comparative Example 5 0.6 Fe 0.4 The charge and discharge specific capacity of PO4 / C material was tested at discharge rates of 1C and 2C, and the results are shown in Table 1.

[0108] Comparative Example 6

[0109] Comparative Example 6 is a method for preparing lithium manganese iron phosphate composite positive electrode materials based on the sol-gel method. The difference between this method and Example 1 is only in step S15, which adopts conventional high-temperature sintering. The other steps and reaction conditions are the same as those in Example 1.

[0110] The specific step S15 of this comparative example 6 is as follows: the precursor powder is sintered at 600°C for 10 hours under an argon atmosphere and a pressure of 0.1 MPa; and then cooled in the furnace to obtain the final powder product LiMn 0.6 Fe 0.4 PO4 / C.

[0111] The LiMn prepared in this comparative example 6 0.6 Fe 0.4 The charge and discharge specific capacity of PO4 / C material was tested at discharge rates of 1C and 2C, and the results are shown in Table 1.

[0112] In order to conduct the charge and discharge specific capacity test, it is usually necessary to make the prepared lithium manganese iron phosphate composite positive electrode material into an electrode plate and then assemble it into a battery; specifically, the lithium manganese iron phosphate composite positive electrode material, the binder PVDF and the conductive agent acetylene black are mixed in a high-energy ball mill in a mass ratio of 8:1:1 to obtain a uniformly mixed black slurry, and then the black slurry is evenly coated on the current collector aluminum foil using a coating machine, and finally dried at 100°C in a vacuum drying oven for 12 hours to obtain a plate; after flattening, it is cut into a circular plate with a diameter of 15 mm; the circular plate is used as the positive electrode, the metal lithium sheet is used as the negative electrode, Celgard 2400 is used as the diaphragm, and 1M LiPF6 EC:DMC (1:1 Vol%) is used as the electrolyte according to the positive electrode shell → positive electrode plate → diaphragm → lithium sheet → gasket, spring sheet → negative electrode shell, and assembled into a button battery, and electrochemical tests such as charge and discharge specific capacity are carried out on a Land electrochemical instrument.

[0113] The button batteries made of lithium manganese iron phosphate composite positive electrode materials prepared in Examples 1 to 3 and Comparative Examples 1 to 6 were tested for charge and discharge specific capacity at discharge rates of 1C and 2C. The initial discharge capacity and capacity retention results obtained after 100 cycles are shown in Table 1.

[0114] Table 1

[0115]

[0116] As can be seen from the data in Table 1, the lithium manganese iron phosphate composite positive electrode materials prepared in Examples 1 to 3 of the present invention have good discharge specific capacity and capacity retention rate, and exhibit excellent dispersion uniformity, electronic conductivity and stability.

[0117] Comparisons of Comparative Examples 1 to 3 with Example 1 show that the lithium manganese iron phosphate composite cathode materials prepared using single microwave, ultrasonic, and microwave-ultrasonic synergistic treatments during the sol-gel preparation process all exhibit lower discharge specific capacities and capacity retention rates than the ultrasonic, ultrasonic-microwave synergistic, and sequential microwave treatments of the present invention. Furthermore, the effect of single microwave-ultrasonic synergistic treatment > single ultrasonic treatment ≈ single microwave treatment.

[0118] Comparisons of Comparative Examples 4 and 5 with Example 1 show that the lithium manganese iron phosphate composite cathode material produced using stepped microwave sintering exhibits better discharge specific capacity and capacity retention than single-power microwave sintering. This is because directly using high-power microwaves for sintering can cause rapid temperature fluctuations, leading to structural damage, while directly using low-power microwaves for sintering results in low reaction efficiency.

[0119] Comparing Comparative Example 6 with Example 1, it can be seen that the electrochemical performance of the lithium manganese iron phosphate composite positive electrode material prepared by microwave sintering is better than that by conventional sintering, and the time consumption is greatly shortened.

[0120] The above-described embodiments merely illustrate several implementations of the present invention, and while their descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the present invention. It should be noted that a person skilled in the art would be able to make numerous variations and improvements without departing from the spirit of the present invention, all of which fall within the scope of protection of the present invention. Therefore, the scope of protection of the present invention shall be determined by the appended claims.

Claims

1. A method for preparing a lithium manganese iron phosphate composite positive electrode material based on a sol-gel method, characterized in that: The method comprises the following steps: S1, preheating a lithium salt solution, a manganese salt solution, a ferrous salt solution, a phosphorus-containing compound solution, and a complexing agent solution to a preset temperature; the phosphorus-containing compound is selected from phosphate or phosphoric acid; S2, while continuously stirring and keeping warm, firstly mix the manganese salt solution and the complexing agent solution, then sequentially add the ferrous salt solution, the phosphorus compound solution and the lithium salt solution, and dropwise add concentrated ammonia water to obtain a reaction solution with a pH of 2 to 5; S3, placing the reaction solution in an ultrasonic-microwave combined reaction system, and sequentially performing ultrasonic treatment alone, ultrasonic-microwave synergistic treatment, and microwave treatment alone to obtain a fluffy honeycomb xerogel; the specific operation is as follows: Place the reaction vessel containing the reaction solution in the ultrasonic-microwave combined reaction system. First, only turn on the ultrasonic dispersion device to perform ultrasonic treatment on the reaction solution. The ultrasonic power is 200W~400W and the treatment time is 10min~30min. Then, the microwave heating device is turned on for ultrasonic-microwave synergistic treatment, the microwave power is set to 250W-500W, and the reaction time is 30min-60min; When the reaction solution turns into a yellow sol, the ultrasonic dispersion device is turned off and the microwave heating treatment is continued until a fluffy honeycomb xerogel is formed; S4, thermally decomposing the xerogel, mixing the obtained product with a carbon source in a predetermined ratio, and then grinding and demagnetizing the mixture to obtain a precursor powder; S5, in the microwave reaction system, the precursor powder was subjected to microwave gradient sintering at 200W~450W under protective atmosphere conditions, and the powder product LiMn was obtained after cooling in the furnace. x Fe 1-x PO4 / C is a lithium manganese iron phosphate composite cathode material; the specific operation of microwave gradient sintering is as follows: First, sinter at a microwave power of 200W~250W and a temperature of 300℃~450℃ for 15min; Sintering at a microwave power of 350W~450W and a temperature of 800℃~1000℃ for 20min~30min; Among them, the pressure during the sintering process is controlled at 0.01MPa~0.5MPa.

2. The method for preparing lithium manganese iron phosphate composite positive electrode material based on the sol-gel method according to claim 1, characterized in that: The ultrasonic-microwave combined reaction system is also provided with a container containing deionized water.

3. The method for preparing lithium manganese iron phosphate composite positive electrode material based on the sol-gel method according to claim 1, characterized in that: The dry gel is thermally decomposed, and the obtained product is mixed with a carbon source in a preset ratio, and then ground and demagnetized in sequence to obtain a precursor powder. The specific operation is as follows: The dry gel was placed in a muffle furnace and heated to 300°C~400°C for thermal decomposition treatment for 3h~6h; After cooling to room temperature in the furnace, the pyrolysis product is taken out, and the pyrolysis product is mixed with the carbon source in a mass ratio of 8 to 12:1 and ground to obtain a uniform mixed powder; The mixed powder is placed in a rotary demagnetizer and demagnetized in a magnetic field of 4000 GS to 6000 GS for 8 minutes to 12 minutes to obtain a precursor powder.

4. The method for preparing lithium manganese iron phosphate composite positive electrode material based on the sol-gel method according to claim 3, characterized in that: The carbon source is selected from glucose or sucrose.

5. The method for preparing lithium manganese iron phosphate composite positive electrode material based on the sol-gel method according to claim 1, characterized in that: The lithium salt is selected from lithium hydroxide monohydrate, lithium nitrate, lithium acetate, lithium chloride or lithium oxalate; The manganese salt is selected from manganese acetate, manganese carbonate, manganese sulfate, manganese nitrate or manganese chloride; The ferrous salt is selected from ferrous nitrate, ferrous sulfate or ferrous chloride; The phosphate is selected from ammonium dihydrogen phosphate or triammonium phosphate; According to the molar ratio, lithium salt: phosphorus-containing compound: manganese salt: ferrous salt = 1: (1.1~1.3): x: (1~x), 0.6≦x≦0.

8.

6. The method for preparing lithium manganese iron phosphate composite positive electrode material based on the sol-gel method according to claim 5, characterized in that: The complexing agent is selected from citric acid; the molar ratio of the lithium salt to the complexing agent is 1:1-2.

7. The method for preparing lithium manganese iron phosphate composite positive electrode material based on the sol-gel method according to claim 1, characterized in that: The preset temperature for preheating is 70℃~90℃.

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