A method for preparing lithium manganese iron phosphate by taking manganese iron ammonium phosphate as a base body

Abstract

The application specifically discloses a method for preparing lithium manganese iron phosphate by taking manganese iron ammonium phosphate as a base body, which adopts a reverse addition co-precipitation method to prepare divalent manganese iron ammonium phosphate, and then prepares lithium manganese iron phosphate by a solvothermal method with the divalent manganese iron ammonium phosphate and a soluble lithium salt, and finally obtains carbon-coated lithium manganese iron phosphate positive electrode material with high purity, good crystallinity and excellent electrochemical performance after mixing with a carbon source and sintering. The method not only retains the advantages of the solvothermal method, but also reduces the use amount of lithium salt, thereby effectively increasing the production capacity, so that the solvothermal method for preparing lithium manganese iron phosphate is expected to realize industrialization, and provides a theoretical basis for promoting the further development of lithium manganese iron phosphate.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of lithium ion battery material preparation, and particularly relates to a method for preparing lithium manganese iron phosphate by taking manganese ammonium iron phosphate as a matrix. BACKGROUND

[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 the advantages of lithium iron phosphate such as safety, stability and low cost. It is the most promising positive electrode material for lithium ion batteries at present and has realized preliminary industrial production in many enterprises.

[0003] The existing preparation methods of lithium manganese iron phosphate mainly include solid phase method and liquid phase method. In the solid phase method, the insoluble manganese source, iron source, phosphorus source, lithium source and carbon source are subjected to high-energy ball milling or sand milling treatment, and a solid phase reaction occurs in the sintering process to obtain lithium manganese iron phosphate material. The patent application No. 202311682742.2 prepares lithium manganese iron phosphate by ball milling, sand milling, spray drying and high-temperature calcination processes, and adds a composite carbon source and a metal dopant to effectively solve the problem of low conductivity of lithium manganese iron phosphate, which has the advantages of simple process and low cost. However, the solid phase diffusion between different components is slow, resulting in uneven product composition and easy phase separation. The liquid phase method includes sol-gel method, spray drying method and solvothermal method, which can achieve atomic-level distribution of each component. The solvothermal method can obtain lithium manganese iron phosphate with high purity without heat treatment, and the product has high crystallinity and conductivity. The patent application No. 202410535547.5 drops a lithium salt aqueous solution into a solution containing lithium salt, phosphorus salt, manganese salt and iron salt, and then transfers it into a solvothermal reaction kettle to obtain lithium manganese iron phosphate. Further, the patent application No. 202410450833.1 introduces a surfactant PVP to obtain lithium manganese iron phosphate crystals with few defects and good orientation. However, these solvothermal studies are based on Li3PO4 as the matrix, and the amount of lithium salt used is about 3 times that of the phosphate salt, so the lithium salt cannot be fully utilized. More importantly, the current lithium ion battery market is controlled by the price of lithium salt, so the solvothermal method for producing lithium manganese iron phosphate cannot realize real industrialization. SUMMARY

[0004] In view of the technical defects of the existing preparation methods of lithium manganese iron phosphate, the application provides a method for preparing lithium manganese iron phosphate by taking manganese ammonium iron phosphate as a matrix. This method not only retains the advantages of the solvothermal method, but also reduces the amount of lithium hydroxide used, providing a theoretical basis for the industrialization of the solvothermal method for preparing lithium manganese iron phosphate. 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 in the manganese-iron mixed solution is one or more of manganese sulfate monohydrate, manganese chloride tetrahydrate or manganese acetate tetrahydrate, 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; the precipitant solution is one or more of ammonia water or sodium hydroxide solution;

[0006] S200, placing the prepared phosphate solution into a reaction container, and adding the prepared manganese-iron mixed solution and the precipitant solution into the reaction container in parallel flow under stirring, controlling the pH of the solution in the reaction container to be constant, and after the manganese-iron mixed solution is completely added, stirring and aging to obtain an ammonium manganese iron phosphate suspension;

[0007] S300, performing solid-liquid separation on the obtained ammonium manganese iron phosphate suspension, removing the supernatant, and repeatedly washing the solid with deionized water and anhydrous ethanol until the conductivity of the washing liquid is reduced to below 500 μS / cm, and then drying the solid under vacuum to obtain ammonium manganese iron phosphate monohydrate precursor;

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

[0009] S500, performing solid-liquid separation on the obtained manganese iron lithium phosphate suspension, removing the supernatant, and repeatedly washing the solid with deionized water until the conductivity of the washing liquid is reduced to below 500 μS / cm, and then washing with anhydrous ethanol and drying under vacuum, to obtain manganese iron lithium phosphate.

[0010] In some embodiments, in the step S100, the total concentration of the manganese salt and the 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 the step S200, the stirring intensity is 200-1000 r / min, the pH 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℃, the feeding speed of the manganese-iron mixed solution is 20-200 ml / min, the stirring time after the manganese-iron mixed solution is completely added is 1-5 h, and the aging time is 4-20 h.

[0012] In some embodiments, in the step S300, the drying temperature under vacuum condition is 60-100℃, 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 in the ammonium manganese iron phosphate monohydrate precursor is 0.95-1.05, and the specific surface area is 15-25m 2 / g.

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

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

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

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

[0017] In some embodiments, the sintering temperature is 600-800℃, and the sintering time is 4-8h.

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

[0019] Compared with the prior art, the ammonium manganese iron phosphate matrix is first prepared by the coprecipitation method, and then the lithium manganese iron phosphate is obtained by reacting the obtained ammonium manganese iron phosphate matrix with a lithium salt solution. In the subsequent solvothermal 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 materials, makes the lithium salt fully utilized, and thus improves the production efficiency and reduces the production cost. Therefore, the method for preparing the lithium manganese iron phosphate with the ammonium manganese iron phosphate as the matrix provided in the application combines the advantages of the coprecipitation method and the solvothermal method, provides a theoretical basis for the solvothermal industrial production of the lithium manganese iron phosphate, and has a simple process, high crystallinity of the prepared lithium manganese iron phosphate, pure phase without sintering, and easy storage and transportation. BRIEF DESCRIPTION OF DRAWINGS

[0020] In order to more clearly illustrate the technical solutions in the specific embodiments of the present application or the prior art, the drawings needed in the description of the specific embodiments or the prior art will be briefly introduced below. Obviously, the drawings in the following description are some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative effort.

[0021] Figure 1 is a flow chart of a method for preparing lithium manganese iron phosphate based on manganese iron ammonium phosphate in the present application;

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

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

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

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

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

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

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

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

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

[0033] Figure 13 A cycle performance graph of the carbon-coated lithium manganese iron phosphate prepared in Embodiment 2 of the present application at 1C rate is given;

[0034] Figure 14 A charge-discharge curve graph of the carbon-coated lithium manganese iron phosphate prepared in Embodiment 3 of the present application at 0.1C rate is given;

[0035] Figure 15 A charge-discharge curve graph of the carbon-coated lithium manganese iron phosphate prepared in Embodiment 3 of the present application at 1C rate is given;

[0036] Figure 16 A rate performance graph of the carbon-coated lithium manganese iron phosphate prepared in Embodiment 3 of the present application is given;

[0037] Figure 17 A cycle performance graph of the carbon-coated lithium manganese iron phosphate prepared in Embodiment 3 of the present application at 1C rate is given;

[0038] Figure 18 A charge-discharge curve graph of the carbon-coated lithium manganese iron phosphate prepared in Comparative Example 1 of the present application at 0.1C rate is given;

[0039] Figure 19 A charge-discharge curve graph of the carbon-coated lithium manganese iron phosphate prepared in Comparative Example 1 of the present application at 1C rate is given;

[0040] Figure 20 A rate performance graph of the carbon-coated lithium manganese iron phosphate prepared in Comparative Example 1 of the present application is given;

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

[0042] "ranges" disclosed herein are defined by both a lower and an upper limit, and the particular range is defined by selecting a lower limit and an upper limit that defines the boundaries of the particular range. Ranges defined by the combination of any lower and upper limit are expressly intended to be encompassed by the disclosure. For example, if the ranges 60-120 and 80-110 are listed, it is intended that the ranges 60-110 and 80-120 are also expressly listed. Moreover, if the minimum range values 1 and 2 are listed, and if the maximum range values 3, 4, and 5 are listed, then the following ranges are all expressly intended: 1-3, 1-4, 1-5, 2-3, 2-4, and 2-5. In the present application, unless otherwise stated, the numerical ranges "a-b" are intended to mean a shorthand way of describing each and every individual integer between the lower limit a and the upper limit b, wherein a and b are both integers. For example, the numerical range "0-5" is intended to mean that all the individual integers between 0 and 5, i.e. 0, 1, 2, 3, 4, and 5 are expressly stated herein. Additionally, when a parameter is stated to be an integer > 2, it is equivalent to disclose that the parameter is an integer 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, etc.

[0043] Unless otherwise indicated, the terms "including", "includes" and "contain" and variations thereof in the present application are intended to be open-ended and also to encompass the other components not listed. For example, the terms "including", "includes" and "contain" can mean that the listed components are included, but other components are also included or can mean that only the listed components are included.

[0044] Reference Figure 1 As shown, the embodiments of the present application provide a method for preparing lithium manganese iron phosphate based on manganese iron ammonium phosphate, which comprises the following steps:

[0045] S100, a manganese iron mixed solution, a phosphate solution and a precipitant solution are prepared, wherein the manganese salt in the manganese iron mixed solution is one or more of manganese sulfate monohydrate, manganese chloride tetrahydrate or manganese acetate tetrahydrate, 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; 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, the prepared phosphate solution is placed in a reaction container, and the prepared manganese-iron mixed solution and the precipitant solution are added into the reaction container under stirring, the pH of the solution in the reaction container is controlled to be constant, after the manganese-iron mixed solution is dropped completely, the manganese-iron ammonium phosphate suspension is obtained after stirring and aging;

[0048] In this step, the reaction container is a reaction kettle, the stirring intensity is 200-1000 r / min, the pH 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℃, the feeding speed of the manganese-iron mixed solution is 20-200 ml / min, the stirring time after the manganese-iron mixed solution is dropped completely is 1-5 h, and the aging time is 4-20 h.

[0049] S300, the obtained manganese-iron ammonium phosphate suspension is subjected to solid-liquid separation, the upper clear liquid is taken and repeatedly washed with deionized water and anhydrous ethanol until the conductivity of the upper clear liquid decreases to below 500 μS / cm, and then the upper clear liquid is dried under vacuum to obtain manganese-iron ammonium phosphate monohydrate precursor;

[0050] In this step, the drying temperature under vacuum is 60-100℃, the drying time is 12-36 h, the vacuum pressure is -0.8 MPa to -1 MPa, and the molecular formula of the manganese-iron ammonium phosphate monohydrate precursor after drying is NH4Mn x Fe 1-x PO4·H2O, wherein the atomic ratio (Mn+Fe) / P in the manganese-iron ammonium phosphate monohydrate precursor is 0.95-1.05, and the specific surface area is 15-25 m 2 / g.

[0051] S400, the obtained manganese-iron ammonium phosphate monohydrate precursor and a lithium salt solution are poured into a polytetrafluoroethylene reaction container and heated to react, and a manganese-iron lithium phosphate suspension is obtained, 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 reaction kettle, the ratio of lithium ions to iron and manganese ions in the lithium salt is 1-1.5:1, the reaction temperature after heating of the reaction container is 180-220℃, and the reaction time is 16-24 h.

[0052] S500, the obtained manganese-iron lithium phosphate suspension is subjected to solid-liquid separation, the upper clear liquid is removed, and the solid is repeatedly washed with deionized water until the conductivity of the washing liquid decreases to below 500 μS / cm, and then the solid is washed with anhydrous ethanol and vacuum dried, and the manganese-iron lithium phosphate is obtained, wherein the drying temperature under vacuum is 60-100℃, the drying time is 12-36 h, and the vacuum pressure is -0.8 MPa to -1 MPa.

[0053] In the above embodiment, firstly, the ammonium manganese iron phosphate matrix is prepared by the coprecipitation method; then, the ammonium manganese iron phosphate matrix obtained is reacted with a lithium salt solution to obtain lithium manganese iron phosphate, and in the subsequent solvothermal 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, makes the lithium salt fully utilized, and thus improves the production efficiency and reduces the production cost. Therefore, the method for preparing lithium manganese iron phosphate with ammonium manganese iron phosphate as the matrix provided in the application combines the advantages of the coprecipitation method and the solvothermal method, provides a theoretical basis for the solvothermal industrial production of lithium manganese iron phosphate process route, and the method has a simple process, the prepared lithium manganese iron phosphate has high crystallinity, a pure phase can be obtained without sintering, and is easy to store and transport.

[0054] In some embodiments, the step S600 of dispersing and mixing the obtained lithium manganese iron phosphate with a carbon source is further included, and then the mixed material is sintered 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, polyvinylpyrrolidone and pitch, the sintering temperature is 600-800°C, the sintering time is 4-8h, and the inert atmosphere is one or more of a nitrogen atmosphere, an argon atmosphere or a helium atmosphere.

[0056] In order to further illustrate the technical principles and technical effects of the application, the following embodiments more specifically describe the disclosed content of the application, which are only used for illustrative description, and various modifications and changes within the scope of the disclosed content of the application are obvious to those skilled in the art. Unless otherwise stated, all reagents and raw materials used in the embodiments are commercially available or synthesized according to conventional methods, and the instruments and equipment used in the embodiments are commercially available.

[0057] Embodiment 1

[0058] 1. 25.88 g (0.225 mol) of ammonium dihydrogen phosphate was dissolved in 150 ml of deionized water, and was transferred into a reaction kettle for stirring at 400 r / min, the temperature was adjusted to 40°C, and then 13.38 mol / L concentrated ammonia water was pumped into the reaction kettle until the pH was 6;

[0059] 2. 29.19 g (0.105 mol) of ferrous sulfate heptahydrate and 7.61 g (0.045 mol) of manganese sulfate monohydrate were dissolved in 150 ml of deionized water, and were pumped into the reaction kettle at a speed of 30 ml / min, and the rotation speed of the reaction kettle was adjusted to 600 r / min, while the concentrated ammonia water was pumped in parallel to control the pH to be constant at 6, after the metal ion mixture was dropped, it was stirred for 3 h, and aged for 12 h to obtain an ammonium manganese iron phosphate suspension;

[0060] 3, the obtained manganese iron ammonium phosphate suspension is placed in a centrifuge and washed with deionized water at a speed of 4000 r / min for 8 min, and then washed twice with anhydrous ethanol after the conductivity of the washing solution is reduced to 500 μS / cm, and then transferred into a vacuum drying oven at 100 ℃ for drying for 24 h under a drying pressure of -0.8 MPa, to obtain high-purity manganese iron ammonium phosphate monohydrate powder with a molecular formula of NH4Mn 0.3 Fe 0.7 PO4·H2O;

[0061] 4, 4.66 g of manganese iron ammonium phosphate monohydrate is ultrasonically dispersed in 60 ml of ethylene glycol and poured into a 100 ml solvent thermal reaction kettle, 1.07 g of LiOH·H2O is dissolved in 20 ml of deionized water and then dropped into the reaction kettle, and the reaction kettle is placed in a forced air drying oven at 180 ℃ for 20 h, to obtain a manganese iron lithium phosphate suspension;

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

[0063] 6, 0.3 g of glucose is dissolved in 20 ml of anhydrous ethanol, and the obtained manganese iron lithium phosphate powder is ultrasonically dispersed for 10 min, stirred in an oil bath at 80 ℃ until the ethanol is completely volatilized, and finally transferred into a vacuum drying oven for drying at 100 ℃ for 12 h under a drying pressure of -1 MPa;

[0064] 7, the dried material is placed in a tube furnace and heated at 350 ℃ for 5 h and at 700 ℃ for 4 h to obtain a carbon-coated manganese iron lithium phosphate material with a molecular formula of LiMn 0.6 Fe 0.4 PO4 / C, wherein the heating rate in the tube furnace is 5 ℃ / min and the atmosphere is argon.

[0065] Example 2

[0066] 1, 25.88 g (0.225 mol) of ammonium dihydrogen phosphate is dissolved in 150 ml of deionized water, and then transferred into a reaction kettle and stirred at 400 r / min, and the temperature is adjusted to 40 ℃, and then concentrated ammonia solution with a concentration of 13.38 mol / L is pumped into the reaction kettle 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, dissolve them in 150 ml of deionized water, pump them into the reactor at a speed of 200 ml / min, adjust the rotation speed of the reactor to 600 r / min, pump concentrated ammonia water in parallel to control the pH to be constant at 6, after the metal ion mixture is dropped, stir for 3 h, and age for 12 h to obtain a manganese iron ammonium phosphate suspension;

[0068] 3. The obtained manganese iron ammonium phosphate suspension is placed in a centrifuge and washed with deionized water at a rotation speed of 4000 r / min for 8 min, after the conductivity of the washing liquid is reduced to 500 μS / cm, it is washed twice with anhydrous ethanol, and then transferred into a vacuum drying oven at 100°C for drying for 24 h, and the drying pressure is -1 MPa, to obtain a high-purity manganese iron ammonium phosphate monohydrate powder with a molecular formula of NH4Mn 0.5 Fe 0.5 PO4·H2O;

[0069] 4. Ultrasonically disperse 4.66 g of manganese iron ammonium phosphate monohydrate in 60 ml of ethylene glycol and pour it into a 100 ml solvent thermal reactor, dissolve 1.07 g of LiOH·H2O in 20 ml of deionized water and drop it into the reactor, and place the reactor in a blast drying oven at 180°C for 20 h to obtain a manganese lithium phosphate suspension;

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

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

[0072] 7. Place the dried material in a tube furnace, and heat at 350°C for 5 h and at 700°C for 4 h to obtain a carbon-coated manganese lithium phosphate material with a molecular formula of LiMn 0.6 Fe 0.4 PO4 / C, wherein the heating rate in the tube furnace is 5°C / min, and the atmosphere is argon.

[0073] Example 3

[0074] 1. Weigh 25.88 g (0.225 mol) of ammonium dihydrogen phosphate into 150 ml of deionized water, and transfer it into a reaction kettle to be stirred at 400 r / min, adjust the temperature to 40℃, pump 13.38 mol / L concentrated ammonia water into the reaction kettle to control the pH value to be 6;

[0075] 2. Weigh 16.68 g (0.06 mol) of ferrous sulfate heptahydrate and 15.21 g (0.09 mol) of manganese sulfate monohydrate into 150 ml of deionized water, pump it into the reaction kettle at a speed of 100 ml / min, and adjust the rotation speed of the reaction kettle to 600 r / min. At the same time, pump in concentrated ammonia water in parallel to control the pH value to be 6, and after the metal ion mixture is dropped, stir for 3 h, and age for 12 h to obtain a manganese iron ammonium phosphate suspension;

[0076] 3. The obtained manganese iron ammonium phosphate suspension is placed in a centrifuge and washed with deionized water at a rotation speed of 4000 r / min for 8 min, and after the conductivity of the washing liquid is reduced to 500 μS / cm, it is washed twice with anhydrous ethanol, and then transferred into a vacuum drying oven at 100℃ for drying for 24 h, and the drying pressure is -0.8 MPa, to obtain a high-purity manganese iron ammonium phosphate monohydrate powder with a molecular formula of NH4Mn 0.6 Fe 0.4 PO4·H2O;

[0077] 4. Ultrasonically disperse 4.66 g of manganese iron ammonium phosphate monohydrate in 60 ml of ethylene glycol and pour it into a 100 ml solvent thermal reaction kettle, dissolve 1.07 g of LiOH·H2O in 20 ml of deionized water and drop it into the reaction kettle, and place the reaction kettle in a blast drying oven at 180℃ for 20 h to obtain a manganese iron lithium phosphate suspension;

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

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

[0080] 7. Place the dried material into a tube furnace, and heat it at 350℃ for 5 h and at 700℃ for 4 h to obtain a carbon-coated manganese iron lithium phosphate material with a molecular formula of LiMn 0.6 Fe 0.4 PO4 / C, wherein the heating rate in the tube furnace is 5℃ / min, and the atmosphere is argon.

[0081] Comparative Example 1

[0082] 1. Weigh 25.88 g (0.225 mol) of ammonium dihydrogen phosphate into 150 ml of deionized water, and transfer it into a reaction kettle to be stirred at 400 r / min, adjust the temperature to 40°C, then pump concentrated ammonia solution of 13.38 mol / L into the reaction kettle until the pH is 6;

[0083] 2. Weigh 16.68 g (0.06 mol) of ferrous sulfate heptahydrate and 15.21 g (0.09 mol) of manganese sulfate monohydrate into 150 ml of deionized water, pump it into the reaction kettle at a speed of 80 ml / min, and adjust the rotation speed of the reaction kettle to 600 r / min. At the same time, pump in concentrated ammonia solution to control the pH to be constant at 6, and after the metal ion mixture is dropped, stir for 3 h, and age for 12 h to obtain a manganese iron ammonium phosphate suspension;

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

[0085] 4. 4.66 g of manganese iron ammonium phosphate monohydrate is ultrasonically dispersed in 60 ml of ethylene glycol and poured into a 100 ml solvent thermal reaction kettle, 1.605 g of LiOH·H2O is dissolved in 20 ml of deionized water and then dropped into the reaction kettle, and the reaction kettle is placed in a blast drying oven at 180°C for 20 h to obtain a manganese iron lithium phosphate suspension;

[0086] 5. The prepared manganese iron lithium phosphate suspension is subjected to solid-liquid separation, washed repeatedly with deionized water until the conductivity of the washing liquid is reduced to below 500 μS / cm, then washed twice with anhydrous ethanol and dried to obtain a manganese iron lithium phosphate powder, the molecular formula of which is NH4Mn 0.6 Fe 0.4 PO4·H2O;

[0087] 6. 0.3 g of glucose is dissolved in 20 ml of anhydrous ethanol, and the obtained manganese iron lithium phosphate powder is added and ultrasonically treated for 10 min, stirred in an oil bath at 80°C until the ethanol is completely volatilized, and finally transferred into a vacuum drying oven for drying at 100°C for 12 h, and the drying pressure is -0.8 MPa;

[0088] 7. The dried material is placed in a tube furnace and heat treated at 350°C for 5 h and at 700°C for 4 h to obtain a carbon-coated manganese iron lithium phosphate material, the molecular formula of which is LiMn 0.6 Fe 0.4PO4 / C, wherein the heating rate in the tube furnace is 5℃ / min, and the atmosphere is argon.

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

[0090] From Figures 2-3 it can be concluded that the manganese iron ammonium phosphate prepared by the coprecipitation method and the subsequent solvent thermal preparation of manganese iron lithium phosphate are both free of impurities and have good crystallinity. From Figures 4-5 it can be observed that the manganese iron ammonium phosphate exhibits excellent dispersibility in two-dimensional sheet form, and the grain growth becomes thicker after the solvent thermal reaction.

[0091] The carbon-coated manganese iron lithium phosphate materials prepared in Examples 1-3 and Comparative Example 1 were subjected to rate performance and charge-discharge performance tests, and the test results are shown in Table 1 and Figures 6-21 .

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

[0093] Example 0.1 C specific capacity (mAh / g) 1 C specific capacity (mAh / g) 1 C cycle 100 cycle capacity retention (%) 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 Figures 6-21 , it can be seen that the carbon-coated manganese iron lithium phosphate materials prepared in Examples 1-3 all have excellent rate cycle performance at 0.1C and 1C rates, and the manganese-iron ratio is continuously adjustable, wherein 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, and 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, with a cycle retention rate of 96.1%. The carbon-coated manganese iron lithium 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 a cycle retention rate of only 81.7%. Therefore, the manganese iron lithium phosphate prepared by the method provided in the present application, which uses manganese iron ammonium phosphate as the base, has the characteristics of high purity and good crystallinity, and excellent electrochemical performance; it combines the advantages of the coprecipitation method and the solvent thermal method, and provides a theoretical basis for the solvent thermal industrial production of manganese iron lithium phosphate process routes.

[0095] The above describes in detail the method for preparing lithium manganese iron phosphate with manganese iron ammonium phosphate as a base provided by the present application. The principles and implementation manners of the present application are described by using specific examples, and the above description of the examples is only used to help understand the core idea of the present application. It should be pointed out that, for those skilled in the art, some improvements and modifications can be made to the present application without departing from the principles of the present application, and these improvements and modifications also fall within the protection scope 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 vessel, adding the prepared ferromanganese mixed solution and the precipitant solution to the reaction vessel in parallel under stirring, controlling the pH of the solution in the reaction vessel to be constant, and after the ferromanganese mixed solution is completely dripped, stirring and aging to obtain an ammonium ferromanganese phosphate suspension; S300, performing solid-liquid separation on the obtained ammonium manganese ferric phosphate suspension, removing the supernatant, 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 ferric 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, thereby obtaining 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, performing solid-liquid separation on the obtained lithium manganese iron phosphate suspension, removing the supernatant, and repeatedly washing the solid with deionized water until the conductivity of the washing liquid drops below 500 μS / cm, 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 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 step S200, the stirring intensity is 200-1000 r / 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 manganese-iron mixed solution is 20-200 ml / min, the stirring time after the manganese-iron mixed solution is completely dripped is 1-5 hours, and the aging time is 4-20 hours.

4. The method according to claim 1, wherein In step S300, 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. 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 manganese iron ammonium phosphate monohydrate precursor is 0.95-1.05, and the specific surface area is 15-25m 2 / g.

5. The method according to claim 1, wherein 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.

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 of 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.

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