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

Abstract

The method comprises the following steps: preparing an ammonium manganese ferric phosphate monohydrate precursor from a manganese source, an iron source, a doped ion source and a phosphorus source through a reverse addition coprecipitation method, then uniformly mixing the ammonium manganese ferric phosphate monohydrate precursor with a lithium salt and a carbon source, and carrying out sanding, spray drying and sintering to obtain the modified lithium manganese ferric phosphate. Further, the carbon-coated modified lithium manganese iron phosphate with good crystallinity is obtained. According to the present invention, the generation of the impure phase can be substantially reduced by using the inverse addition method so as to improve the purity of the ammonium manganese iron phosphate precursor, and the metal ion in-situ doping can make the element distribution more uniform and the doping effect better so as to effectively stabilize the crystal structure of the lithium manganese iron phosphate and improve the electrochemical performance.

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 modified lithium manganese iron phosphate using ammonium manganese iron phosphate as a matrix. Background Art

[0002] Lithium iron phosphate has become the mainstream positive electrode material in the energy field due to its stable structure, low cost, and environmental friendliness. However, its low voltage platform (3.4V) can no longer meet the market demand for energy density. Lithium iron manganese phosphate inherits its advantages and greatly improves the energy density. It is the next generation product of lithium iron phosphate and will surely occupy a large market share in the future.

[0003] However, due to the 3+ Disproportionation reactions are prone to occur in aqueous solutions, and it is impossible to directly use the traditional co-precipitation production route to prepare trivalent manganese iron phosphate, and then prepare lithium iron manganese phosphate. The use of organic solvents will increase the risk of preparation and production costs. The solid-phase method is simple, but it cannot achieve uniform distribution of elements, and the electrochemical performance of the prepared lithium iron manganese phosphate is poor. The solvent thermal method, which has been studied more, is limited by high equipment requirements and the solubility of many raw materials. The preparation scale is small and the lithium source needs to be excessive, which is not suitable for industrial production. Therefore, it is necessary to develop a route that can use traditional lithium iron phosphate equipment to prepare lithium iron manganese phosphate, realize its large-scale production, and accelerate the industrialization process of lithium iron manganese phosphate.

[0004] In addition to the difficulty in preparing lithium manganese iron phosphate cathode materials, their low intrinsic electronic conductivity and Mn 3+ The Jahn-Teller effect leads to an unstable crystal structure, which in turn causes a significant attenuation of rate performance and cycle stability, which limits the development of lithium manganese iron phosphate with a high Mn atomic ratio. Summary of the invention

[0005] In view of the technical defects existing in the existing lithium iron manganese phosphate preparation method, the present application provides a method for preparing modified lithium iron manganese phosphate using ammonium iron manganese phosphate as a matrix, the method improves the purity of the ammonium iron manganese phosphate precursor by a reverse addition coprecipitation method, and at the same time introduces divalent doping metal ions in the coprecipitation reaction stage to achieve atomic-level distribution of metal ions, thereby effectively improving the energy density and cycle stability of lithium iron manganese phosphate. The method comprises the following steps:

[0006] S100, preparing a metal ion mixed solution, a phosphate solution and a precipitant solution, wherein the metal ion mixed solution contains a manganese salt, an iron salt and a doping salt, the manganese salt 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 doping salt is one or more of nickel sulfate, nickel chloride, cobalt sulfate, cobalt chloride, magnesium sulfate and magnesium chloride; 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;

[0007] S200, placing the prepared phosphate solution in a reaction container, adding a precipitant under stirring conditions and controlling the temperature and pH value of the solution in the reaction container to be constant, then adding the prepared metal ion mixed solution and the precipitant solution to the reaction container in parallel, maintaining the pH value of the solution constant until the solution is dripped away, and then stirring and aging to obtain an ammonium manganese iron phosphate suspension;

[0008] S300, separating the obtained ammonium manganese iron phosphate suspension from 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 500μS / cm, and then drying it under vacuum conditions to obtain a doped ammonium manganese iron phosphate monohydrate precursor; S400, mixing the obtained doped ammonium manganese iron phosphate monohydrate precursor with a lithium salt and a carbon source and frosting the mixture to obtain a uniformly mixed lithium manganese iron phosphate suspension; S500, spray drying the obtained lithium manganese iron phosphate suspension to obtain a dry spherical powder, and then sintering the dry spherical powder under an inert atmosphere to obtain modified lithium manganese iron phosphate.

[0009] In some embodiments, in step S100, the total concentration of manganese salt, iron salt and doping salt in the metal ion mixed solution is 0.5-2 mol / L, the concentration of phosphate solution is 0.5-4 mol / L, and the concentration of precipitant is 1-15 mol / L.

[0010] 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.

[0011] 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 NH4Mn x Fey M 1-x-y PO4·H2O, where the atomic ratio (Mn+Fe+M) / P is 0.95-1.05 and the specific surface area is 15-25m 2 In some embodiments, the lithium salt is one or more of lithium carbonate, lithium hydroxide, lithium oxalate, and lithium citrate.

[0012] In some embodiments, the carbon source is one or more of glucose, sucrose, phenolic epoxy resin, polyvinyl pyrrolidone, and asphalt.

[0013] In some embodiments, in step S500, the sintering temperature is 600-800° C., and the sintering time is 4-8 hours.

[0014] In some embodiments, the inert atmosphere is one or more of a nitrogen atmosphere, an argon atmosphere, or a helium atmosphere.

[0015] Compared with the prior art, the present application prepares an ammonium manganese iron phosphate monohydrate precursor by mixing a manganese source, an iron source, a doping ion source and a phosphorus source through a reverse addition coprecipitation method. The reverse addition method can greatly reduce the formation of impurities, thereby improving the purity of the ammonium manganese iron phosphate precursor. At the same time, divalent doping metal ions are introduced in the coprecipitation reaction stage to achieve atomic-level distribution of metal ions, effectively improving the energy density and cycle stability of lithium manganese iron phosphate. The prepared ammonium manganese iron phosphate monohydrate precursor is then evenly mixed with a lithium salt and a carbon source, and then sand milled, spray dried and sintered to finally obtain a carbon-coated modified lithium manganese iron phosphate with good crystallinity and excellent electrochemical properties. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] 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.

[0017] Figure 1 This is a flow chart of a method for preparing modified lithium manganese iron phosphate using ammonium manganese iron phosphate as a matrix in the present application;

[0018] Figure 2 The scanning electron microscope image of the modified lithium manganese iron phosphate prepared in Example 1 of the present application is given;

[0019] Figure 3 The charge and discharge curve of the modified lithium manganese iron phosphate prepared in Example 1 of the present application at a rate of 0.1C is given;

[0020] Figure 4The charge and discharge curve of the modified lithium manganese iron phosphate prepared in Example 1 of the present application at a 1C rate is given;

[0021] Figure 5 The rate performance diagram of the modified lithium manganese iron phosphate prepared in Example 1 of the present application is given;

[0022] Figure 6 The cycle performance diagram of the modified lithium manganese iron phosphate prepared in Example 1 of the present application at a 1C rate is given;

[0023] Figure 7 The scanning electron microscope image of the modified lithium iron manganese phosphate prepared in Example 2 of the present application is given;

[0024] Figure 8 The charge and discharge curve of the modified lithium manganese iron phosphate prepared in Example 2 of the present application at a rate of 0.1C is given;

[0025] Fig. 9 The charge and discharge curve of the modified lithium manganese iron phosphate prepared in Example 2 of the present application at a 1C rate is given;

[0026] Fig.10 The rate performance diagram of the modified lithium manganese iron phosphate prepared in Example 2 of the present application is given;

[0027] Fig.11 The cycle performance diagram of the modified lithium manganese iron phosphate prepared in Example 2 of the present application at a 1C rate is given;

[0028] Fig.12 The scanning electron microscope image of the modified lithium iron manganese phosphate prepared in Example 3 of the present application is given;

[0029] Fig.13 The charge and discharge curve of the modified lithium manganese iron phosphate prepared in Example 3 of the present application at a rate of 0.1C is given;

[0030] Fig.14 The charge and discharge curve of the modified lithium manganese iron phosphate prepared in Example 3 of the present application at a 1C rate is given;

[0031] Fig.15 The rate performance diagram of the modified lithium manganese iron phosphate prepared in Example 3 of the present application is given;

[0032] Fig.16 The cycle performance diagram of the modified lithium manganese iron phosphate prepared in Example 3 of the present application at a 1C rate is given;

[0033] Fig.17 The scanning electron microscope image of the lithium manganese iron phosphate prepared in Comparative Example 1 of the present application is given;

[0034] Fig.18The charge and discharge curve of the lithium manganese iron phosphate prepared in Comparative Example 1 of the present application at a rate of 0.1C is given;

[0035] Fig.19 The charge and discharge curve of the lithium manganese iron phosphate prepared in Comparative Example 1 of the present application at a 1C rate is given;

[0036] Fig. 20 The rate performance diagram of lithium manganese iron phosphate prepared in Comparative Example 1 of the present application is given;

[0037] Fig.21 A cycle performance diagram of the lithium manganese iron phosphate prepared in Comparative Example 1 of the present application at a 1C rate is given. DETAILED DESCRIPTION

[0038] "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.

[0039] 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.

[0040] refer to Figure 1 As shown, the embodiment of the present application provides a method for preparing modified lithium manganese iron phosphate using ammonium manganese iron phosphate as a matrix, and the method comprises the following steps:

[0041] S100, preparing a metal ion mixed solution, a phosphate solution and a precipitant solution, wherein the metal ion mixed solution contains a manganese salt, an iron salt and a doping salt, the manganese salt 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 doping salt is one or more of nickel sulfate, nickel chloride, cobalt sulfate, cobalt chloride, magnesium sulfate and magnesium chloride; 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;

[0042] In this step, the total concentration of manganese salt, iron salt and doping salt in the metal ion mixed solution is 0.5-2 mol / L, the concentration of phosphate solution is 0.5-4 mol / L, and the concentration of precipitant is 1-15 mol / L.

[0043] S200, placing the prepared phosphate solution in a reaction container, adding a precipitant under stirring conditions and controlling the temperature and pH value of the solution in the reaction container to be constant, then adding the prepared metal ion mixed solution and the precipitant solution to the reaction container in parallel, maintaining the pH value of the solution constant until the solution is dripped away, and then stirring and aging to obtain an ammonium manganese iron phosphate suspension;

[0044] 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.

[0045] 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 a doped ammonium manganese iron phosphate monohydrate precursor;

[0046] 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, and the molecular formula of the ammonium manganese iron phosphate monohydrate precursor after drying is NH4Mn x Fe y M 1-x-y PO4·H2O, and the morphology of the dried ammonium manganese iron phosphate precursor is a uniform two-dimensional sheet, wherein the atomic ratio (Mn+Fe+M) / P is 0.95-1.05, and the specific surface area is 15-25m 2 / g.

[0047] S400, mixing the obtained doped ammonium manganese iron phosphate monohydrate precursor with a lithium salt and a carbon source and performing a frosting treatment to obtain a uniformly mixed lithium manganese iron phosphate suspension;

[0048] In this step, the lithium salt is one or more of lithium carbonate, lithium hydroxide, lithium oxalate, and lithium citrate, and the carbon source is one or more of glucose, sucrose, phenolic epoxy resin, polyvinyl pyrrolidone, and asphalt.

[0049] S500, spray drying the obtained lithium manganese iron phosphate suspension to obtain dry spherical powder, and then sintering the dry spherical powder under an inert atmosphere to obtain modified lithium manganese iron phosphate.

[0050] In this step, the lithium manganese iron phosphate suspension is dried under vacuum conditions, the drying temperature is 60-100°C, the drying time is 12-36h, and the vacuum pressure is -0.8MPa to -1MPa; 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.

[0051] In the above embodiment, the manganese source, iron source, doping ion source and phosphorus source are mixed and reacted to prepare the ammonium manganese iron phosphate monohydrate precursor by the reverse addition coprecipitation method. The reverse addition method can greatly reduce the generation of impurities, thereby effectively improving the purity of the ammonium manganese iron phosphate precursor. At the same time, divalent doped metal ions are introduced in the coprecipitation reaction stage to achieve the atomic-level distribution of metal ions, effectively improving the energy density and cycle stability of lithium manganese iron phosphate; then the prepared ammonium manganese iron phosphate monohydrate precursor is mixed with lithium salt and carbon source, and then sand milled, spray dried and sintered to obtain carbon-coated modified lithium manganese iron phosphate with good crystallinity. The method is simple in process, and the prepared lithium manganese iron phosphate has good rate performance.

[0052] 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.

[0053] Example 1

[0054] 1. Weigh 172.55g (1.5mol) of ammonium dihydrogen phosphate and dissolve it in 1000ml of deionized water, transfer it into a reactor and stir it at a speed of 600r / min, pump 13.38mol / L of concentrated ammonia water into the reactor and control the pH of the solution in the reactor to 6, and adjust the solution temperature to 40°C;

[0055] 2. Weigh 105.64g (0.38mol) of ferrous sulfate heptahydrate, 101.412g (0.6mol) of manganese sulfate monohydrate and 5.26g (0.02mol) of nickel sulfate hexahydrate and dissolve them in 1000ml of deionized water, pump them into the reactor at a speed of 40ml / min, and adjust the stirring speed of the reactor to 500r / min. At the same time, pump concentrated ammonia water to control the pH value of the solution to be constant at 6. After the metal ion mixture is dripped, stir for 3h and age for 12h to obtain ammonium manganese ferric phosphate suspension;

[0056] 3. The obtained ammonium manganese ferric phosphate suspension is placed in a centrifuge for solid-liquid separation, and the washing liquid is repeatedly washed with deionized water and anhydrous ethanol to reduce the conductivity of the washing liquid to below 500 μS / cm, and then dried under vacuum conditions to obtain a nickel-doped ammonium manganese ferric phosphate monohydrate precursor with a molecular formula of NH4Mn 0.6 Fe 0.38 Ni 0.02 PO4·H2O;

[0057] 4. Weigh 1.57 g of sucrose and dissolve it in 100 ml of deionized water, then add 18.63 g (0.1 mol) of dried nickel-doped ammonium manganese iron phosphate monohydrate precursor and 3.79 g (0.105 mol) of lithium carbonate and stir and disperse them evenly, then transfer them to a sand mill and sand grind them until the particle size is less than 400 nm to obtain a uniformly mixed lithium manganese iron phosphate suspension, wherein the sand mill speed is 2000 r / min and the sand grinding time is 30 min;

[0058] 5. The sand-milled lithium iron manganese phosphate suspension is spray dried. The feed rate of the spray dryer is 650 ml / h and the inlet temperature is 220 ° C. The dried powder is then placed in a tube furnace and heated to 350 ° C at a rate of 5 ° C / min for 5 hours, and then heated to 700 ° C for 4 hours to obtain nickel-doped modified lithium iron manganese phosphate, whose molecular formula is LiMn 0.6 Fe 0.38 Ni 0.02 PO4 / C.

[0059] Example 2

[0060] 1. Weigh 172.55g (1.5mol) of ammonium dihydrogen phosphate and dissolve it in 1000ml of deionized water, transfer it into a reactor and stir it at a speed of 600r / min, pump 13.38mol / L of concentrated ammonia water into the reactor and control the pH of the solution in the reactor to 6, and adjust the solution temperature to 60°C;

[0061] 2. Weigh 105.64 g (0.38 mol) of ferrous sulfate heptahydrate, 101.412 g (0.6 mol) of manganese sulfate monohydrate and 5.62 g (0.02 mol) of cobalt sulfate heptahydrate and dissolve them in 1000 ml of deionized water, pump them into the reactor at a speed of 80 ml / min, and adjust the speed of the reactor to 1000 r / min. At the same time, pump concentrated ammonia water to control the pH value of the solution 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;

[0062] 3. The ammonium manganese ferric phosphate suspension is placed in a centrifuge for solid-liquid separation, and the washing liquid is repeatedly washed with deionized water and anhydrous ethanol to reduce the conductivity of the washing liquid to below 500 μS / cm. After drying under vacuum conditions, a cobalt-doped ammonium manganese ferric phosphate monohydrate precursor is obtained, and its molecular formula is NH4Mn 0.6 Fe 0.38 Co 0.02 PO4 / C·H2O;

[0063] 4. Weigh 1.57 g of sucrose and dissolve it in 100 ml of deionized water, then add 18.63 g (0.1 mol) of dried cobalt-doped ammonium manganese iron phosphate monohydrate precursor and 3.79 g of lithium carbonate, stir and disperse them evenly, and then transfer them to a sand mill and sand grind them until the particle size is less than 400 nm to obtain a uniformly mixed lithium manganese iron phosphate suspension, wherein the sand mill speed is 2000 r / min and the sand grinding time is 30 min;

[0064] 5. The sand-milled lithium iron manganese phosphate suspension is spray dried. The feed rate of the spray dryer is 650 ml / h and the inlet temperature is 220 ° C. The dried powder is then placed in a tube furnace and heated to 350 ° C at a rate of 5 ° C / min for 5 hours, and then heated to 800 ° C for 4 hours to obtain cobalt-doped modified lithium iron manganese phosphate, whose molecular formula is LiMn 0.6 Fe 0.38 Co 0.02 PO4 / C.

[0065] Example 3

[0066] 1. Weigh 172.55g (1.5mol) of ammonium dihydrogen phosphate and dissolve it in 1000ml of deionized water, transfer it into a reactor and stir it at a speed of 600r / min, pump 13.38mol / L of concentrated ammonia water into the reactor to control the pH of the solution in the reactor to 6, and adjust the solution temperature to 40°C;

[0067] 2. Weigh 105.64g (0.38mol) of ferrous sulfate heptahydrate, 101.412g (0.6mol) of manganese sulfate monohydrate and 2.41g (0.02mol) of anhydrous magnesium sulfate and dissolve them in 1000ml of deionized water, pump them into the reactor at a speed of 100ml / min, and adjust the speed of the reactor to 800r / min. At the same time, pump concentrated ammonia water to control the pH value of the solution to be constant at 6. After the metal ion mixture is dripped, stir for 3h and age for 12h to obtain ammonium manganese ferrous phosphate suspension;

[0068] 3. The ammonium manganese ferric phosphate suspension is placed in a centrifuge for solid-liquid separation, and the washing liquid is repeatedly washed with deionized water and anhydrous ethanol to reduce the conductivity of the washing liquid to below 500 μS / cm. After drying under vacuum conditions, the magnesium-doped ammonium manganese ferric phosphate monohydrate precursor is obtained, and its molecular formula is NH4Mn 0.6 Fe 0.38 Mg 0.02 PO4·H2O;

[0069] 4. Weigh 1.57 g of sucrose and dissolve it in 100 ml of deionized water, then add 18.63 g (0.1 mol) of dried magnesium-doped ammonium manganese iron phosphate monohydrate precursor and 3.79 g of lithium carbonate and stir and disperse them evenly, then transfer them to a sand mill and sand grind them until the particle size is less than 400 nm to obtain a uniformly mixed lithium manganese iron phosphate suspension, wherein the sand mill speed is 2000 r / min and the sand grinding time is 30 min;

[0070] 5. The sand-milled lithium iron manganese phosphate suspension is spray dried. The feed rate of the spray dryer is 650 ml / h and the inlet temperature is 220 ° C. The dried powder is then placed in a tube furnace and heated to 350 ° C at a rate of 5 ° C / min for 5 hours, and then heated to 500 ° C for 4 hours to obtain magnesium-doped modified lithium iron manganese phosphate, whose molecular formula is LiMn 0.6 Fe 0.38 Mg 0.02 PO4 / C.

[0071] Comparative Example 1

[0072] 1. Weigh 172.55g (1.5mol) of ammonium dihydrogen phosphate and dissolve it in 1000ml of deionized water, transfer it into a reactor and stir it at a speed of 600r / min, pump 13.38mol / L of concentrated ammonia water into the reactor and control the pH of the solution in the reactor to 6, and adjust the solution temperature to 40°C;

[0073] 2. Weigh 111.20 g (0.4 mol) of ferrous sulfate heptahydrate and 101.412 g (0.6 mol) of manganese sulfate monohydrate, dissolve them in 1000 ml of deionized water, and pump them into the reactor at a speed of 40 ml / min. Adjust the stirring speed of the reactor to 500 r / min, and simultaneously pump concentrated ammonia water to control the pH value to be constant at 6. After the metal ion mixture is dripped, stir for 3 h and age for 12 h to obtain ammonium manganese ferric phosphate suspension;

[0074] 3. Place the precursor suspension in a centrifuge for solid-liquid separation, wash repeatedly with deionized water and anhydrous ethanol until the conductivity of the washing liquid drops below 500 μS / cm, and then dry under vacuum to obtain ammonium manganese ferrous phosphate monohydrate precursor, whose molecular formula is NH4Mn 0.6 Fe 0.4 PO4·H2O;

[0075] 4. Weigh 1.57 g of sucrose and dissolve it in 100 ml of deionized water, then add 18.63 g (0.1 mol) of dried doped ammonium manganese iron phosphate monohydrate precursor and 3.79 g of lithium carbonate and stir and disperse them evenly, then transfer them to a sand mill and sand grind them until the particle size is less than 400 nm to obtain a uniformly mixed lithium manganese iron phosphate suspension, wherein the sand mill speed is 2000 r / min and the sand grinding time is 30 min;

[0076] 5. The sand-milled lithium iron manganese phosphate suspension was spray dried. The feed rate of the spray dryer was 650 ml / h and the inlet temperature was 220 ° C. The dried powder was then placed in a tube furnace and heated to 350 ° C at a rate of 5 ° C / min for 5 h, and then heated to 700 ° C for 4 h to obtain lithium iron manganese phosphate with a molecular formula of LiMn 0.6 Fe 0.4 PO4 / C.

[0077] The lithium manganese iron phosphate positive electrode materials prepared in Examples 1-3 and Comparative Example 1 were subjected to scanning electron microscopy, energy dispersive X-ray spectroscopy, rate performance and cycle performance tests, as shown in Tables 1 and Figure 2-21 As shown, all the prepared materials appear as spherical particles with good dispersion, indicating that doping will not affect their spherical morphology. In the energy spectrum, it can be observed that the distribution of each component element and the doping element is relatively uniform, demonstrating the superiority of the co-precipitation method and in-situ doping.

[0078] Table 1 Test data of the modified lithium manganese iron phosphate prepared in Examples 1-3 and Comparative Example 1 at 0.1C and 1C rates, discharge medium voltage and 1C cycle 100 cycle capacity retention rate

[0079]

[0080] It can be seen from the electrochemical performance diagram and Table 1 that the lithium manganese iron phosphate material prepared in Comparative Example 1 has a discharge specific capacity of 147.0 mAh / g at a rate of 0.1C, and a discharge medium voltage of 3.864 V; and a discharge specific capacity of 128.0 mAh / g at a rate of 1C, and a discharge medium voltage of 3.612 V, and the performance is average.

[0081] The modified lithium manganese iron phosphate materials prepared in Examples 1-3 have a discharge specific capacity of more than 151 mAh / g at a rate of 0.1C, and a discharge medium voltage of more than 3.9 V. The discharge specific capacity at a rate of 1C is more than 131 mAh / g, and the discharge medium voltage is more than 3.63 V. Compared with Comparative Example 1, the modified lithium manganese iron phosphate prepared in Examples 1-3 has a better rate performance and a higher discharge medium voltage.

[0082] At the same time, the capacity retention rates of the modified lithium manganese iron phosphate prepared in Examples 1-3 after 100 cycles at 1C rate are all greater than 90.8%, with the highest being 94.6% in Example 3; while the cycle retention rate of Comparative Example 1 is only 84.4%; thus, it can be seen that the modified lithium manganese iron phosphate material has better cycle stability.

[0083] In summary, by doping divalent metal ions in the process of preparing ammonium manganese iron phosphate precursor by the reverse co-precipitation method, the atomic-level distribution of metal ions can be achieved, thereby improving the energy density and cycle stability of the modified lithium manganese iron phosphate, and making the modified lithium manganese iron phosphate material have better rate performance at 0.1C and 1C rates.

[0084] The above is a detailed introduction to a method for preparing modified lithium manganese iron phosphate with ammonium manganese iron phosphate as the matrix provided by the present application. Specific examples are used herein to illustrate the principles and implementation methods of the present application. 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 modified lithium manganese iron phosphate using ammonium manganese iron phosphate as a matrix, characterized in that: The method comprises the following steps: S100, preparing a metal ion mixed solution, a phosphate solution and a precipitant solution, wherein the metal ion mixed solution contains a manganese salt, an iron salt and a doping salt, the manganese salt 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 doping salt is one or more of nickel sulfate, nickel chloride, cobalt sulfate, cobalt chloride, magnesium sulfate and magnesium chloride; 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; S200, placing the prepared phosphate solution in a reaction container, adding a precipitant under stirring conditions and controlling the temperature and pH value of the solution in the reaction container to be constant, then adding the prepared metal ion mixed solution and the precipitant solution to the reaction container in parallel, maintaining the pH value of the solution constant until the solution is dripped away, and then stirring and aging to obtain an ammonium manganese iron 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 a doped ammonium manganese iron phosphate monohydrate precursor; S400, mixing the obtained doped ammonium manganese iron phosphate monohydrate precursor with a lithium salt and a carbon source and performing a frosting treatment to obtain a uniformly mixed lithium manganese iron phosphate suspension; S500, spray drying the obtained lithium manganese iron phosphate suspension to obtain dry spherical powder, and then sintering the dry spherical powder under an inert atmosphere to obtain modified lithium manganese iron phosphate.

2. The method according to claim 1, characterized in that: In the step S100, the total concentration of manganese salt, iron salt and doping salt in the metal ion mixed solution is 0.5-2 mol / L, the concentration of phosphate solution is 0.5-4 mol / L, and the concentration of 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 doped ammonium manganese iron phosphate monohydrate precursor after drying is NH4Mn x Fe y M 1-x-y PO4·H2O, where the atomic ratio (Mn+Fe+M) / P 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 The lithium salt is one or more of lithium carbonate, lithium hydroxide, lithium oxalate and lithium citrate.

6. The method according to claim 1, characterized in that The carbon source is one or more of glucose, sucrose, phenolic epoxy resin, polyvinyl pyrrolidone and asphalt.

7. The method according to claim 1, characterized in that In the step S500, the sintering temperature is 600-800°C, and the sintering time is 4-8h.

8. The method according to any one of claims 1 to 7, characterized in that: The inert atmosphere is one or more of a nitrogen atmosphere, an argon atmosphere or a helium atmosphere.

Images