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

Abstract

The application provides a lithium iron manganese phosphate positive electrode material and a preparation method thereof, and relates to the technical field of lithium ion batteries. The method uses ammonia water as a complexing agent, mixes a soluble manganese iron solution and rare earth doping elements, and uses an oxalic acid solution for precipitation, so that iron, manganese and rare earth elements are uniformly dispersed at an atomic level, the problem of abnormal local molar ratio of Fe / Mn is avoided, the particle size of the material is reduced, the ion conductivity and the electronic conductivity of the material are improved, the electrochemical performance of the material is effectively improved, and a lithium iron manganese phosphate preparation process with commercial application prospect is developed. The prepared precursor has the advantages of uniform distribution, high compaction density and large specific surface area; the preparation process of the precursor ensures uniform mixing of raw materials, uniform growth of crystal nuclei, reduces the preparation time, improves the production efficiency, does not need an additive, is controllable, and is suitable for large-scale industrial production.

Description

Technical Field

[0001] This invention relates to the field of lithium-ion battery technology, and in particular to a lithium manganese iron phosphate cathode material and its preparation method. Background Technology

[0002] Lithium-ion batteries have become one of the most widely used energy storage and conversion devices in recent years due to their high energy density, high operating voltage, long cycle life, and environmental friendliness. As a major component of lithium-ion batteries, cathode materials mainly include layered LiCoO2, LiMnO2, LiNiO2, and ternary composite materials, spinel-type LiM2O4 (M = Ti, V, and Mn) materials, and polyanionic LiFePO4. Among these, LiFePO4, with its olivine structure, has become one of the main cathode materials used in electric vehicles due to its high safety, high-rate discharge performance, and high cycle stability. The theoretical discharge specific capacity of LiFePO4 is 170 mAh g / g. -1 The discharge voltage plateau is 3.4V (vs Li + / Li). However, LiFePO4 has a low energy density, and improving its energy density has become a bottleneck for its further development. In contrast, LiMnPO4 has the same olivine structure as LiFePO4, with a theoretical specific capacity of 171 mAh g. -1 However, the discharge voltage plateau reached 4.1V (vs Li + / Li); LiFe obtained by mixing Fe and Mn in solid solution form. x Mn 1-x PO4 materials exhibit higher energy density and voltage plateau. Theoretically, for every 0.1 decrease in Fe content x, the LiFe... x Mn 1-x The energy density of PO4 will increase by 2.13%.

[0003] Currently, LiFe x Mn 1-x The main synthesis methods for PO4 materials include high-temperature solid-state methods, carbothermal reduction methods, solvothermal methods, sol-gel methods, and coprecipitation methods. Furthermore, because the ionic and electronic conductivity decreases after Mn replaces Fe, and because Mn itself has a dissolution problem, it easily leads to a reduction in the electrochemical performance of the material. Therefore, bulk doping or surface coating methods are often used to improve this performance.

[0004] CN 103151521 B discloses a lithium manganese iron phosphate cathode material co-coated with lithium titanate and carbon, which suppresses the deformation of the cathode material during charge and discharge, improves the cycle performance, overcharge and over-discharge resistance, and also enhances the material's conductivity and electrochemical properties. First, lithium, iron, manganese, phosphorus, and dopant sources are mixed, and then subjected to two sintering and cooling processes under a protective atmosphere to obtain LiFe. 1-x-y Mn x N y PO4 precursor; followed by lithium titanate and LiFe 1-x-y Mn x N y PO4 and a carbon source are mixed and wet-milled in a molar ratio of (0.01:0.3):1:(0~0.3). After drying, the mixture is sintered at 400~1050℃ under a protective atmosphere to obtain lithium manganese iron phosphate cathode material. Elemental doping is employed during co-coating, with N preferably being one or a combination of at least two of V, Ti, Mg, Al, Cu, or Mo. CN 106486668 B discloses a vanadium manganese iron phosphate precursor, lithium manganese iron vanadium phosphate / carbon cathode material, and its preparation method. The method involves adding a mixed solution of trivalent iron source, divalent manganese source, pentavalent vanadium source, and phosphorus source during precursor preparation, followed by a co-precipitation process to obtain the vanadium manganese iron phosphate precursor material. This precursor material is then uniformly mixed with a lithium source and a carbon source, dried, and calcined in an inert atmosphere to obtain lithium manganese iron vanadium phosphate / carbon (LiFe) cathode material. 1-x Mn 2x / 3 V x / 3 (PO4 / C) cathode material. CN 104710302 B discloses a method for preparing a gradient-doped manganese ferric manganese oxalate precursor. The main steps are: first, dissolve soluble manganese and iron sources to prepare a manganese ferric mixed solution; then adjust the pH with ammonium oxalate; introduce inert gas into the reaction vessel and heat and stir until the ammonium oxalate is completely dissolved; gradually add dopant ion solution while stirring, adjusting the addition rate to 0.005-2 L / h and gradually increasing it, so that the content of dopant element gradually increases from the center of the precursor along the radius from the inside to the outside; after the addition is complete, age the precursor to obtain a gradient-doped manganese ferric manganese oxalate precursor. CN107697899 B discloses a method for preparing battery-grade manganese ferric manganese phosphate, which uses an oxidation-precipitation process in a single aqueous solution system to prepare manganese ferric manganese phosphate. An oxidant is added to a mixed salt solution of divalent manganese salt and divalent iron salt and the pH is adjusted; then a soluble phosphorus source solution is added and the reaction continues to obtain the final product Mn. x Fe 1-xPO4·yH2O. CN 113213545B discloses a spherical ferromanganese carbonate and its preparation method. A solution of soluble ferrous salt and soluble ferrous salt, a carbonate solution, and an alkaline solution are added concurrently to a reaction vessel containing a reaction substrate. The reaction is then carried out under a protective atmosphere and a stable pH value to obtain a ferromanganese carbonate precursor. CN 114057177B discloses a method for preparing ferrous manganese phosphate. First, ferrous and ferrous salts are dissolved in water to obtain a mixed salt solution of manganese and iron. A phosphorus source is added to the mixed salt solution. The resulting precursor solution is then co-precipitated with an alkali to obtain a ferrous manganese phosphate slurry. The ferrous manganese phosphate slurry is washed, filtered, and dried to obtain the ferrous manganese phosphate precursor.

[0005] However, during the preparation of the aforementioned precursor and cathode materials, micro-agglomeration of the materials is prone to occur, leading to material irregularities and uneven mixing of Fe / Mn at the atomic level, resulting in poor electrochemical performance. Furthermore, the compaction density of the material cannot be effectively improved. Therefore, the above methods cannot effectively solve the problems existing in lithium manganese iron phosphate materials. Summary of the Invention

[0006] This invention provides a lithium manganese iron phosphate cathode material and its preparation method, the purpose of which is to solve the above-mentioned problems existing in the background art.

[0007] To achieve the above objectives, embodiments of the present invention provide a lithium manganese iron phosphate cathode material and its preparation method. This method uses ammonia as a complexing agent, mixes a soluble manganese iron solution with rare earth dopant elements, and precipitates the mixture using oxalic acid solution. This achieves uniform dispersion of iron, manganese, and rare earth elements at the atomic level, avoiding the problem of abnormal local Fe / Mn molar ratios, reducing the particle size of the material, and improving its ionic and electronic conductivity. This effectively enhances the electrochemical performance of the material and develops a lithium manganese iron phosphate preparation process with commercial application prospects. Furthermore, by improving the compaction density and conductivity of the material through element doping, lithium manganese iron phosphate cathode materials with different Mn / Fe molar ratios can be prepared according to market demand, providing a new approach for improving the electrochemical performance and industrial application of lithium manganese iron phosphate.

[0008] An embodiment of the present invention provides a method for preparing lithium manganese iron phosphate cathode material, comprising the following steps:

[0009] S1: Dissolve a mixture of soluble divalent manganese and divalent iron salts completely in water and pass in an inert gas to prepare a manganese-iron mixed solution; dissolve a soluble ionic compound of rare earth elements completely in water to prepare a rare earth doped ion solution; add oxalic acid to a reaction vessel containing water and pass in an inert gas, heat and stir until the oxalic acid is completely dissolved to prepare an oxalic acid solution.

[0010] S2: Inert gas is introduced into the reactor, and a manganese-iron mixed solution, rare earth doped ion solution, ammonia water and antioxidant are added. The complex solution is prepared by heating and stirring.

[0011] S3: Add oxalic acid solution to the complex solution, and add ammonia water to control the pH value and stirring speed. Heat and mix evenly. After stirring, age the mixture. After aging, cool it naturally to room temperature, wash, filter, and dry to obtain the precursor.

[0012] S4: The precursor is mixed with lithium source and phosphorus source in proportion, and then carbon source is added. Carbothermic reduction reaction is carried out in an inert gas atmosphere. After natural cooling to room temperature and pulverization, lithium manganese iron phosphate cathode material is obtained.

[0013] Preferably, in step S1, the divalent manganese salt is one or more of manganese sulfate, manganese nitrate, and manganese chloride; the divalent ferric salt is one or more of ferrous sulfate, ferrous oxalate, ferrous acetate, and ferrous nitrate; and the soluble ionic compound is one or more of rare earth chloride, rare earth carbonate, rare earth hydroxide, rare earth fluoride, rare earth nitrate, rare earth sulfate, rare earth acetate, rare earth oxalate, and rare earth phosphate.

[0014] Preferably, the concentration of manganese-iron metal ions in the manganese-iron mixed solution in step S1 is 100–1000 g / L. -1 The molar ratio of the divalent manganese salt to the divalent iron salt is Mn / Fe = 1–4; the concentration of the oxalic acid solution is 5–30 wt.%.

[0015] Preferably, the antioxidant in step S2 is one or more of ascorbic acid, hydrazine hydrate, and sodium sulfite, and the content of the antioxidant is 1 to 2 wt.% of the total mass of the manganese-iron mixed solution and the rare earth doped ion solution.

[0016] Preferably, the precursor in step S3 has the chemical formula Mn. x Fe 1-x-y Me y C2O4·z H2O, where Me is a rare earth dopant element, 0.5≤x≤0.8, 0.01≤y≤0.1, and z is 0~4. Me is selected from one or more of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, and Sc. The molar ratio of oxalic acid to iron and manganese is: oxalic acid / (Fe+Mn)=1~1.1.

[0017] Preferably, in step S3, the aging time is 2-8 hours; the pH value is 7-10; the solvent used for washing is one or more of deionized water, ethanol, and acetone; the drying temperature is 60-80°C; and the drying time is 6-12 hours.

[0018] Preferably, in step S4, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium oxalate, lithium phosphate, lithium nitrate, and lithium dihydrogen phosphate; the phosphorus source is at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, phosphorus pentoxide, lithium phosphate, and lithium dihydrogen phosphate; the carbon source is at least one of glucose, sucrose, polyethylene glycol, citric acid, and phenolic resin, and the content of the carbon source is 1-2 wt.% of the total mass of the precursor, lithium source, and phosphorus source; the molar ratio of the precursor to the lithium source and the phosphorus source is 1:(1.01-1.05):1.

[0019] Preferably, the temperature of the carbothermic reduction reaction in step S4 is 300–800°C, and the time is 8–20 h.

[0020] Preferably, the inert gas includes one or more of helium, nitrogen, and argon; the heating temperature is 30–70°C; and the stirring speed is 500–2500 rpm. -1 .

[0021] Based on a general inventive concept, embodiments of the present invention also provide lithium manganese iron phosphate cathode materials prepared by the above-described preparation method.

[0022] The above-described solution of the present invention has the following beneficial effects:

[0023] 1. The manganese iron oxalate precursor prepared by this invention has the advantages of uniform distribution, high compaction density, and large specific surface area. Iron, manganese, and doped rare earth elements are uniformly mixed at the atomic level, avoiding large differences in the local Fe / Mn molar ratio in the manganese iron oxalate precursor. The addition of rare earth elements can effectively reduce the particle size, increase the compaction density, improve the material conductivity, reduce the structural distortion caused by the Jahn-Teller effect, and enhance the stability of the lithium manganese iron phosphate material structure, thereby further improving the energy density and electrochemical performance of the lithium manganese iron phosphate cathode material.

[0024] 2. The preparation method of the manganese ferric oxalate precursor of the present invention does not require additives during the reaction process, and the equipment and production process are simple and highly controllable. The method of rapidly adding pH adjuster ensures uniform mixing of raw materials and uniform growth of crystal nuclei, while reducing preparation time and improving production efficiency. It can stably prepare manganese ferric oxalate precursor with high compaction density and large specific surface area, and is suitable for large-scale industrial production.

[0025] 3. This invention can, according to the requirements of the target cathode material, obtain manganese iron oxalate (Mn) with different Mn / Fe molar ratios and rare earth element contents by adjusting the molar ratio of manganese salt and iron salt as well as the amount of different rare earth element compounds added. x Fe 1-x-y Me yC2O4·z H2O yields lithium iron manganese phosphate cathode materials with different properties, which can be adapted to various application scenarios and have a wide range of applications. Attached Figure Description

[0026] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a SEM image of the lithium manganese iron phosphate cathode material prepared in Example 1 of the present invention.

[0028] Figure 2 This is the XRD pattern of the lithium manganese iron phosphate cathode material prepared in Example 1 of the present invention. Detailed Implementation

[0029] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0030] Unless otherwise defined, all technical terms used herein have the same meaning as commonly understood by those skilled in the art. The technical terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the scope of the invention.

[0031] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0032] Currently, LiFe x Mn 1-x The synthesis methods for PO4 materials mainly include high-temperature solid-state methods, carbothermal reduction methods, solvothermal methods, sol-gel methods, and co-precipitation methods. Furthermore, due to the decrease in ionic and electronic conductivity after Mn replaces Fe, and the inherent problem of Mn dissolution, the electrochemical performance of the material is easily reduced. Therefore, bulk doping or surface coating methods are often used to improve this. However, in the preparation processes of existing precursor materials and cathode materials, micro-agglomeration of materials is prone to occur, leading to material irregularities and uneven mixing of Fe / Mn at the atomic level, resulting in poor electrochemical performance; in addition, the compaction density of the material cannot be effectively improved. The above-mentioned existing methods cannot effectively solve the problems existing in lithium manganese iron phosphate materials. Based on this, the present invention provides a method for preparing lithium manganese iron phosphate cathode materials, including the following steps:

[0033] S1: Dissolve a mixture of soluble divalent manganese and divalent iron salts completely in water and pass in an inert gas to prepare a manganese-iron mixed solution; dissolve a soluble ionic compound of rare earth elements completely in water to prepare a rare earth doped ion solution; add oxalic acid to a reaction vessel containing water and pass in an inert gas, heat and stir until the oxalic acid is completely dissolved to prepare an oxalic acid solution.

[0034] S2: Inert gas is introduced into the reactor, and a manganese-iron mixed solution, rare earth doped ion solution, ammonia water and antioxidant are added. The complex solution is prepared by heating and stirring.

[0035] S3: Add oxalic acid solution to the complex solution, and add ammonia water to control the pH value and stirring speed. Heat and mix evenly. After stirring, age the mixture. After aging, cool it naturally to room temperature, wash, filter, and dry to obtain the precursor.

[0036] S4: The precursor is mixed with lithium source and phosphorus source in proportion, and then carbon source is added. Carbothermic reduction reaction is carried out in an inert gas atmosphere. After natural cooling to room temperature and pulverization, lithium manganese iron phosphate cathode material is obtained.

[0037] Preferably, in step S1, the divalent manganese salt is one or more of manganese sulfate, manganese nitrate, and manganese chloride; the divalent ferric salt is one or more of ferrous sulfate, ferrous oxalate, ferrous acetate, and ferrous nitrate; and the soluble ionic compound is one or more of rare earth chloride, rare earth carbonate, rare earth hydroxide, rare earth fluoride, rare earth nitrate, rare earth sulfate, rare earth acetate, rare earth oxalate, and rare earth phosphate.

[0038] Preferably, the concentration of manganese-iron metal ions in the manganese-iron mixed solution in step S1 is 100–1000 g / L. -1 The molar ratio of the divalent manganese salt to the divalent iron salt is Mn / Fe = 1–4; the concentration of the oxalic acid solution is 5–30 wt.%.

[0039] Preferably, the antioxidant in step S2 is one or more of ascorbic acid, hydrazine hydrate, and sodium sulfite, and the content of the antioxidant is 1 to 2 wt.% of the total mass of the manganese-iron mixed solution and the rare earth doped ion solution.

[0040] Preferably, the precursor in step S3 has the chemical formula Mn. x Fe 1-x-y Me yC2O4·z H2O, where Me is a rare earth dopant element, 0.5≤x≤0.8, 0.01≤y≤0.1, and z is 0~4. Me is selected from one or more of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, and Sc. The molar ratio of oxalic acid to iron and manganese is: oxalic acid / (Fe+Mn)=1~1.1.

[0041] Preferably, in step S3, the aging time is 2-8 hours; the pH value is 7-10; the solvent used for washing is one or more of deionized water, ethanol, and acetone; the drying temperature is 60-80°C; and the drying time is 6-12 hours.

[0042] Preferably, in step S4, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium oxalate, lithium phosphate, lithium nitrate, and lithium dihydrogen phosphate; the phosphorus source is at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, phosphorus pentoxide, lithium phosphate, and lithium dihydrogen phosphate; the carbon source is at least one of glucose, sucrose, polyethylene glycol, citric acid, and phenolic resin, and the content of the carbon source is 1-2 wt.% of the total mass of the precursor, lithium source, and phosphorus source; the molar ratio of the precursor to the lithium source and the phosphorus source is 1:(1.01-1.05):1.

[0043] Preferably, the temperature of the carbothermic reduction reaction in step S4 is 300–800°C, and the time is 8–20 h.

[0044] Preferably, the inert gas includes one or more of helium, nitrogen, and argon; the heating temperature is 30–70°C; and the stirring speed is 500–2500 rpm. -1 .

[0045] Based on a general inventive concept, embodiments of the present invention also provide lithium manganese iron phosphate cathode materials prepared by the above-described preparation method.

[0046] Compared with the prior art, the present invention has the following advantages:

[0047] 1. The manganese iron oxalate precursor prepared by this invention has the advantages of uniform distribution, high compaction density, and large specific surface area. Iron, manganese, and doped rare earth elements are uniformly mixed at the atomic level, avoiding large differences in the local Fe / Mn molar ratio in the manganese iron oxalate precursor. The addition of rare earth elements can effectively reduce the particle size, increase the compaction density, improve the material conductivity, reduce the structural distortion caused by the Jahn-Teller effect, and enhance the stability of the lithium manganese iron phosphate material structure, thereby further improving the energy density and electrochemical performance of the lithium manganese iron phosphate cathode material.

[0048] 2. The preparation method of the manganese ferric oxalate precursor of the present invention does not require additives during the reaction process, and the equipment and production process are simple and highly controllable. It ensures uniform mixing of raw materials and uniform growth of crystal nuclei, while reducing preparation time and improving production efficiency. It can stably prepare manganese ferric oxalate precursors with high compaction density and large specific surface area, and is suitable for large-scale industrial production.

[0049] 3. This invention can, according to the requirements of the target cathode material, obtain manganese iron oxalate (Mn) with different Mn / Fe molar ratios and rare earth element contents by adjusting the molar ratio of manganese salt and iron salt as well as the amount of different rare earth element compounds added. x Fe 1-x-y Me y C2O4·z H2O yields lithium iron manganese phosphate cathode materials with different properties, which can be adapted to various application scenarios and have a wide range of applications.

[0050] The following will be explained through specific embodiments.

[0051] Example 1

[0052] A method for preparing a lithium manganese iron phosphate cathode material includes the following steps:

[0053] S1. Weigh 118.8 g of manganese chloride tetrahydrate and 59.7 g of ferrous chloride tetrahydrate solution and add them to the reaction vessel. Add 1.785 L of deionized water and purge with nitrogen gas. Heat at 30 °C and stir at a rate of 500 rpm. -1 Mix under the conditions to form 100g L -1 A mixed solution of manganese and iron; weigh 31.8 g of lanthanum chloride hexahydrate and add it to the reaction vessel, then add 0.318 L of deionized water and stir at 800 rpm. -1 Stir at a rate sufficient to form 100g L -1 The doped ion solution B; weigh 126g of oxalic acid dihydrate and add it to the reaction vessel, add 0.9L of deionized water, and purge with nitrogen gas. Heat at 30℃ and stir at a rate of 1000 rpm. -1 Mix under the conditions to form 100g L -1 Oxalic acid solution C;

[0054] S2. Heat at 30℃ for 800 rpm. -1 Under stirring, a manganese-iron mixed solution A and rare earth doped ion solution B are added to the reactor, along with ammonia and 1 wt.% ascorbic acid. Under inert gas N2 conditions, a complex solution D containing manganese-iron mixed solution A and rare earth doped ion solution B is formed.

[0055] S3. Add the prepared oxalic acid solution C to the complex solution D, and simultaneously add ammonia water to control the pH to 8-9. Maintain a stirring speed of 1500 rpm, heat to 30°C, and continue stirring for 60 minutes to ensure the reaction products are thoroughly mixed. After stirring, allow to age for 2 hours. After aging, allow the manganese ferric oxalate slurry to cool naturally to room temperature, wash with deionized water, and dry at 80°C for 6 hours to obtain the product Mn. 0.6 Fe 0.3 La 0.1 C2O4·4H2O.

[0056] S4. Mn 0.6 Fe 0.3 La 0.1 C2O4·4H2O precursor was mixed with lithium carbonate and phosphoric acid in a molar ratio of 1:1.01:1, and 1 wt.% of sucrose was added as a carbon source. Carbothermic reduction was carried out under a N2 atmosphere at 700℃ for 10 h. After natural cooling to room temperature, LiMn was finally obtained by pulverization. 0.6 Fe 0.3 La 0.1 PO4 finished product. SEM image of this lithium manganese iron phosphate cathode material is shown below. Figure 1 As shown, the XRD pattern is as follows Figure 2 As shown.

[0057] Example 2

[0058] A method for preparing a lithium manganese iron phosphate cathode material includes the following steps:

[0059] S1. Weigh 3.049 kg of manganese chloride tetrahydrate and 1.095 kg of ferrous chloride tetrahydrate solution and add them to the reaction vessel. Add 8.288 L of deionized water and purge with nitrogen gas. Heat at 70 °C and stir at 800 rpm. -1 Mix under the conditions to form 500g L -1 A mixed solution of manganese and iron (A); 133.1 g of rubidium chloride was weighed and added to the reaction vessel, and 0.266 L of deionized water was added at 800 rpm. -1 Stir at a rate sufficient to form 500g L -1 The doped ion solution B; 2.772 kg of oxalic acid dihydrate was weighed and added to a 30 L reactor, along with 5.544 L of deionized water, and nitrogen gas was introduced. The reactor was heated to 70 °C and stirred at a rate of 1000 rpm. -1 Mix under the conditions to form 500g L -1 Oxalic acid solution C;

[0060] S2. Heat at 70℃ for 1000 rpm. -1Under stirring, a manganese-iron mixed solution A and rare earth doped ion solution B are added to the reactor, along with ammonia and 1.5 wt.% hydrazine hydrate. Under inert gas N2 conditions, a complex solution D containing manganese-iron mixed solution A and rare earth doped ion solution B is formed.

[0061] S3. Add the prepared oxalic acid solution C to the complex solution D, and simultaneously add ammonia water to control the pH to 7-8. Stir at 1000 rpm, heat to 70℃, and continue stirring for 30 minutes to ensure the reaction products are thoroughly mixed. After stirring, age for 4 hours. The aged manganese ferric oxalate slurry is then allowed to cool naturally to room temperature, washed with deionized water, and dried at 80℃ for 8 hours to obtain the product Mn. 0.7 Fe 0.25 Rb 0.05 C2O4·4H2O.

[0062] S4. Mn 0.7 Fe 0.25 Rb 0.05 The C2O4·4H2O precursor was mixed with lithium hydroxide and ammonium dihydrogen phosphate in a molar ratio of 1:1.05:1, and 1.5 wt.% of citric acid was added as a carbon source. Carbothermic reduction was carried out under an Ar atmosphere at 750℃ for 8 hours, followed by natural cooling to room temperature. Finally, LiMn was obtained by pulverization. 0.7 Fe 0.25 Rb 0.05 PO4 finished product.

[0063] Example 3

[0064] A method for preparing a lithium manganese iron phosphate cathode material includes the following steps:

[0065] S1. Weigh 135.2 g of manganese sulfate monohydrate and 41.7 g of ferrous sulfate heptahydrate solution and add them to the reaction vessel. Add 0.442 L of deionized water and purge with nitrogen gas. Heat at 30 °C and stir at 400 rpm. -1 Mixed under the conditions to form 400g L -1 A mixed solution of manganese and iron; weigh 39.1 g of lutetium sulfate octahydrate and add it to the reaction vessel, then add 0.391 L of deionized water and stir at 400 rpm. -1 Stir at a rate sufficient to form 100g L -1 The doped ion solution B; weigh 126g of oxalic acid dihydrate and add it to the reaction vessel, add 1.26L of deionized water, and purge with nitrogen gas. Heat at 30℃ and stir at a rate of 800 rpm. -1 Mix under the conditions to form 100g L -1 Oxalic acid solution C;

[0066] S2. Heat at 30℃ for 1000 rpm. -1 Under stirring, a manganese-iron mixed solution A and rare earth doped ion solution B are added to the reactor, along with ammonia and 2 wt.% sodium sulfite. Under inert gas N2 conditions, a complex solution D containing manganese-iron mixed solution A and rare earth doped ion solution B is formed.

[0067] S3. Add the prepared oxalic acid solution C to the complex solution D, and simultaneously add ammonia water to control the pH to 9-10. Stir at 800 rpm, maintain the temperature at 30°C, and continue stirring for 40 minutes to ensure the reaction products are thoroughly mixed. After stirring, age for 3 hours. The aged manganese ferric oxalate slurry is then allowed to cool naturally to room temperature, washed with deionized water, and dried at 60°C for 10 hours to obtain the product Mn. 0.8 Fe 0.15 Lu 0.05 C2O4·H2O.

[0068] S4. Mn 0.8 Fe 0.15 Lu 0.05 C2O4·H2O precursor was mixed with lithium oxalate and ammonium phosphate in a molar ratio of 1:1.02:1:0.4, and 1.5 wt.% of glucose was added as a carbon source. Carbothermic reduction was carried out under a N2 atmosphere at 700℃ for 10 h. After natural cooling to room temperature, LiMn was obtained by pulverization. 0.8 Fe 0.15 Lu 0.05 PO4 finished product.

[0069] Comparative Example

[0070] The difference between this comparative example and Example 1 is that step 2) of preparing the complex is not included; it is directly mixed with oxalic acid. The other steps are the same as in Example 1.

[0071] The electrochemical performance of the lithium manganese iron phosphate electrode materials prepared in Examples 1-3 and the comparative example was determined using button cells. The positive electrode active material was the lithium manganese iron phosphate positive electrode material prepared in Examples 1-3 and the comparative example, the negative electrode was a lithium sheet, the conductive agent was SuperP, the binder was polyvinylidene fluoride, the separator was a Celgard 2500 composite membrane, and the electrolyte was a 1 mol / L lithium hexafluorophosphate solution as the lithium salt, with a 1:1 volume ratio of dimethyl carbonate (DMC) and ethylene carbonate (EC) solution as the solvent. The mass ratio of the positive electrode active material, conductive agent, and binder was 8:1:1. The button cells were assembled in a glove box under an argon protective atmosphere. The electrochemical performance of the button cells was tested using a blue electric test cabinet, with the test voltage range set from 2V to 4.5V.

[0072]

[0073] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A method for preparing a lithium manganese iron phosphate cathode material, characterized in that, Includes the following steps: S1: A mixed solution of soluble divalent manganese and divalent iron salts is completely dissolved in water, and an inert gas is introduced to prepare a manganese-iron mixed solution; a rare earth element-doped ion solution is completely dissolved in water; oxalic acid is added to a reaction vessel containing water, and an inert gas is introduced, and the mixture is heated and stirred until the oxalic acid is completely dissolved to prepare an oxalic acid solution; wherein the concentration of manganese-iron metal ions in the manganese-iron mixed solution is 100 ~ 1000 g / L. -1 The molar ratio of the divalent manganese salt to the divalent iron salt is Mn / Fe = 1~4; the concentration of the oxalic acid solution is 5~30 wt.%. S2: Inert gas is introduced into the reactor, and a manganese-iron mixed solution, rare earth doped ion solution, ammonia water, and antioxidant are added. The mixture is heated and stirred to prepare a complex solution. The antioxidant is one or more of ascorbic acid, hydrazine hydrate, and sodium sulfite, and the antioxidant content is 1-2 wt.% of the total mass of the manganese-iron mixed solution and the rare earth doped ion solution. S3: Oxalic acid solution is added to the complex solution, and ammonia water is added simultaneously to control the pH value. The stirring speed is controlled, and the mixture is heated and stirred until homogeneous. After stirring, the mixture is aged and then naturally cooled to room temperature. After washing, filtering, and drying, the precursor is obtained. The chemical formula of the precursor is Mn. x Fe 1-x-y Me y C2O4·zH2O, where Me is a rare earth dopant element, 0.5 ≤ x ≤ 0.8, 0.01 ≤ y ≤ 0.1, and z is 0~4. Me is selected from one or more of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y, and Sc. The molar ratio of oxalic acid to iron and manganese is: oxalic acid / (Fe+Mn) = 1~1.

1. S4: The precursor is mixed with lithium source and phosphorus source in proportion, and then carbon source is added. Carbothermic reduction reaction is carried out in an inert gas atmosphere. After natural cooling to room temperature and pulverization, lithium manganese iron phosphate cathode material is obtained.

2. The method for preparing a lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step S1, the divalent manganese salt is one or more of manganese sulfate, manganese nitrate, and manganese chloride; the divalent ferric salt is one or more of ferrous sulfate, ferrous oxalate, ferrous acetate, and ferrous nitrate; and the soluble ionic compound is one or more of rare earth chloride, rare earth carbonate, rare earth hydroxide, rare earth fluoride, rare earth nitrate, rare earth sulfate, rare earth acetate, rare earth oxalate, and rare earth phosphate.

3. The method for preparing a lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step S3, the aging time is 2-8 h; the pH value is 7-10; the washing solvent is one or more of deionized water, ethanol, and acetone; the drying temperature is 60-80 ℃, and the drying time is 6-12 h.

4. The method for preparing a lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step S4, the lithium source is at least one of lithium carbonate, lithium hydroxide, lithium oxalate, lithium phosphate, lithium nitrate, and lithium dihydrogen phosphate; the phosphorus source is at least one of phosphoric acid, ammonium dihydrogen phosphate, ammonium phosphate, phosphorus pentoxide, lithium phosphate, and lithium dihydrogen phosphate; the carbon source is at least one of glucose, sucrose, polyethylene glycol, citric acid, and phenolic resin, and the content of the carbon source is 1 to 2 wt.% of the total mass of the precursor, lithium source, and phosphorus source; the molar ratio of the precursor to the lithium source and the phosphorus source is 1: (1.01~1.05):

1.

5. The method for preparing a lithium manganese iron phosphate cathode material according to claim 1, characterized in that, In step S4, the temperature of the carbothermic reduction reaction is 300~800 ℃, and the time is 8~20 h.

6. The method for preparing a lithium manganese iron phosphate cathode material according to claim 1, characterized in that, The inert gas includes one or more of helium, nitrogen, and argon; the heating temperature is 30~70℃; and the stirring speed is 500~2500 rmin. -1 .

7. The lithium manganese iron phosphate cathode material prepared by the preparation method according to any one of claims 1 to 6.

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