Method for preparing lithium manganese iron phosphate by taking ammonium manganese iron phosphate as matrix

Abstract

The invention particularly discloses a method for preparing lithium manganese iron phosphate by taking ammonium manganese iron phosphate as a matrix, which comprises the following steps: preparing divalent ammonium manganese iron phosphate by adopting a reverse addition coprecipitation method, and preparing the lithium manganese iron phosphate from the divalent ammonium manganese iron phosphate and soluble lithium salt by adopting a solvothermal method. And finally, mixing and sintering with a carbon source to obtain the carbon-coated lithium manganese iron phosphate positive electrode material with high purity, good crystallinity and excellent electrochemical performance. The method not only retains the advantages of the solvothermal method, but also reduces the use amount of the lithium salt, so that the productivity is effectively improved, the route for preparing the lithium manganese iron phosphate by the solvothermal method is expected to realize industrialization, and a theoretical basis is provided for promoting the further development of the lithium manganese iron phosphate.

Description

Technical Field

[0001] The present application belongs to the technical field of lithium ion battery material preparation, and in particular, relates to a method for preparing lithium manganese iron phosphate using ammonium manganese iron phosphate as a matrix. Background Art

[0002] As the next generation product of lithium iron phosphate, lithium manganese iron phosphate can greatly improve the energy density of lithium-ion batteries while retaining many advantages of lithium iron phosphate such as safety, stability and low cost. It is currently the most promising lithium-ion battery positive electrode material and has achieved initial industrial production in many companies.

[0003] The existing preparation methods of lithium iron manganese phosphate are mainly solid-phase method and liquid-phase method. Among them, the solid-phase method is to subject the insoluble manganese source, iron source, phosphorus source, lithium source and carbon source to high-energy ball milling or sand milling, and a solid-phase reaction occurs during the sintering process to obtain lithium iron manganese phosphate material. For example, the invention patent with application number 202311682742.2 prepares lithium iron manganese phosphate from various raw materials through processes such as ball milling, sand milling, spray drying and high-temperature calcination, and adds a composite carbon source and a metal dopant to effectively solve the low conductivity problem of lithium iron manganese phosphate. It has the advantages of simple process and low cost, but the solid-phase diffusion between different components is slow, resulting in uneven product composition and easy phase separation. Liquid phase methods include sol-gel method, spray drying method and solvothermal method, which can achieve atomic-level distribution of each component. Solvothermal method can obtain lithium iron manganese phosphate with high purity without heat treatment, and the product has high crystallinity and conductivity. For example, the invention patent with application number 202410535547.5 drips a lithium salt aqueous solution into a solution containing lithium salt, phosphate salt, manganese salt and iron salt, and then transfers it to a solvothermal reactor to obtain lithium iron manganese phosphate. Furthermore, the invention patent with application number 202410450833.1 introduces the surfactant PVP to obtain lithium iron manganese phosphate crystals with fewer defects and good orientation, but these solvothermal studies are all based on Li 3 PO 4 The reaction is a matrix reaction, in which the amount of lithium salt used is about three times that of phosphate, and the lithium salt cannot be fully utilized. More importantly, the current lithium-ion battery market is controlled by the price of lithium salts. Therefore, the solvothermal method for producing lithium manganese iron phosphate cannot be truly industrialized. Summary of the invention

[0004] In view of the technical defects of the existing lithium iron manganese phosphate preparation method, the present application provides a method for preparing lithium iron manganese phosphate using ammonium iron manganese phosphate as a matrix. The method not only retains the advantages of the solvent thermal method, but also reduces the amount of lithium hydroxide used, and provides a theoretical basis for the industrialization of the process route for preparing lithium iron manganese phosphate by the solvent thermal method. The method comprises the following steps:

[0005] S100, preparing a manganese-iron mixed solution, a phosphate solution and a precipitant solution, wherein the manganese salt of the manganese-iron mixed solution is one or more of manganese sulfate monohydrate, manganese chloride tetrahydrate or manganese acetate tetrahydrate, and the iron salt is one or more of ferrous sulfate heptahydrate or ferrous chloride tetrahydrate; the phosphate solution is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate or ammonium phosphate; and the precipitant solution is one or more of ammonia water or sodium hydroxide solution;

[0006] S200, placing the prepared phosphate solution in a reaction container, adding the prepared ferromanganese mixed solution and the precipitant solution to the reaction container in parallel under stirring conditions, controlling the pH of the solution in the reaction container to be constant, and after the ferromanganese mixed solution is dripped away, stirring and aging to obtain an ammonium ferromanganese phosphate suspension;

[0007] S300, separating the obtained ammonium manganese iron phosphate suspension into solid and liquid, removing the upper clear liquid, and repeatedly washing the solid with deionized water and anhydrous ethanol until the conductivity of the washing liquid drops below 500 μS / cm, and then drying it under vacuum conditions to obtain an ammonium manganese iron phosphate monohydrate precursor;

[0008] S400, pouring the obtained ammonium manganese iron phosphate monohydrate precursor and lithium salt solution into a polytetrafluoroethylene reaction container and heating them to react, so as to obtain a lithium manganese iron phosphate suspension, wherein the lithium salt in the lithium salt solution is one or more of lithium hydroxide, lithium oxalate, and lithium citrate;

[0009] S500, separating the obtained lithium manganese iron phosphate suspension into solid and liquid, removing the upper clear liquid, and repeatedly washing the solid with deionized water until the conductivity of the washing liquid drops below 500 μS / cm, and then washing with anhydrous ethanol and vacuum drying to obtain lithium manganese iron phosphate.

[0010] In some embodiments, in step S100, the total concentration of manganese salt and iron salt in the manganese-iron mixed solution is 0.5-2 mol / L, the concentration of the phosphate solution is 0.5-4 mol / L, and the concentration of the precipitant is 1-15 mol / L.

[0011] In some embodiments, in step S200, the stirring intensity is 200-1000r / min, the pH value of the solution in the reaction container is controlled to be 4-10, the temperature of the solution in the reaction container is 20-80°C, the feeding rate of the ferromanganese mixed solution is 20-200ml / min, the stirring time after the ferromanganese mixed solution is dripped is 1-5h, and the aging time is 4-20h.

[0012] In some embodiments, in step S300, the drying temperature under vacuum conditions is 60-100° C., the drying time is 12-36 hours, the vacuum pressure is -0.8 MPa to -1 MPa, and the molecular formula of the ammonium manganese iron phosphate monohydrate precursor after drying is NH 4 Mn x Fe 1-x PO 4 ·H 2 O, wherein the atomic ratio (Mn+Fe) / P in the monohydrate ammonium manganese iron phosphate precursor is 0.95-1.05, and the specific surface area is 15-25m 2 / g.

[0013] In some embodiments, in step S400, the ratio of lithium ions to iron and manganese ions in the lithium salt is 1-1.5:1, the reaction temperature after the reaction container is heated is 180-220° C., and the reaction time is 16-24 hours.

[0014] In some embodiments, in step S500, the drying temperature under vacuum conditions is 60-100° C., the drying time is 12-36 hours, and the vacuum pressure is -0.8 MPa to -1 MPa.

[0015] In some embodiments, the process further includes step S600, dispersing the obtained lithium manganese iron phosphate and mixing it with a carbon source, and then sintering the mixed material under an inert atmosphere to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0016] In some embodiments, the carbon source is one or more of glucose, sucrose, phenolic epoxy resin, polyvinyl pyrrolidone, and asphalt.

[0017] In some embodiments, the sintering temperature is 600-800° C., and the sintering time is 4-8 hours.

[0018] In some embodiments, the inert atmosphere is one or more of a nitrogen atmosphere, an argon atmosphere, or a helium atmosphere.

[0019] Compared with the prior art, the present application first prepares an ammonium manganese iron phosphate matrix by a coprecipitation method; then, the obtained ammonium manganese iron phosphate matrix is ​​reacted with a lithium salt solution to obtain lithium manganese iron phosphate. In the subsequent solvent thermal reaction process, only the solubility of the lithium salt needs to be considered, and the lithium salt does not need to be excessive, which greatly reduces the amount of raw material input, so that the lithium salt is fully utilized, thereby improving production efficiency and reducing production costs. Therefore, the method for preparing lithium manganese iron phosphate using ammonium manganese iron phosphate as a matrix provided by the present application combines the advantages of the coprecipitation method and the solvent thermal method, and provides a theoretical basis for the solvent thermal industrial production of lithium manganese iron phosphate process route, and the method is simple in process, the prepared lithium manganese iron phosphate has high crystallinity, and a pure phase can be obtained without sintering, which is easy to store and transport. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] In order to more clearly illustrate the specific implementation methods of the present invention or the technical solutions in the prior art, the drawings required for use in the specific implementation methods or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are some implementation methods of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying creative work.

[0021] Figure 1 This is a flow chart of a method for preparing lithium iron manganese phosphate using ammonium iron manganese phosphate as a matrix in the present application;

[0022] Figure 2 The X-ray diffraction pattern of ammonium manganese ferrous phosphate prepared in Example 3 of the present application is given;

[0023] Figure 3 The X-ray diffraction pattern of the carbon-coated lithium manganese iron phosphate prepared in Example 3 of the present application is given;

[0024] Figure 4 A scanning electron microscope image of the ammonium manganese ferric phosphate prepared in Example 3 of the present application is given;

[0025] Figure 5 A scanning electron microscope image of the carbon-coated lithium manganese iron phosphate prepared in Example 3 of the present application is given;

[0026] Figure 6 The charge and discharge curve of the carbon-coated lithium manganese iron phosphate prepared in Example 1 of the present application at a rate of 0.1C is given;

[0027] Figure 7 The charge and discharge curve of the carbon-coated lithium manganese iron phosphate prepared in Example 1 of the present application at a 1C rate is given;

[0028] Figure 8 The rate performance diagram of the carbon-coated lithium manganese iron phosphate prepared in Example 1 of the present application is given;

[0029] Fig. 9 The cycle performance diagram of the carbon-coated lithium manganese iron phosphate prepared in Example 1 of the present application at a 1C rate is given;

[0030] Fig.10 The charge and discharge curve of the carbon-coated lithium manganese iron phosphate prepared in Example 2 of the present application at a rate of 0.1C is given;

[0031] Fig.11 The charge and discharge curve of the carbon-coated lithium manganese iron phosphate prepared in Example 2 of the present application at a 1C rate is given;

[0032] Fig.12The rate performance diagram of the carbon-coated lithium manganese iron phosphate prepared in Example 2 of the present application is given;

[0033] Fig.13 The cycle performance diagram of the carbon-coated lithium manganese iron phosphate prepared in Example 2 of the present application at a 1C rate is given;

[0034] Fig.14 The charge and discharge curve of the carbon-coated lithium manganese iron phosphate prepared in Example 3 of the present application at a rate of 0.1C is given;

[0035] Fig.15 The charge and discharge curve of the carbon-coated lithium manganese iron phosphate prepared in Example 3 of the present application at a 1C rate is given;

[0036] Fig.16 The rate performance diagram of the carbon-coated lithium manganese iron phosphate prepared in Example 3 of the present application is given;

[0037] Fig.17 The cycle performance diagram of the carbon-coated lithium manganese iron phosphate prepared in Example 3 of the present application at a 1C rate is given;

[0038] Fig.18 The charge and discharge curve of the carbon-coated lithium manganese iron phosphate prepared in Comparative Example 1 of the present application at a rate of 0.1C is given;

[0039] Fig.19 The charge and discharge curve of the carbon-coated lithium manganese iron phosphate prepared in Comparative Example 1 of the present application at a 1C rate is given;

[0040] Fig. 20 The rate performance diagram of the carbon-coated lithium manganese iron phosphate prepared in Comparative Example 1 of the present application is given;

[0041] Fig.21 A cycle performance diagram of the carbon-coated lithium manganese iron phosphate prepared in Comparative Example 1 of the present application at a 1C rate is given. DETAILED DESCRIPTION

[0042] "Scope" disclosed in the present application is limited in the form of lower limit and upper limit, and a given range is limited by selecting a lower limit and an upper limit, and the selected lower limit and upper limit define the boundary of a special range. The scope limited in this way can be including end values ​​or not including end values, and can be arbitrarily combined, that is, any lower limit can be combined with any upper limit to form a scope. For example, if the scope of 60-120 and 80-110 is listed for a specific parameter, it is understood that the scope 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 scope can be all expected: 1-3, 1-4, 1-5, 2-3, 2-4 and 2-5. In the present application, unless otherwise specified, the numerical range "ab" represents the abbreviation of any real number combination between a and b, wherein a and b are real numbers. For example, the numerical range "0-5" means that all real numbers between "0-5" are listed in this document, 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 an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0043] If there is no special explanation, the "include" and "comprising" mentioned in this application are open-ended or closed-ended. For example, the "include" and "comprising" may mean 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.

[0044] refer to Figure 1 As shown, the embodiment of the present application provides a method for preparing lithium manganese iron phosphate using ammonium manganese iron phosphate as a matrix, and the method comprises the following steps:

[0045] S100, preparing a manganese-iron mixed solution, a phosphate solution and a precipitant solution, wherein the manganese salt of the manganese-iron mixed solution is one or more of manganese sulfate monohydrate, manganese chloride tetrahydrate or manganese acetate tetrahydrate, and the iron salt is one or more of ferrous sulfate heptahydrate or ferrous chloride tetrahydrate; the phosphate solution is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate or ammonium phosphate; and the precipitant solution is one or more of ammonia water or sodium hydroxide solution;

[0046] In this step, the total concentration of manganese salt and iron salt in the manganese-iron mixed solution is 0.5-2 mol / L, the concentration of the phosphate solution is 0.5-4 mol / L, and the concentration of the precipitant is 1-15 mol / L.

[0047] S200, placing the prepared phosphate solution in a reaction container, adding the prepared ferromanganese mixed solution and the precipitant solution to the reaction container in parallel under stirring conditions, controlling the pH of the solution in the reaction container to be constant, and after the ferromanganese mixed solution is dripped away, stirring and aging to obtain an ammonium ferromanganese phosphate suspension;

[0048] In this step, the reaction container is a reactor, the stirring intensity is 200-1000r / min, the pH value of the solution in the reaction container is controlled to be 4-10, the temperature of the solution in the reaction container is 20-80°C, the feeding rate of the ferromanganese mixed solution is 20-200ml / min, the stirring time after the ferromanganese mixed solution is dripped is 1-5h, and the aging time is 4-20h.

[0049] S300, separating the obtained ammonium manganese iron phosphate suspension into solid and liquid, taking the supernatant and repeatedly washing it with deionized water and anhydrous ethanol until the conductivity of the supernatant drops below 500 μS / cm, and then drying it under vacuum conditions to obtain an ammonium manganese iron phosphate monohydrate precursor;

[0050] In this step, the drying temperature under vacuum conditions is 60-100°C, the drying time is 12-36h, the vacuum pressure is -0.8MPa to -1MPa, and the molecular formula of the ammonium manganese iron phosphate monohydrate precursor after drying is NH 4 Mn x Fe 1-x PO 4 ·H 2 O, wherein the atomic ratio (Mn+Fe) / P in the monohydrate ammonium manganese iron phosphate precursor is 0.95-1.05, and the specific surface area is 15-25m 2 / g.

[0051] S400, pouring the obtained ammonium manganese iron phosphate monohydrate precursor and lithium salt solution into a polytetrafluoroethylene reaction container and heating them to react, so as to obtain a lithium manganese iron phosphate suspension, wherein the lithium salt in the lithium salt solution is one or more of lithium hydroxide, lithium oxalate, and lithium citrate; in this step, the polytetrafluoroethylene reaction container is a polytetrafluoroethylene reactor, the ratio of lithium ions to iron manganese ions in the lithium salt is 1-1.5:1, the reaction temperature after the reaction container is heated is 180-220°C, and the reaction time is 16-24h.

[0052] S500, separating the obtained lithium manganese iron phosphate suspension into solid-liquid, removing the upper clear liquid, and repeatedly washing the solid with deionized water until the conductivity of the washing liquid drops below 500 μS / cm, and then washing with anhydrous ethanol and vacuum drying to obtain lithium manganese iron phosphate, wherein the drying temperature under vacuum conditions is 60-100° C., the drying time is 12-36 hours, and the vacuum pressure is -0.8Mpa to -1Mpa.

[0053] In the above embodiment, first, an ammonium manganese iron phosphate matrix is ​​prepared by a coprecipitation method; then, the obtained ammonium manganese iron phosphate matrix is ​​reacted with a lithium salt solution to obtain lithium manganese iron phosphate. In the subsequent solvent thermal reaction process, only the solubility of the lithium salt needs to be considered, and the lithium salt does not need to be excessive, which greatly reduces the amount of raw material input, so that the lithium salt is fully utilized, thereby improving production efficiency and reducing production costs. Therefore, the method for preparing lithium manganese iron phosphate using ammonium manganese iron phosphate as a matrix provided in this application combines the advantages of coprecipitation and solvent thermal methods, and provides a theoretical basis for the solvent thermal industrial production of lithium manganese iron phosphate process route, and the method is simple in process, the prepared lithium manganese iron phosphate has high crystallinity, and a pure phase can be obtained without sintering, which is easy to store and transport.

[0054] In some embodiments, the process further includes step S600, dispersing the obtained lithium manganese iron phosphate and mixing it with a carbon source, and then sintering the mixed material under an inert atmosphere to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

[0055] In the above embodiment, the carbon source is one or more of glucose, sucrose, phenolic epoxy resin, polyvinyl pyrrolidone, and asphalt, the sintering temperature is 600-800°C, the sintering time is 4-8h, and the inert atmosphere is one or more of nitrogen atmosphere, argon atmosphere, or helium atmosphere.

[0056] In order to further illustrate the technical principle and technical effect of the present application, the following describes the contents disclosed in the present application in more detail with several examples, which are only used for illustrative purposes, because it is obvious to those skilled in the art to make various modifications and changes within the scope of the present application. Unless otherwise stated, all reagents and raw materials used in the examples are commercially available or synthesized according to conventional methods, and the instruments and equipment used in the examples are commercially available.

[0057] Example 1

[0058] 1. Weigh 25.88g (0.225mol) of ammonium dihydrogen phosphate and dissolve it in 150ml of deionized water. Transfer it into the reactor and stir it at 400r / min. Adjust the temperature to 40°C. Then pump 13.38mol / L of concentrated ammonia water into the reactor until the pH is 6.

[0059] 2. Weigh 29.19 g (0.105 mol) of ferrous sulfate heptahydrate and 7.61 g (0.045 mol) of manganese sulfate monohydrate and dissolve them in 150 ml of deionized water, pump them into the reactor at a speed of 30 ml / min, and adjust the speed of the reactor to 600 r / min. At the same time, pump concentrated ammonia water in parallel to control the pH value to be constant at 6. After the metal ion mixture is dripped, stir for 3 hours and age for 12 hours to obtain ammonium manganese ferric phosphate suspension;

[0060] 3. The obtained ammonium manganese ferric phosphate suspension was placed in a centrifuge and washed with deionized water at a speed of 4000r / min for 8min. After the conductivity of the washing liquid was reduced to 500μS / cm, it was washed twice with anhydrous ethanol and then transferred to a vacuum drying oven at 100℃ for 24h. The drying pressure was -0.8MPa, and high-purity ammonium manganese ferric phosphate monohydrate powder was obtained. The molecular formula is NH 4 Mn 0.3 Fe 0.7 PO 4 ·H 2 O;

[0061] 4. Ultrasonic disperse 4.66g of ammonium manganese iron phosphate monohydrate in 60ml of ethylene glycol and pour into a 100ml solvent thermal reactor. Then add 1.07g of LiOH·H 2 O is dissolved in 20 ml of deionized water and then dripped into the reactor, and the reactor is placed in a forced air drying oven at 180°C for 20 hours to obtain a lithium manganese iron phosphate suspension;

[0062] 5. The prepared lithium iron manganese phosphate suspension is subjected to solid-liquid separation, and is repeatedly washed with deionized water to reduce the conductivity of the washing liquid to below 500 μS / cm, and then washed twice with anhydrous ethanol and dried to obtain lithium iron manganese phosphate powder;

[0063] 6. Dissolve 0.3 g of glucose in 20 ml of anhydrous ethanol, add the obtained lithium iron manganese phosphate powder and ultrasonicate for 10 min, stir in an oil bath at 80 ° C until the ethanol is completely volatilized, and finally transfer to a vacuum drying oven and dry at 100 ° C for 12 h, with a drying pressure of -1 MPa;

[0064] 7. Place the dried material into a tube furnace and keep it at 350℃ for 5h and 700℃ for 4h to obtain carbon-coated lithium manganese iron phosphate material with the molecular formula of LiMn 0.6 Fe 0.4 PO 4 / C, wherein the heating rate in the tube furnace is 5°C / min and the atmosphere is argon.

[0065] Example 2

[0066] 1. Weigh 25.88g (0.225mol) of ammonium dihydrogen phosphate and dissolve it in 150ml of deionized water. Transfer it into the reactor and stir it at 400r / min. Adjust the temperature to 40°C. Then pump 13.38mol / L of concentrated ammonia water into the reactor until the pH value is 6.

[0067] 2. Weigh 20.85 g (0.075 mol) of ferrous sulfate heptahydrate and 12.68 g (0.075 mol) of manganese sulfate monohydrate and dissolve them in 150 ml of deionized water, pump them into the reactor at a speed of 200 ml / min, and adjust the speed of the reactor to 600 r / min. At the same time, pump concentrated ammonia water in parallel to control the pH value to be constant at 6. After the metal ion mixture is dripped, stir for 3 hours and age for 12 hours to obtain ammonium manganese ferric phosphate suspension;

[0068] 3. The obtained ammonium manganese ferric phosphate suspension was placed in a centrifuge and washed with deionized water at a speed of 4000r / min for 8min. After the conductivity of the washing liquid was reduced to 500μS / cm, it was washed twice with anhydrous ethanol and then transferred to a vacuum drying oven at 100℃ for 24h. The drying pressure was -1MPa, and high-purity ammonium manganese ferric phosphate monohydrate powder was obtained. The molecular formula is NH 4 Mn 0.5 Fe 0.5 PO 4 ·H 2 O;

[0069] 4. Ultrasonic disperse 4.66g of ammonium manganese iron phosphate monohydrate in 60ml of ethylene glycol and pour into a 100ml solvent thermal reactor. Then add 1.07g of LiOH·H 2 O is dissolved in 20 ml of deionized water and then dripped into the reactor, and the reactor is placed in a forced air drying oven at 180°C for 20 hours to obtain a lithium manganese iron phosphate suspension;

[0070] 5. The prepared lithium iron manganese phosphate suspension is subjected to solid-liquid separation, and is repeatedly washed with deionized water to reduce the conductivity of the washing liquid to below 500 μS / cm, and then washed twice with anhydrous ethanol and dried to obtain lithium iron manganese phosphate powder;

[0071] 6. Dissolve 0.3 g of glucose in 20 ml of anhydrous ethanol, add the obtained lithium iron manganese phosphate powder and ultrasonicate for 10 min, stir in an oil bath at 80 ° C until the ethanol is completely volatilized, and finally transfer to a vacuum drying oven and dry at 100 ° C for 12 h, with a drying pressure of -0.8 MPa;

[0072] 7. Put the dried material into a tube furnace and keep it at 350℃ for 5h and 700℃ for 4h to obtain carbon-coated lithium manganese iron phosphate material with the molecular formula of LiMn 0.6 Fe 0.4 PO 4 / C, wherein the heating rate in the tube furnace is 5°C / min and the atmosphere is argon.

[0073] Example 3

[0074] 1. Weigh 25.88g (0.225mol) of ammonium dihydrogen phosphate and dissolve it in 150ml of deionized water. Transfer it into the reactor and stir it at 400r / min. Adjust the temperature to 40℃. Pump 13.38mol / L of concentrated ammonia water into the reactor to control the pH value to 6.

[0075] 2. Weigh 16.68g (0.06mol) of ferrous sulfate heptahydrate and 15.21g (0.09mol) of manganese sulfate monohydrate and dissolve them in 150ml of deionized water, pump them into the reactor at a speed of 100ml / min, and adjust the speed of the reactor to 600r / min. At the same time, pump concentrated ammonia water in parallel to control the pH to 6. After the metal ion mixture drips out, stir for 3h and age for 12h to obtain ammonium manganese ferric phosphate suspension;

[0076] 3. The obtained ammonium manganese ferric phosphate suspension was placed in a centrifuge and washed with deionized water at a speed of 4000r / min for 8min. After the conductivity of the washing liquid was reduced to 500μS / cm, it was washed twice with anhydrous ethanol and then transferred to a vacuum drying oven at 100℃ for 24h. The drying pressure was -0.8MPa, and high-purity ammonium manganese ferric phosphate monohydrate powder was obtained. The molecular formula is NH 4 Mn 0.6 Fe 0.4 PO 4 ·H 2 O;

[0077] 4. Ultrasonic disperse 4.66g of ammonium manganese iron phosphate monohydrate in 60ml of ethylene glycol and pour into a 100ml solvent thermal reactor. Then add 1.07g of LiOH·H 2 O is dissolved in 20 ml of deionized water and then dripped into the reactor, and the reactor is placed in a forced air drying oven at 180°C for 20 hours to obtain a lithium manganese iron phosphate suspension;

[0078] 5. The prepared lithium iron manganese phosphate suspension is subjected to solid-liquid separation, and is repeatedly washed with deionized water to reduce the conductivity of the washing liquid to below 500 μS / cm, and then washed twice with anhydrous ethanol and dried to obtain lithium iron manganese phosphate powder;

[0079] 6. Dissolve 0.3 g of glucose in 20 ml of anhydrous ethanol, add the obtained lithium iron manganese phosphate powder and ultrasonicate for 10 min, stir in an oil bath at 80 ° C until the ethanol is completely volatilized, and finally transfer to a vacuum drying oven and dry at 100 ° C for 12 h, with a drying pressure of -0.8 MPa;

[0080] 7. Put the dried material into a tube furnace and keep it at 350℃ for 5h and 700℃ for 4h to obtain carbon-coated lithium manganese iron phosphate material with the molecular formula of LiMn 0.6 Fe0.4 PO 4 / C, wherein the heating rate in the tube furnace is 5°C / min and the atmosphere is argon.

[0081] Comparative Example 1

[0082] 1. Weigh 25.88g (0.225mol) of ammonium dihydrogen phosphate and dissolve it in 150ml of deionized water. Transfer it into the reactor and stir it at 400r / min. Adjust the temperature to 40°C. Then pump 13.38mol / L of concentrated ammonia water into the reactor until the pH is 6.

[0083] 2. Weigh 16.68g (0.06mol) of ferrous sulfate heptahydrate and 15.21g (0.09mol) of manganese sulfate monohydrate and dissolve them in 150ml of deionized water, pump them into the reactor at a speed of 80ml / min, and adjust the speed of the reactor to 600r / min. At the same time, pump concentrated ammonia water in parallel to control the pH to 6. After the metal ion mixture is dripped, stir for 3h and age for 12h to obtain ammonium manganese ferrous phosphate suspension;

[0084] 3. After the reaction, the suspension was placed in a centrifuge and washed with deionized water at a speed of 4000 r / min for 8 min. After the conductivity of the washing liquid was reduced to 500 μS / cm, it was washed twice with anhydrous ethanol, and then transferred to a vacuum drying oven at 100 ° C for 24 h to obtain high-purity ammonium manganese iron phosphate monohydrate powder, wherein the drying pressure is -0.8 MPa;

[0085] 4. Ultrasonic disperse 4.66g of ammonium manganese iron phosphate monohydrate in 60ml of ethylene glycol and pour into a 100ml solvent thermal reactor. Then add 1.605g of LiOH·H 2 O is dissolved in 20 ml of deionized water and then dripped into the reactor, and the reactor is placed in a forced air drying oven at 180°C for 20 hours to obtain a lithium manganese iron phosphate suspension;

[0086] 5. The prepared lithium manganese iron phosphate suspension is subjected to solid-liquid separation, and the cleaning liquid is repeatedly washed with deionized water to reduce the conductivity of the cleaning liquid to below 500 μS / cm, and then washed twice with anhydrous ethanol and dried to obtain lithium manganese iron phosphate powder with a molecular formula of NH 4 Mn 0.6 Fe 0.4 PO 4 ·H 2 O;

[0087] 6. Dissolve 0.3 g of glucose in 20 ml of anhydrous ethanol, add the obtained lithium iron manganese phosphate powder and ultrasonicate for 10 min, stir in an oil bath at 80 ° C until the ethanol is completely volatilized, and finally transfer to a vacuum drying oven and dry at 100 ° C for 12 h, with a drying pressure of -0.8 MPa;

[0088] 7. Put the dried material into a tube furnace and keep it at 350℃ for 5h and 700℃ for 4h to obtain carbon-coated lithium manganese iron phosphate material with the molecular formula of LiMn 0.6 Fe 0.4 PO 4 / C, wherein the heating rate in the tube furnace is 5°C / min and the atmosphere is argon.

[0089] The ammonium manganese iron phosphate and carbon-coated lithium manganese iron phosphate in Example 3 were taken and subjected to X-ray diffraction analysis and electron microscope scanning analysis, respectively. Figure 2-Figure 5 shown.

[0090] from Figure 2-Figure 3 It can be concluded that the ammonium manganese iron phosphate prepared by coprecipitation and the lithium manganese iron phosphate prepared by subsequent solvent thermal method have no impurities and good crystallinity. Figure 4-Figure 5 It can be observed that ammonium manganese ferric phosphate presents a two-dimensional flake with excellent dispersibility, and the grains grow thicker after the solvothermal reaction.

[0091] The carbon-coated lithium manganese iron phosphate materials prepared in Examples 1-3 and Comparative Example 1 were tested for rate performance and charge-discharge performance, and the test results are shown in Tables 1 and Figure 6-Figure 21 shown.

[0092] Table 1 Test data of the discharge specific capacity and 1C cycle 100-week capacity retention rate of the modified lithium manganese iron phosphate prepared in Examples 1-3 and Comparative Example 1 at 0.1C and 1C rates at medium discharge pressure

[0093] example 0.1C specific capacity (mAh / g) 1C specific capacity (mAh / g) 1C cycle 100 weeks capacity retention rate (%) Example 1 151.8 141.2 98.2 Example 2 154.5 140.0 96.4 Example 3 154.9 135.6 96.1 Comparative Example 1 139 122.7 81.7

[0094] From Table 1 and Figure 6-Figure 21 It can be seen that the carbon-coated lithium manganese iron phosphate materials prepared in Examples 1-3 have excellent rate cycle performance at 0.1C and 1C rates, and achieve continuously adjustable manganese-iron ratio. Example 1 with a manganese-iron ratio of 3:7 exhibits 151.8 mAh / g and 141.2 mAh / g at 0.1C and 1C rates, while Example 3 with a manganese-iron ratio of 6:4 exhibits 154.9 mAh / g and 135.6 mAh / g at 0.1C and 1C rates, and the cycle retention rate reaches 96.1%. The carbon-coated lithium manganese iron phosphate material prepared in Comparative Example 1 has a discharge capacity of only 139 mAh / g and 122.7 mAh / g at 0.1C rate, and the cycle retention rate is only 81.7%. It can be seen that the lithium manganese iron phosphate prepared by the method for preparing lithium manganese iron phosphate using ammonium manganese iron phosphate as the matrix provided in the present application has the characteristics of high purity and good crystallinity, and excellent electrochemical properties; it combines the advantages of co-precipitation method and solvothermal method, and provides a theoretical basis for the solvothermal industrial production process route of lithium manganese iron phosphate.

[0095] The above is a detailed introduction to a method for preparing lithium manganese iron phosphate using ammonium manganese iron phosphate as a matrix provided by the present application. Specific examples are used herein to illustrate the principles and implementation methods of the present application, and the description of the above embodiments is only used to help understand the core idea of ​​the present application. It should be pointed out that for ordinary technicians in this technical field, without departing from the principles of the present application, several improvements and modifications can be made to the present application, and these improvements and modifications also fall within the scope of protection of the claims of the present application.

Claims

1. A method for preparing lithium manganese iron phosphate using ammonium manganese iron phosphate as a matrix, characterized in that: The method comprises the following steps: S100, preparing a manganese-iron mixed solution, a phosphate solution and a precipitant solution, wherein the manganese salt of the manganese-iron mixed solution is one or more of manganese sulfate monohydrate, manganese chloride tetrahydrate or manganese acetate tetrahydrate, and the iron salt is one or more of ferrous sulfate heptahydrate or ferrous chloride tetrahydrate; the phosphate solution is one or more of ammonium dihydrogen phosphate, diammonium hydrogen phosphate or ammonium phosphate; and the precipitant solution is one or more of ammonia water or sodium hydroxide solution; S200, placing the prepared phosphate solution in a reaction container, adding the prepared ferromanganese mixed solution and the precipitant solution to the reaction container in parallel under stirring conditions, controlling the pH of the solution in the reaction container to be constant, and after the ferromanganese mixed solution is dripped away, stirring and aging to obtain an ammonium ferromanganese phosphate suspension; S300, separating the obtained ammonium manganese iron phosphate suspension into solid and liquid, removing the upper clear liquid, and repeatedly washing the solid with deionized water and anhydrous ethanol until the conductivity of the washing liquid drops below 500 μS / cm, and then drying it under vacuum conditions to obtain an ammonium manganese iron phosphate monohydrate precursor; S400, pouring the obtained ammonium manganese iron phosphate monohydrate precursor and lithium salt solution into a polytetrafluoroethylene reaction container and heating them to react, so as to obtain a lithium manganese iron phosphate suspension, wherein the lithium salt in the lithium salt solution is one or more of lithium hydroxide, lithium oxalate, and lithium citrate; S500, separating the obtained lithium manganese iron phosphate suspension into solid and liquid, removing the upper clear liquid, and repeatedly washing the solid with deionized water until the conductivity of the washing liquid drops below 500 μS / cm, and then washing with anhydrous ethanol and vacuum drying to obtain lithium manganese iron phosphate.

2. The method according to claim 1, characterized in that In the step S100, the total concentration of manganese salt and iron salt in the manganese-iron mixed solution is 0.5-2 mol / L, the concentration of the phosphate solution is 0.5-4 mol / L, and the concentration of the precipitant is 1-15 mol / L.

3. The method according to claim 1, characterized in that In the step S200, the stirring intensity is 200-1000r / min, the pH value of the solution in the reaction container is controlled to be 4-10, the temperature of the solution in the reaction container is 20-80°C, the feeding speed of the ferromanganese mixed solution is 20-200ml / min, the stirring time after the ferromanganese mixed solution is completely dripped is 1-5h, and the aging time is 4-20h.

4. The method according to claim 1, characterized in that: In the step S300, the drying temperature under vacuum conditions is 60-100°C, the drying time is 12-36h, the vacuum pressure is -0.8MPa to -1MPa, and the molecular formula of the ammonium manganese iron phosphate monohydrate precursor after drying is NH4Mn x Fe 1-x PO4·H2O, wherein the atomic ratio (Mn+Fe) / P of the precursor of manganese iron ammonium phosphate monohydrate is 0.95-1.05, and the specific surface area is 15-25m 2 / g.

5. The method according to claim 1, characterized in that In the step S400, the ratio of lithium ions to iron and manganese ions in the lithium salt is 1-1.5:1, the reaction temperature after the reaction container is heated is 180-220° C., and the reaction time is 16-24 hours.

6. The method according to claim 5, characterized in that In the step S500, the drying temperature under vacuum conditions is 60-100°C, the drying time is 12-36 hours, and the vacuum pressure is -0.8 MPa to -1 MPa.

7. The method according to claim 6, characterized in that The method further includes step S600, dispersing the obtained lithium manganese iron phosphate and mixing it with a carbon source, and then sintering the mixed material under an inert atmosphere to obtain a carbon-coated lithium manganese iron phosphate positive electrode material.

8. The method according to claim 7, characterized in that The carbon source is one or more of glucose, sucrose, phenolic epoxy resin, polyvinyl pyrrolidone and asphalt.

9. The method according to claim 7, characterized in that: The sintering temperature is 600-800° C., and the sintering time is 4-8 hours.

10. The method according to claim 7, characterized in that The inert atmosphere is one or more of a nitrogen atmosphere, an argon atmosphere or a helium atmosphere.

Images