Method for preparing lithium manganese iron phosphate material and application thereof

Abstract

The application discloses a method for preparing a lithium manganese iron phosphate material and application thereof. The method comprises the following steps: respectively pre-burning lithium iron phosphate and lithium manganese phosphate into nuclei, mixing the pre-burned lithium iron phosphate and lithium manganese phosphate by using a eutectic solvent method, then adding a carbon source and a magnesium source after heating and stirring, and finally high-temperature sintering to obtain a carbon-coated lithium manganese iron phosphate material. The lithium manganese iron phosphate material is prepared by using the eutectic solvent method, mechanical stress existing at a two-phase interface of the lithium manganese iron phosphate can be effectively reduced, and lattice distortion caused by the mechanical stress can be reduced; meanwhile, the doping of magnesium can reduce the dissolution of Mn, improve the structural stability in the charging and discharging process, and prolong the cycle life. The lithium manganese iron phosphate provided by the application has the advantages of higher rate performance and longer cycle life.

Description

Technical Field

[0001] This invention belongs to the field of lithium-ion battery manufacturing technology, specifically relating to a method for preparing lithium manganese iron phosphate material and its application. Background Technology

[0002] With the development of electric vehicles and energy storage projects, the requirements for power batteries are also increasing, especially in terms of energy density and cycle life. However, the ternary lithium iron phosphate battery materials commonly found on the market either lack sufficient energy density or fail to meet the required cycle life.

[0003] Lithium manganese iron phosphate (LFP) materials have a high voltage plateau and a stable crystal structure, and can be regarded as a combination of LFP and LMP. Therefore, they have the dual characteristics of LFP and LMP, with both high energy density and long cycle life.

[0004] However, the cell volume changes of the lithium insertion / extraction phases in LMP are relatively large, with ΔV of LiMnPO4 / MnPO4 being 9.0-11%. Furthermore, during continuous charging and discharging, lithium manganese iron phosphate will cause surface Mn dissolution due to the Jan Taylor effect, leading to changes in the positive electrode structure and the precipitation of negative electrode metals, resulting in an irreversible decrease in the capacity of the power battery and affecting the cycle performance.

[0005] In view of this, the present invention is hereby proposed. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing lithium manganese iron phosphate materials using a eutectic solvent method. The eutectic solvent method can effectively reduce the mechanical stress at the interface between the two phases of lithium manganese iron phosphate and reduce the resulting lattice distortion. At the same time, magnesium doping can reduce the dissolution of Mn, improve the structural stability during charge and discharge, and enhance the cycle life.

[0007] To achieve the above objectives, the present invention provides the following technical solution:

[0008] In a first aspect, the present invention provides a method for preparing lithium manganese iron phosphate material, comprising the following steps:

[0009] S1. Mix choline chloride and ethylene glycol and heat to obtain a choline chloride / ethylene glycol eutectic solvent.

[0010] S2. Add lithium iron phosphate and lithium manganese phosphate to the choline chloride / ethylene glycol eutectic solvent, heat and stir, and add carbon source and magnesium source;

[0011] S3. Sinter the material obtained in step S2 to obtain carbon-coated lithium manganese iron phosphate material.

[0012] In step S1, the molar ratio of choline chloride to ethylene glycol is 1:(1-2).

[0013] The heating temperature is 70–80°C.

[0014] In step S2, the molar ratio of Mn / Fe in the mixed system is (1-4):1, preferably 1.5:1.

[0015] In step S2, the heating temperature is 100-130°C.

[0016] In step S2, the stirring conditions are: stirring speed of 400-500 rpm and stirring time of 0.5-1 h.

[0017] In step S2, the carbon source is selected from one or more of glucose, sucrose, and starch. The amount of the carbon source used is 8-14% of the sum of the mass of the lithium iron phosphate and the lithium manganese phosphate, preferably 10%.

[0018] In step S2, the magnesium source is selected from one or more of magnesium oxide, magnesium carbonate, and magnesium acetate. The amount of the magnesium source used is 1-3% of the sum of the mass of the lithium iron phosphate and the lithium manganese phosphate.

[0019] In step S2, the lithium iron phosphate and the lithium manganese phosphate are prepared according to the following operation:

[0020] (1) Dissolve the iron source and lithium source in water to obtain solution A; dissolve the manganese source, lithium source and phosphoric acid in water to obtain solution B;

[0021] The iron source and the lithium source are mixed at a Fe:Li molar ratio of 1:1, and then stirred for 1-2 hours.

[0022] The manganese source, the lithium source, and the phosphoric acid are mixed in a Mn:Li:P molar ratio of 1:1:1, and then stirred for 1-2 hours.

[0023] (2) The solution A and the solution B are ball-milled and dried to obtain dried material A and dried material B;

[0024] The ball milling conditions are as follows: ball-to-material ratio of 1:(5-10), rotation speed of 400-500 rpm, and time of 20-30 h.

[0025] The D particles milled into solution A 50 The particle size (D) in solution B is 0.4–0.6 μm. 50 The size is 0.3–0.5 μm.

[0026] (3) The dried material A and the dried material B are pre-sintered to obtain the lithium iron phosphate and the lithium manganese phosphate;

[0027] The pre-sintering is carried out in a tube furnace; the pre-sintering conditions are as follows: under an inert atmosphere, the temperature rise is 2.5-3℃ / min, the pre-sintering temperature of the dried material A is 350-400℃, and the sintering time is 4-5h; the pre-sintering temperature of the dried material B is 300-350℃, and the sintering time is 3-4h.

[0028] In step S3, the sintering is carried out in a tube furnace; the sintering conditions are: under an inert atmosphere, the sintering temperature is 650-750℃, the sintering time is 8-12h, and the temperature rise is 2.5-3℃ / min.

[0029] Step S3 is performed as follows: the material obtained in step S2 is centrifuged and washed with water 1 to 3 times, and then centrifuged and washed with ethanol 1 to 3 times. The centrifugation speed is 1200 to 1500 rpm, the centrifugation time is 20 to 30 min, the drying temperature is 90 to 100℃, and the drying time is 12 to 24 h; then it is sintered to obtain carbon-coated lithium manganese iron phosphate material.

[0030] Secondly, the present invention further provides the lithium manganese iron phosphate material (LiMn) obtained by the above preparation method. x Fe 1-x P / C y x ranges from 0.6 to 0.8, and y ranges from 0.08 to 0.14.

[0031] Thirdly, the present invention further provides a lithium-ion battery, comprising a positive electrode and a negative electrode; wherein the positive electrode is made using the aforementioned lithium manganese iron phosphate material.

[0032] Compared with the prior art, the beneficial effects achieved by the present invention are:

[0033] This invention involves pre-calcining lithium iron phosphate (LFP) and lithium manganese phosphate (LMP) into nuclei, then mixing the nucleated LFP and LMP using a eutectic solvent method. The mixture is then heated and stirred while adding a carbon source and a magnesium source, and finally sintered at high temperature to obtain carbon-coated lithium manganese iron phosphate material (LiMn). x Fe 1-x P / C y ).

[0034] This invention utilizes a eutectic solvent method to prepare lithium manganese iron phosphate (LFP) materials, which effectively reduces the mechanical stress at the two-phase interface and the resulting lattice distortion. Simultaneously, magnesium doping reduces Mn dissolution, improves structural stability during charge and discharge, and extends cycle life. The LFP provided by this invention achieves advantages such as higher rate performance and longer cycle life. Attached Figure Description

[0035] Figure 1Lithium manganese iron phosphate (LiMn) provided in Example 3 0.6 Fe 0.4 P / C 0.1 Scanning electron microscope image. Detailed Implementation

[0036] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to the following embodiments.

[0037] Unless otherwise specified, the experimental methods used in the following examples are conventional methods.

[0038] Unless otherwise specified, all reagents, materials, instruments, etc. used in the following examples are commercially available.

[0039] This invention specifically provides a method for preparing lithium manganese iron phosphate, including steps such as dissolution and mixing, ball milling and drying, pre-sintering, eutectic mixing, centrifugal washing, and high-temperature sintering.

[0040] The specific steps are as follows:

[0041] Step 1: Dissolve and mix:

[0042] Iron phosphate and lithium carbonate were dissolved in deionized water at a Fe:Li molar ratio of 1:1, and the mixture was magnetically stirred for 1 hour to obtain solution A.

[0043] Manganese sulfate, lithium hydroxide, and phosphoric acid were dissolved in deionized water at a molar ratio of Mn:Li:P of 1:1:1, and the solution was magnetically stirred for 1 hour to obtain solution B.

[0044] Choline chloride and ethylene glycol were mixed in a beaker at a molar ratio of 1:2, heated to 80°C, and stirred to obtain a colorless and transparent liquid. After cooling, the eutectic solvent C of choline chloride and ethylene glycol was obtained.

[0045] Step 2: Ball milling and drying:

[0046] Solution A was transferred to a planetary ball mill with a ball-to-material ratio of 1:8, a rotation speed of 400 rpm, and a milling time of 25 hours. After milling, the particle size distribution of the particles in solution A was reduced to D. 50 It is 0.5μm;

[0047] Solution B was transferred to a planetary ball mill with a ball-to-material ratio of 1:8, a rotation speed of 400 rpm, and a milling time of 30 hours. After milling, the particle size distribution of the particles in solution B was reduced to D. 50 It is 0.4μm;

[0048] Solution A and solution B were spray dried separately to obtain dried material A and dried material B.

[0049] Step 3, Pre-sintering:

[0050] Dry material A and dry material B were transferred to a tube furnace. Under N2 atmosphere, the temperature rise was 2.5℃ / min. The pre-sintering temperature of dry material A was 400℃ and the sintering time was 5h. The pre-sintering temperature of dry material B was 350℃ and the sintering time was 4h, thus obtaining material A and material B.

[0051] Step 4: Eutectic Mixing

[0052] The pre-sintered materials A and B were added to the choline chloride / ethylene glycol eutectic solvent C at a ratio of 1:(1.5-4), and sucrose and magnesium oxide were added, with the addition amounts being 8-14% and 1% of the total mass of materials A and B, respectively. The mixed solution was heated to 120°C, and the stirring speed was 400 rpm for 1 hour.

[0053] Step 5: Centrifuge, wash, and dry.

[0054] The product after the eutectic mixing in step four was washed by centrifugation three times with deionized water and ethanol, respectively, at a speed of 1500 rpm for 20 min, and then placed in an oven at 100℃ for 24 h to dry.

[0055] Step Six: High-Temperature Sintering

[0056] The dried material from step five was transferred to a tube furnace and sintered at 680℃ for 10 hours under a N2 atmosphere at a temperature rise of 2.5℃ / min, to obtain lithium manganese iron phosphate material LiMn. x Fe 1-x P / C y x ranges from 0.6 to 0.8, and y ranges from 0.08 to 0.14.

[0057] The 2.4Ah soft pack used in the following tests was prepared according to the following procedure:

[0058] Weigh out the positive electrode active material, conductive carbon black, and PVDF in a mass ratio of 96:1.5:2.5, and add a certain amount of NMP solvent to adjust the viscosity of the slurry. After the slurry is fully mixed, transfer it to a clean aluminum foil with a thickness of 0.13 mm and coat it with a coating machine (coating accuracy 200 μm). Dry it at 120℃ for 12 h to obtain a dried electrode sheet. Then roll, cut, weigh it, and assemble it into a soft-pack battery.

[0059] Example 1

[0060] Step 1: Dissolving and Mixing: Dissolve iron phosphate and lithium carbonate in deionized water at a Fe:Li molar ratio of 1:1 and stir magnetically for 1 hour to obtain solution A; dissolve manganese sulfate, lithium hydroxide, and phosphoric acid in deionized water at a Mn:Li:P molar ratio of 1:1:1 and stir magnetically for 1 hour to obtain solution B; mix choline chloride and ethylene glycol in a beaker at a molar ratio of 1:2, heat to 80°C, and stir to obtain a colorless and transparent liquid. After cooling, obtain choline chloride / ethylene glycol eutectic solvent C.

[0061] Step 2, Ball Milling and Drying: Transfer solution A to a planetary ball mill with a ball-to-material ratio of 1:8, a rotation speed of 400 rpm, and a milling time of 25 hours. After milling, the particle size distribution of the particles in solution A is reduced to D. 50 To achieve a particle size of 0.5 μm, solution B was transferred to a planetary ball mill with a ball-to-material ratio of 1:8, a rotation speed of 400 rpm, and a milling time of 30 h. After milling, the particle size of solution B was reduced to D. 50 The solution was spray-dried to a depth of 0.4 μm, and solutions A and B were spray-dried to obtain dried material A and dried material B, respectively.

[0062] Step 3, Pre-sintering: Transfer the dried materials to a tube furnace: Under N2 atmosphere, the temperature rise is 2.5℃ / min, the pre-sintering temperature of dried material A is 400℃, the sintering time is 5h, and the pre-sintering temperature of dried material B is 350℃, the sintering time is 4h.

[0063] Step 4, Eutectic Mixing: Add the pre-sintered materials A and B to the eutectic solvent C of choline chloride / ethylene glycol at a ratio of 0.2:0.8, and add sucrose and magnesium oxide, respectively, at 10% and 1% of the total mass of materials A and B. Heat the mixed solution to 120°C, stir at 400 rpm for 1 hour.

[0064] Step 5, centrifugal washing: Wash the product after eutectic mixing by centrifugation three times with deionized water and ethanol, respectively. The centrifugation speed is 1500 rpm and the centrifugation time is 20 min. Then place it in an oven at 100℃ and dry for 24 h.

[0065] Step Six: High-Temperature Sintering: The dried material is transferred to a tube furnace. Under a N2 atmosphere, the temperature rise is 2.5℃ / min, the sintering temperature is 680℃, and the calcination time is 10h to obtain lithium manganese iron phosphate material LiMn. 0.8 Fe 0.2 P / C 0.1 .

[0066] Example 2

[0067] Step 1: Dissolving and Mixing: Dissolve iron phosphate and lithium carbonate in deionized water at a Fe:Li molar ratio of 1:1 and stir magnetically for 1 hour to obtain solution A; dissolve manganese sulfate, lithium hydroxide, and phosphoric acid in deionized water at a Mn:Li:P molar ratio of 1:1:1 and stir magnetically for 1 hour to obtain solution B; mix choline chloride and ethylene glycol in a beaker at a molar ratio of 1:2, heat to 80°C, and stir to obtain a colorless and transparent liquid. After cooling, obtain choline chloride / ethylene glycol eutectic solvent C.

[0068] Step 2, Ball Milling and Drying: Transfer solution A to a planetary ball mill with a ball-to-material ratio of 1:8, a rotation speed of 400 rpm, and a milling time of 25 hours. After milling, the particle size distribution of the particles in solution A is reduced to D. 50 To achieve a particle size of 0.5 μm, solution B was transferred to a planetary ball mill with a ball-to-material ratio of 1:8, a rotation speed of 400 rpm, and a milling time of 30 h. After milling, the particle size of solution B was reduced to D. 50 The solution was spray-dried to a depth of 0.4 μm, and solutions A and B were spray-dried to obtain dried material A and dried material B, respectively.

[0069] Step 3, Pre-sintering: Transfer the dried materials to a tube furnace: Under N2 atmosphere, the temperature rise is 2.5℃ / min, the pre-sintering temperature of dried material A is 400℃, the sintering time is 5h, and the pre-sintering temperature of dried material B is 350℃, the sintering time is 4h.

[0070] Step 4, Eutectic Mixing: Add the pre-sintered materials A and B to the eutectic solvent C of choline chloride / ethylene glycol at a ratio of 0.3:0.7, and add sucrose and magnesium oxide, respectively, at 10% and 1% of the total mass of materials A and B. Heat the mixed solution to 120°C, stir at 400 rpm for 1 hour.

[0071] Step 5, centrifugal washing: Wash the product after eutectic mixing by centrifugation three times with deionized water and ethanol, respectively. The centrifugation speed is 1500 rpm and the centrifugation time is 20 min. Then place it in an oven at 100℃ and dry for 24 h.

[0072] Step Six: High-Temperature Sintering: The dried material is transferred to a tube furnace. Under a N2 atmosphere, the temperature rise is 2.5℃ / min, the sintering temperature is 680℃, and the calcination time is 10h to obtain lithium manganese iron phosphate material LiMn. 0.7 Fe 0.3 P / C 0.1 .

[0073] Example 3

[0074] Step 1: Dissolving and Mixing: Dissolve iron phosphate and lithium carbonate in deionized water at a Fe:Li molar ratio of 1:1 and stir magnetically for 1 hour to obtain solution A; dissolve manganese sulfate, lithium hydroxide, and phosphoric acid in deionized water at a Mn:Li:P molar ratio of 1:1:1 and stir magnetically for 1 hour to obtain solution B; mix choline chloride and ethylene glycol in a beaker at a molar ratio of 1:2, heat to 80°C, and stir to obtain a colorless and transparent liquid. After cooling, obtain choline chloride / ethylene glycol eutectic solvent C.

[0075] Step 2, Ball Milling and Drying: Transfer solution A to a planetary ball mill with a ball-to-material ratio of 1:8, a rotation speed of 400 rpm, and a milling time of 25 hours. After milling, the particle size distribution of the particles in solution A is reduced to D. 50 To achieve a particle size of 0.5 μm, solution B was transferred to a planetary ball mill with a ball-to-material ratio of 1:8, a rotation speed of 400 rpm, and a milling time of 30 h. After milling, the particle size of solution B was reduced to D. 50 The solution was spray-dried to a depth of 0.4 μm, and solutions A and B were spray-dried to obtain dried material A and dried material B, respectively.

[0076] Step 3, Pre-sintering: Transfer the dried materials to a tube furnace: Under N2 atmosphere, the temperature rise is 2.5℃ / min, the pre-sintering temperature of dried material A is 400℃, the sintering time is 5h, and the pre-sintering temperature of dried material B is 350℃, the sintering time is 4h.

[0077] Step 4, Eutectic Mixing: Add the pre-sintered materials A and B to the eutectic solvent C of choline chloride / ethylene glycol at a ratio of 0.4:0.6, and add sucrose and magnesium oxide, respectively, at 10% and 1% of the total mass of materials A and B. Heat the mixed solution to 120°C, stir at 400 rpm for 1 hour.

[0078] Step 5, centrifugal washing: Wash the product after eutectic mixing by centrifugation three times with deionized water and ethanol, respectively. The centrifugation speed is 1500 rpm and the centrifugation time is 20 min. Then place it in an oven at 100℃ and dry for 24 h.

[0079] Step Six: High-Temperature Sintering: The dried material is transferred to a tube furnace. Under a N2 atmosphere, the temperature rise is 2.5℃ / min, the sintering temperature is 680℃, and the calcination time is 10h to obtain lithium manganese iron phosphate material LiMn. 0.6 Fe 0.4 P / C 0.1 .

[0080] Example 4

[0081] Step 1: Dissolving and Mixing: Dissolve iron phosphate and lithium carbonate in deionized water at a Fe:Li molar ratio of 1:1 and stir magnetically for 1 hour to obtain solution A; dissolve manganese sulfate, lithium hydroxide, and phosphoric acid in deionized water at a Mn:Li:P molar ratio of 1:1:1 and stir magnetically for 1 hour to obtain solution B; mix choline chloride and ethylene glycol in a beaker at a molar ratio of 1:2, heat to 80°C, and stir to obtain a colorless and transparent liquid. After cooling, obtain choline chloride / ethylene glycol eutectic solvent C.

[0082] Step 2, Ball Milling and Drying: Transfer solution A to a planetary ball mill with a ball-to-material ratio of 1:8, a rotation speed of 400 rpm, and a milling time of 25 hours. After milling, the particle size distribution of the particles in solution A is reduced to D. 50 To achieve a particle size of 0.5 μm, solution B was transferred to a planetary ball mill with a ball-to-material ratio of 1:8, a rotation speed of 400 rpm, and a milling time of 30 h. After milling, the particle size of solution B was reduced to D. 50 The solution was spray-dried to a depth of 0.4 μm, and solutions A and B were spray-dried to obtain dried material A and dried material B, respectively.

[0083] Step 3, Pre-sintering: Transfer the dried materials to a tube furnace: Under N2 atmosphere, the temperature rise is 2.5℃ / min, the pre-sintering temperature of dried material A is 400℃, the sintering time is 5h, and the pre-sintering temperature of dried material B is 350℃, the sintering time is 4h.

[0084] Step 4, Eutectic Mixing: Add the pre-sintered materials A and B to the eutectic solvent C of choline chloride / ethylene glycol at a ratio of 0.4:0.6, and add sucrose and magnesium oxide, respectively, at 8% and 1% of the total mass of materials A and B. Heat the mixed solution to 120°C, stir at 400 rpm for 1 hour.

[0085] Step 5, centrifugal washing: Wash the product after eutectic mixing by centrifugation three times with deionized water and ethanol, respectively. The centrifugation speed is 1500 rpm and the centrifugation time is 20 min. Then place it in an oven at 100℃ and dry for 24 h.

[0086] Step Six: High-Temperature Sintering: The dried material is transferred to a tube furnace. Under a N2 atmosphere, the temperature rise is 2.5℃ / min, the sintering temperature is 680℃, and the calcination time is 10h to obtain lithium manganese iron phosphate material LiMn. 0.6 Fe 0.4 P / C 0.08 .

[0087] Example 5

[0088] Step 1: Dissolving and Mixing: Dissolve iron phosphate and lithium carbonate in deionized water at a Fe:Li molar ratio of 1:1 and stir magnetically for 1 hour to obtain solution A; dissolve manganese sulfate, lithium hydroxide, and phosphoric acid in deionized water at a Mn:Li:P molar ratio of 1:1:1 and stir magnetically for 1 hour to obtain solution B; mix choline chloride and ethylene glycol in a beaker at a molar ratio of 1:2, heat to 80°C, and stir to obtain a colorless and transparent liquid. After cooling, obtain choline chloride / ethylene glycol eutectic solvent C.

[0089] Step 2, Ball Milling and Drying: Transfer solution A to a planetary ball mill with a ball-to-material ratio of 1:8, a rotation speed of 400 rpm, and a milling time of 25 hours. After milling, the particle size distribution of the particles in solution A is reduced to D. 50 To achieve a particle size of 0.5 μm, solution B was transferred to a planetary ball mill with a ball-to-material ratio of 1:8, a rotation speed of 400 rpm, and a milling time of 30 h. After milling, the particle size of solution B was reduced to D. 50 The solution was spray-dried to a depth of 0.4 μm, and solutions A and B were spray-dried to obtain dried material A and dried material B, respectively.

[0090] Step 3, Pre-sintering: Transfer the dried materials to a tube furnace: Under N2 atmosphere, the temperature rise is 2.5℃ / min, the pre-sintering temperature of dried material A is 400℃, the sintering time is 5h, and the pre-sintering temperature of dried material B is 350℃, the sintering time is 4h.

[0091] Step 4, Eutectic Mixing: Add the pre-sintered materials A and B to the eutectic solvent C of choline chloride / ethylene glycol at a ratio of 0.4:0.6, and add sucrose and magnesium oxide, respectively, at 14% and 1% of the total mass of materials A and B. Heat the mixed solution to 120°C, stir at 400 rpm for 1 hour.

[0092] Step 5, centrifugal washing: Wash the product after eutectic mixing by centrifugation three times with deionized water and ethanol, respectively. The centrifugation speed is 1500 rpm and the centrifugation time is 20 min. Then place it in an oven at 100℃ and dry for 24 h.

[0093] Step Six: High-Temperature Sintering: The dried material is transferred to a tube furnace. Under a N2 atmosphere, the temperature rise is 2.5℃ / min, the sintering temperature is 680℃, and the calcination time is 10h to obtain lithium manganese iron phosphate material LiMn. 0.6 Fe 0.4 P / C 0.14 .

[0094] Comparative Example 1

[0095] The difference from Example 3 is that no sucrose was added.

[0096] Comparative Example 2

[0097] The difference from Example 3 is that magnesium oxide was not added.

[0098] Comparative Example 3

[0099] The difference from Example 3 is that step four is carried out as follows: the pre-sintered material A and material B are added to deionized water at a ratio of 0.4:0.6, and then sucrose and magnesium oxide are added, with the addition amounts being 8% and 1% of the total mass of material A and material B, respectively. The mixed solution is heated to 120°C, the stirring speed is 400 rpm, and the stirring time is 1 hour.

[0100] Effect verification:

[0101] 1. Microstructure: composed of Figure 1 As can be seen, the lithium manganese iron phosphate material obtained in Example 3 has distinct particles, exhibits a graded and stacked distribution of large and small particles, and is free of other impurities, demonstrating high purity. This indicates that the lithium manganese iron phosphate material prepared by the eutectic solvent method of this invention can effectively reduce the mechanical stress existing at the interface between the two phases of lithium manganese iron phosphate and reduce the resulting lattice distortion. At the same time, magnesium doping can reduce the dissolution of Mn and improve the structural stability during the charge and discharge process.

[0102] 2. The 2.4Ah pouch batteries made from the lithium manganese iron phosphate materials obtained in Examples 1-5 and Comparative Examples 1-3 were tested, and the results are shown in the table below.

[0103] Table 1 shows the performance test results of the 2.4Ah soft-pack lithium manganese iron phosphate battery manufactured in this invention.

[0104]

[0105] As shown in Table 1, the lithium manganese iron phosphate materials prepared by the eutectic solvent method in Examples 1-5 exhibit high rate performance and long cycle life; among them, the LiMn corresponding to Example 3... 0.6 Fe 0.4 P / C 0.1 The material exhibited the highest electrical performance across all components, with an optimal Mn / Fe ratio of 6:4, and the best cycle retention rate at a carbon content of 1%. Furthermore, because the role of Mg is to suppress Mn dissolution, the high Mn content in Example 1 resulted in a correspondingly high Mn dissolution rate, leading to a significant decrease in cycle retention rate compared to Example 3. In summary, the lithium manganese iron phosphate provided by this invention achieves advantages such as higher rate performance and longer cycle life.

[0106] Comparative Example 1 showed reduced conductivity due to the absence of a carbon source; Comparative Example 2 showed Mn leaching from the surface of the material during continuous charge and discharge due to the absence of magnesium oxide, resulting in a significant decrease in cycle retention; In Comparative Example 3, materials A and B were mixed in deionized water, resulting in greater mechanical stress at the interface between the two phases, which caused lattice distortion and a decrease in electrical performance.

[0107] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A method for preparing lithium manganese iron phosphate material, comprising the following steps: S1. Mix choline chloride and ethylene glycol and heat to obtain a choline chloride / ethylene glycol eutectic solvent. S2. Add lithium iron phosphate and lithium manganese phosphate to the choline chloride / ethylene glycol eutectic solvent, heat and stir, and add carbon source and magnesium source; S3. Sinter the mixture obtained in step S2 to obtain carbon-coated lithium manganese iron phosphate material. In step S2, the lithium iron phosphate and the lithium manganese phosphate are prepared according to the following operation: (1) Dissolve the iron source and lithium source in water to obtain solution A; dissolve the manganese source, lithium source and phosphoric acid in water to obtain solution B; (2) The solution A and the solution B are ball-milled and dried to obtain dried material A and dried material B; (3) The dried material A and the dried material B are pre-sintered to obtain the lithium iron phosphate and the lithium manganese phosphate; In step (2), the ball milling process is carried out until the particles in solution A reach D... 50 The particle size (D) in solution B is 0.4~0.6 μm. 50 The thickness is 0.3~0.5 μm; In step (3), the pre-sintering is carried out in a tube furnace; the pre-sintering conditions are: in an inert atmosphere, the temperature rise is 2.5~3℃. C / min, the pre-sintering temperature of the dried material A is 350~400 C, the sintering time is 4~5 h; the pre-sintering temperature of the dried material B is 300~350 °C. C, sintering time is 3~4 h; In step S2, the molar ratio of Mn / Fe in the mixed system is (1-1.5):

1.

2. The method for preparing lithium manganese iron phosphate material according to claim 1, characterized in that: In step S1, the molar ratio of choline chloride to ethylene glycol is 1:(1~2). The heating temperature is 70~80℃.

3. The method for preparing lithium manganese iron phosphate material according to claim 1 or 2, characterized in that: In step S2, the heating temperature is 100~130℃; The stirring conditions are: stirring speed of 400~500 rpm and stirring time of 0.5~1 h.

4. The method for preparing lithium manganese iron phosphate material according to claim 1 or 2, characterized in that: In step S2, the carbon source is selected from one or more of glucose, sucrose, and starch; The amount of carbon source used is 8-14% of the sum of the mass of the lithium iron phosphate and the lithium manganese phosphate; The magnesium source is selected from one or more of magnesium oxide, magnesium carbonate, and magnesium acetate; The amount of magnesium source used is 1 to 3% of the sum of the masses of lithium iron phosphate and lithium manganese phosphate.

5. The method for preparing lithium manganese iron phosphate material according to claim 4, characterized in that: In step (1), the iron source and the lithium source are mixed at a Fe:Li molar ratio of 1:1 and stirred for 1-2 hours after mixing; the manganese source, the lithium source and the phosphoric acid are mixed at a Mn:Li:P molar ratio of 1:1:1 and stirred for 1-2 hours after mixing. In step (2), the conditions for ball milling are: ball-to-material ratio of 1:(5~10), rotation speed of 400~500 rpm, and time of 20~30h.

6. The method for preparing lithium manganese iron phosphate material according to claim 1 or 2, characterized in that: In step S3, the sintering is carried out in a tube furnace; The sintering conditions are: sintering temperature of 650~750°C under an inert atmosphere. C, sintering time is 8~12 h, temperature rise is 2.5~3 C / min.

7. The method for preparing lithium manganese iron phosphate material according to claim 1 or 2, characterized in that: Step S3 is performed as follows: the material obtained in step S2 is centrifuged and washed with water 1 to 3 times, and then centrifuged and washed with ethanol 1 to 3 times. The centrifugation speed is 1200 to 1500 rpm, the centrifugation time is 20 to 30 min, the drying temperature is 90 to 100℃, and the drying time is 12 to 24 h; then it is sintered to obtain carbon-coated lithium manganese iron phosphate material.

8. The lithium manganese iron phosphate material obtained by the preparation method according to any one of claims 1-7.

9. A lithium-ion battery comprising a positive electrode and a negative electrode; wherein the positive electrode is made using the lithium manganese iron phosphate material as described in claim 8.

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