Preparation method of high-specific-energy lithium manganese iron phosphate positive electrode material

Abstract

The invention relates to the technical field of preparation of lithium ion battery positive electrode materials, and particularly discloses a preparation method of a high-specific-energy lithium iron manganese phosphate positive electrode material. The preparation method comprises the following steps: firstly preparing a lithium manganese iron phosphate precursor, then mixing the precursor with a liquid carbon source ethylene glycol, and finally carrying out carbon coating on the lithium manganese iron phosphate through step-by-step calcination. According to the invention, the lithium manganese iron phosphate is subjected to carbon coating by adopting the organic liquid for the first time, and the prepared lithium manganese iron phosphate material is low in carbon content and has better electrochemical performance.

Description

Technical Field

[0001] The invention relates to the technical field of lithium ion battery positive electrode material preparation, and in particular to a method for preparing a high-energy lithium manganese iron phosphate positive electrode material. Background Art

[0002] In recent years, the application of lithium-ion batteries in consumer electronics, energy storage power stations and electric vehicles has achieved great results and harvests, and cathode materials, as one of the key materials in lithium-ion battery systems, have received widespread attention. Lithium cobalt oxide, lithium iron phosphate, lithium manganese oxide and nickel cobalt manganese oxide ternary cathode materials are currently the most market-oriented materials. As an upgraded version of lithium iron phosphate, lithium manganese iron phosphate (LMFP) improves the voltage platform and energy density by doping manganese elements, approaching the level of ternary lithium batteries, while retaining its advantages of low cost and high safety. However, in actual use, manganese is easily dissolved in the cathode of lithium manganese iron phosphate, resulting in lattice distortion, resulting in disadvantages such as reduced battery capacity and cycle life. Carbon coating is the core modification method to improve the electrochemical performance of lithium manganese iron phosphate. At present, it is mainly achieved through in-situ / ex-situ coating, graphene composite and other methods. The carbon layer forms a conductive network, reduces the internal resistance of the electrode, alleviates the problem of intrinsic low conductivity of lithium manganese iron phosphate, and makes the capacity more fully utilized. However, in the current field of carbon coating, nano-sized particles and double-layer carbon coating require precise process control (such as liquid phase synthesis), and the equipment investment and processing costs are high, which may offset some of the raw material cost advantages. In addition, non-in situ coating can easily lead to uneven distribution of the carbon layer, affecting the electrochemical performance.

[0003] At present, there are three methods in the published literature for carbon coating of LMFP positive electrode materials, namely solid phase method, sol-gel method and spray drying method. The solid phase method is to add a carbon source in the ball milling step, mix and ball mill, and then place it in a high temperature environment for calcination; the sol-gel method is to dissolve the carbon source in a solvent to form a sol and then dry it into a gel, and then calcine it for carbonization; and the spray drying method is to spray the precursor and carbon source solution and then calcine it. In the process of preparing LMFP positive electrode materials by liquid phase method, if you want to coat the carbon coating, you often need to carbon coat the prepared material by ball milling or spray drying. At the same time, there are many types of carbon sources, including sugars, organic acids, biomass carbon sources, etc. But in general, these are all carbon-coated materials through solid phase carbon sources or after dissolving solid phase carbon sources. Whether the preparation process can be simplified by other means in the liquid phase method, such as direct calcination of carbon coating as in the high-temperature solid phase method, and whether there are more carbon sources that can be used for LMFP carbon coating preparation, is one of the key topics of exploration in the current research field. Summary of the invention

[0004] In view of the deficiencies in the above-mentioned prior art, the purpose of the present invention is to provide a method for preparing a high-energy lithium iron manganese phosphate positive electrode material. The method first prepares a lithium iron manganese phosphate precursor, then mixes the precursor with a liquid carbon source ethylene glycol, and finally carbon-coates the lithium iron manganese phosphate by step-by-step calcination. The present invention expands the raw materials for in-situ carbon coating of the existing liquid phase preparation of lithium iron manganese phosphate positive electrode materials from solid to liquid, providing a breakthrough innovative solution for the subsequent exploration of more liquid carbon sources. The carbon content of the LMFP prepared by the present invention by coating the liquid phase carbon source is only 2.74% at most, which is much lower than the carbon content in the current solid phase method for carbon coating, and has better electrochemical performance.

[0005] In order to achieve the above object, the present invention adopts the following technical scheme:

[0006] A method for preparing a high specific energy lithium manganese iron phosphate positive electrode material comprises the following steps:

[0007] (1) preparing a lithium manganese iron phosphate precursor;

[0008] (2) Ethylene glycol is added to the lithium iron manganese phosphate precursor in (1) to fully immerse it, and the resulting mixed solution is placed in a tubular furnace, heated to 180-210° C. under an inert atmosphere, and calcined for 2.5-3 hours, then heated to 600-700° C. and calcined for 5-6 hours to obtain the high specific energy lithium iron manganese phosphate positive electrode material.

[0009] Furthermore, in step (2), the lithium manganese iron phosphate precursor is immersed in ethylene glycol at a solid-liquid ratio of 1 g: (2-8) mL; preferably, it is immersed in ethylene glycol at a solid-liquid ratio of 1 g: (4-6) mL; more preferably, it is immersed in ethylene glycol at a solid-liquid ratio of 1 g: (4.5-5.5) mL.

[0010] Furthermore, in step (2), under an inert atmosphere, the temperature is raised to 200±5° C., kept warm and calcined for 3 hours, and then the temperature is raised to 680-700° C., kept warm and calcined for 5 hours to obtain the high specific energy lithium manganese iron phosphate positive electrode material.

[0011] Furthermore, in step (2), the inert atmosphere is argon.

[0012] Furthermore, in step (2), the heating rate is 1-5°C / min.

[0013] Furthermore, in step (2), the lithium manganese iron phosphate is LM x F 1-x P, x = 0.4 to 0.6, preferably x = 0.4.

[0014] Furthermore, in step (1), the lithium iron manganese phosphate precursor is prepared by a hydrothermal solvothermal method, and preferably the solvent used is an ethylene glycol aqueous solution.

[0015] Furthermore, the hydrothermal solvothermal method for preparing the lithium manganese iron phosphate precursor comprises the following steps:

[0016] S1: dissolving a lithium source, a phosphorus source, a divalent iron source, a divalent manganese source and a reducing agent in a solvent, wherein the solvent is a mixed solution of water and ethylene glycol in a volume ratio of 2:(5-10), and stirring evenly;

[0017] S2: subjecting the obtained uniform solution to a hydrothermal solvent thermal reaction at 180° C. for 12 hours; after the reaction is completed, washing with alcohol, washing with water, and drying to obtain the lithium manganese iron phosphate precursor.

[0018] Further, step S1 includes: dissolving a lithium source and a phosphorus source in an ethylene glycol aqueous solution, stirring evenly to obtain a solution A; dissolving an iron source, a manganese source and a reducing agent in an ethylene glycol aqueous solution, stirring evenly to obtain a solution B; mixing solution A and solution B, and stirring evenly.

[0019] Furthermore, in step S1, the molar ratio of the lithium source, phosphorus source, iron source and manganese source is 30:10:(4-6):(4-6), preferably 30:10:6:4.

[0020] Furthermore, in step S1, the ratio of the amount of the reducing agent to the sum of the amounts of the divalent iron source and the divalent manganese source is 12.5%.

[0021] Furthermore, the lithium source is lithium hydroxide, for example, LiOH·H 2 O, the phosphorus source is 85wt% phosphoric acid, and the divalent iron source is ferrous sulfate, such as FeSO 4 7H 2 O, the divalent manganese source is manganese sulfate, for example, MnSO 4 ·H 2 O, the reducing agent is ascorbic acid.

[0022] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0023] Ethylene glycol is widely used as one of the key raw materials for preparing LMFP positive electrode materials by liquid phase method. This is because in the process of preparing lithium manganese iron phosphate by liquid phase method, it can dissolve raw materials such as lithium salts, iron salts, manganese salts and phosphoric acid very well, so that these raw materials are fully mixed and uniform at the molecular level, which is conducive to the subsequent reaction and ensures that the generated lithium manganese iron phosphate has a uniform composition and structure. However, due to the influence of inherent thinking and the certain danger of organic liquids in high temperature environments, no one has tried to add ethylene glycol as a carbon source to the material for coating as a carbon source so far.

[0024] The advantage of ethylene glycol as a liquid carbon source is that the hydroxyl group in the molecule has a certain coordination ability and can coordinate with metal ions. This effect can control the reactivity and reaction rate of metal ions, make the reaction more controllable, and help to form a uniform precursor, thereby improving the crystallinity and purity of lithium manganese iron phosphate and improving the performance of the material. At the same time, in the process of preparing LMFP positive electrode materials, Mn source and Fe source are used as their divalent salts as raw materials in most cases. During the preparation process, they are easily oxidized when in contact with oxygen under solution conditions. At this time, ethylene glycol can play a certain reducing role, so that the two metal ions can avoid being oxidized to a high valence state, produce unnecessary side reactions during the reaction, and improve the consistency and stability of the product. In addition, ethylene glycol can affect the formation process of lithium manganese iron phosphate crystals and help control the morphology and size of the particles. It can be adsorbed on the surface of the crystal, inhibit the growth of the crystal in certain directions, and promote the growth of the crystal in other directions, so as to obtain lithium manganese iron phosphate particles with specific morphology. These particles can have a large specific surface area and good inter-particle contact, which is conducive to improving the electrochemical properties of the material. However, the current liquid phase method is limited to the use of ethylene glycol in the step of dissolving and preparing precursors. Considering that ethylene glycol has a high boiling point and good thermal stability, it can withstand high reaction temperatures without violent decomposition or volatilization. At the same time, the molecular structure of ethylene glycol contains two hydroxyl groups, which has certain coordination ability and steric hindrance effect. In some material synthesis systems, it can interact with the precursor or the growing crystal surface, affect the growth direction and rate of the crystal, play a certain regulatory role on the morphology and size of the material, and show a function similar to that of a template. The present invention uses ethylene glycol as a liquid carbon source, adds it to a tubular furnace in the calcination step, and carbon-coates the material. The specific advantages are as follows:

[0025] 1. The present invention expands the raw materials for in-situ carbon coating of existing liquid-phase prepared lithium manganese iron phosphate positive electrode materials from solid to liquid, providing a breakthrough innovative solution for subsequent exploration of more liquid carbon sources.

[0026] 2. The lithium manganese iron phosphate positive electrode material prepared in the present invention has excellent electrochemical properties and also shows good stability in cycle performance. The in-situ carbon coating is an option for industrialization.

[0027] 3. In the present invention, EG (ethylene glycol) is used to liquid-seal the material, which innovatively solves the oxidation problem that may exist in the high-temperature calcination process of the easily oxidized lithium iron manganese phosphate positive electrode material, and provides an option for industrial production to reduce the impact of oxidation during the calcination preparation process of lithium iron manganese phosphate.

[0028] 4. Currently, in the modification of lithium manganese iron phosphate positive electrode materials, high-performance materials are obtained for the first time through one-time carbon coating.

[0029] 5. Ethylene glycol itself has good thermal stability and reducibility, is suitable for high-temperature reactions, can achieve long-term reactions without high-pressure equipment, and promotes crystal growth.

[0030] 6. Ethylene glycol is a good structure-directing agent and surface dispersant. It can be adsorbed on the particle surface, prevent agglomeration through steric hindrance effect, and improve dispersibility. It plays a key role in the synthesis of nanoparticles.

[0031] 7. Ethylene glycol is an organic compound widely used in the preparation process of liquid phase method (including hydrothermal, solvothermal, sol-gel and co-precipitation methods). The use of ethylene glycol in carbon coating avoids the introduction of other impurities.

[0032] 8. The carbon content of LMFP prepared by coating with liquid carbon source is only 2.74% at most, which is much lower than the carbon content in the current solid phase carbon coating method, and has better electrochemical performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] Figure 1 This is the XRD diagram of the lithium manganese iron phosphate positive electrode material prepared in Example 1-3.

[0034] Figure 2 The discharge specific capacity cycle performance diagram at 0.1C (a), 1C (b) and 5C (c) rates after the lithium manganese iron phosphate positive electrode material prepared in Example 1-3 is assembled into a lithium ion half-cell.

[0035] Figure 3 LM prepared in Example 1 0.4 F 0.6 SEM image of P material.

[0036] Figure 4 The discharge specific capacity cycle performance diagram at 0.1C (a), 1C (b) and 5C (c) rates after the lithium manganese iron phosphate positive electrode material prepared in Example 4-6 is assembled into a lithium ion half-cell.

[0037] Figure 5 LM prepared in Example 6 0.4 F 0.6 SEM images of P-7 material at different magnifications.

[0038] Figure 6 LM prepared in Example 6 0.4 F 0.6 TEM images of P-7 material at different magnifications.

[0039] Figure 7This is a picture of the appearance of the lithium manganese iron phosphate material prepared in Comparative Example 1. DETAILED DESCRIPTION

[0040] The technical solution of the present application will be described in detail below in conjunction with embodiments, but the scope of protection requested by the claims of the present invention is not limited to these embodiments.

[0041] Embodiment 1 A method for preparing a lithium manganese iron phosphate positive electrode material comprises the following steps:

[0042] (1) Weigh 1.26 g (30 mmol) of LiOH·H 2 O is poured into a conical flask and 10 ml of deionized water is added. After sufficient stirring, 0.68 mL of the mass fraction is 85 wt% and the density is 1.695 g / cm 3 (10mmol) of phosphoric acid, place the conical flask on a magnetic stirrer for stirring at a speed of 1200r / min. After stirring for 30min, add 25ml of EG (ethylene glycol) to obtain solution A;

[0043] (2) Weigh 1.67 g (6 mmol) of FeSO 4 7H 2 O and 0.68 g (4 mmol) of MnSO 4 ·H 2 O and 0.22 g (1.25 mmol) ascorbic acid were placed in a 100 ml conical flask, 10 ml of deionized water was added to dissolve, and after fully dissolved, 25 ml of EG (ethylene glycol) was added to obtain solution B;

[0044] (3) Pour solution A in (1) into solution B in (2), place in a constant temperature water bath, stir at 25°C for 120 min, pour the resulting liquid into a 100 ml polytetrafluoroethylene lined reactor, seal it, and perform a hydrothermal reaction at 180°C for 12 h. After the hydrothermal reaction is completed, take out the reactor and let it stand to room temperature (20-25°C);

[0045] (4) The material obtained in the reaction kettle in (3) was centrifugally washed with anhydrous ethanol and deionized water in sequence, and the washing was repeated three times, and then placed in a vacuum drying oven and dried at 80° C. for 12 h to obtain a lithium manganese iron phosphate precursor;

[0046] (5) The lithium manganese iron phosphate precursor in (4) was taken out and placed in a porcelain boat, which was placed in a tube furnace. Under an argon atmosphere, the temperature was raised to 700°C at a rate of 5°C / min and calcined for 5 h to obtain the final product, which was named LM 0.4 F 0.6 P.

[0047] Example 2 A method for preparing a lithium manganese iron phosphate positive electrode material

[0048] The preparation method is the same as that in Example 1, except that the amount of iron salt and manganese salt used in step (2) is different. In this example, 1.39 g (5 mmol) of FeSO 4 7H 2 O and 0.85 g (5 mmol) of MnSO 4 ·H 2 O.

[0049] The final product was named LM 0.5 F 0.5 P.

[0050] Example 3 A method for preparing a lithium manganese iron phosphate positive electrode material

[0051] The preparation method is the same as that in Example 1, except that the amount of iron salt and manganese salt used in step (2) is different. In this example, 1.11 g (4 mmol) of FeSO 4 7H 2 O and 1.01 g (6 mmol) of MnSO 4 ·H 2 O.

[0052] The final product was named LM 0.6 F 0.4 P.

[0053] Embodiment 4 A method for preparing a high specific energy lithium manganese iron phosphate positive electrode material comprises the following steps:

[0054] (1) Weigh 1.26 g (30 mmol) of LiOH·H 2 O is poured into a conical flask and 10 ml of deionized water is added. After sufficient stirring, 0.68 mL of the mass fraction is 85 wt% and the density is 1.695 g / cm 3 (10mmol) of phosphoric acid, place the conical flask on a magnetic stirrer for stirring at a speed of 1200r / min. After stirring for 30min, add 25ml of EG (ethylene glycol) to obtain solution A;

[0055] (2) Weigh 1.67 g (6 mmol) of FeSO 4 7H 2 O and 0.68 g (4 mmol) of MnSO 4 ·H 2 O and 0.22 g (1.25 mmol) ascorbic acid were placed in a 100 ml conical flask, 10 ml of deionized water was added to dissolve, and after fully dissolved, 25 ml of EG (ethylene glycol) was added to obtain solution B;

[0056] (3) Pour solution A in (1) into solution B in (2), place in a constant temperature water bath, stir at 25°C for 120 min, pour the resulting liquid into a 100 ml polytetrafluoroethylene lined reactor, seal it, and perform a hydrothermal reaction at 180°C for 12 h. After the hydrothermal reaction is completed, take out the reactor and let it stand to room temperature (20-25°C);

[0057] (4) The material obtained in the reaction kettle in (3) was centrifugally washed with anhydrous ethanol and deionized water in sequence, and the washing was repeated three times, and then placed in a vacuum drying oven and dried at 80° C. for 12 h to obtain a lithium manganese iron phosphate precursor;

[0058] (5) The lithium iron manganese phosphate precursor (1.5 g) in (4) was taken out and placed in a porcelain boat. 3 ml of EG (ethylene glycol) was added to the porcelain boat. After the lithium iron manganese phosphate precursor was fully immersed, the porcelain boat was placed in a tube furnace. Under an argon atmosphere, the temperature was raised to 200°C at a rate of 5°C / min. After calcination for 3 hours, the temperature was further raised to 700°C at a rate of 5°C / min. The temperature was calcined for 5 hours to obtain the final product, which was named LM 0.4 F 0.6 P-3, abbreviated as LMFP-3 in the attached figure. After the carbon and sulfur analyzer, LM 0.4 F 0.6 The carbon content of the P-3 sample was 0.79%.

[0059] Example 5 A method for preparing a high specific energy lithium manganese iron phosphate positive electrode material

[0060] The preparation method is the same as that of Example 4, the only difference being the amount of ethylene glycol used in step (5), which in this example is: 5 ml EG (ethylene glycol).

[0061] The final product was named LM 0.4 F 0.6 P-5, abbreviated as LMFP-5 in the attached figure. After the carbon and sulfur analyzer, LM 0.4 F 0.6 The carbon content of the P-5 sample is 1.90%.

[0062] Example 6: Preparation method of a high specific energy lithium manganese iron phosphate positive electrode material

[0063] The preparation method is the same as that of Example 4, the only difference being the amount of ethylene glycol used in step (5), which in this example is: 7 ml EG (ethylene glycol).

[0064] The final product was named LM 0.4 F 0.6 P-7, abbreviated as LMFP-7 in the attached figure. After the carbon and sulfur analyzer, LM 0.4 F 0.6The carbon content of the P-7 sample is 2.74%.

[0065] It can be seen from Examples 1 and 4-6 that in step (5), as the amount of ethylene glycol increases, the carbon content inside the obtained lithium manganese iron phosphate material also increases.

[0066] Comparative Example 1: A method for preparing a lithium manganese iron phosphate positive electrode material

[0067] (1) Weigh 1.26 g (30 mmol) of LiOH·H 2 O is poured into a conical flask and 10 ml of deionized water is added. After sufficient stirring, 0.68 mL of the mass fraction is 85 wt% and the density is 1.695 g / cm 3 (10mmol) of phosphoric acid, place the conical flask on a magnetic stirrer for stirring at a speed of 1200r / min. After stirring for 30min, add 25ml of EG (ethylene glycol) to obtain solution A;

[0068] (2) Weigh 1.67 g (6 mmol) of FeSO 4 7H 2 O and 0.68 g (4 mmol) of MnSO 4 ·H 2 O and 0.22 g (1.25 mmol) ascorbic acid were placed in a 100 ml conical flask, 10 ml of deionized water was added to dissolve, and after fully dissolved, 25 ml of EG (ethylene glycol) was added to obtain solution B;

[0069] (3) Pour solution A in (1) into solution B in (2), place in a constant temperature water bath, stir at 25°C for 120 min, pour the resulting liquid into a 100 ml polytetrafluoroethylene lined reactor, seal it, and perform a hydrothermal reaction at 180°C for 12 h. After the hydrothermal reaction is completed, take out the reactor and let it stand to room temperature (20-25°C);

[0070] (4) The material obtained in the reaction kettle in (3) was centrifugally washed with anhydrous ethanol and deionized water in sequence, and the washing was repeated three times, and then placed in a vacuum drying oven and dried at 80° C. for 12 h to obtain a lithium manganese iron phosphate precursor;

[0071] (5) The lithium manganese iron phosphate precursor (1.5 g) in (4) was taken out and placed in a porcelain boat. 0.14 g of polyethylene glycol 6000 was added to the porcelain boat. After grinding and mixing, the porcelain boat was placed in a tube furnace. Under an argon atmosphere, the temperature was raised to 700° C. at a rate of 5° C. / min and calcined for 5 h to obtain a final product. Figure 7 The dimensions of the porcelain boat in the picture are 5.2cm×2.5cm×1cm.

[0072] Performance testing and characterization

[0073] The lithium iron manganese phosphate positive electrode materials prepared in Examples 1-6 and Comparative Example 1 were assembled into lithium ion half-cells according to the following method.

[0074] Using N-methyl-2-pyrrolidone (NMP) as a solvent, the active material (lithium manganese iron phosphate material prepared in Examples 1-6 and Comparative Example 1), the conductive agent acetylene black and the binder polyvinylidene fluoride (PVDF) are mixed into a slurry in a mass ratio of 8:1:1, and then coated on an aluminum foil. After drying under vacuum conditions at 80°C, the circular pole pieces are taken out and punched into a circular pole piece with a diameter of 12 mm for standby use. The above circular pole pieces are then assembled into lithium-ion half-cells in a glove box (water and oxygen levels are kept below 0.01ppm). First, the positive electrode shell, the above circular pole piece, the diaphragm, the lithium sheet, the nickel foam and the negative electrode shell are stacked in sequence, and packaged after adding appropriate electrolyte. The battery shell used is CR2032 type, the diaphragm is Celgard2400, and the electrolyte is 1mol / LLiPF 6 / EC-DMC(1:1)(LiPF 6 Dissolved in a mixed solution of ethylene carbonate and dimethyl carbonate in a volume ratio of 1:1).

[0075] The above batteries were tested by Blue Power CT3002A test system at 2.0-4.5V (vs. Li + / Li) voltage window, the rate performance test was carried out at 0.1C, 1C and 5C rates, and the results are shown in Table 1.

[0076] Table 1

[0077]

[0078] The XRD pattern of the lithium manganese iron phosphate positive electrode material prepared in Example 1-3 is shown in Figure 1 , (b) is the enlarged view of the 34°-36° XRD diffraction peak in (a). Figure 1 It can be seen that: In the figure, LM 0.23 F 0.77 The standard PDF card of P was used as a reference. The XRD diffraction peak shifts of the three samples prepared were all shifted to the left. This is because in the LMFP of olivine structure, Mn and Fe usually exist in the +2 form. 2+ The octahedral coordination ion radius is larger than that of Fe 2+ The radius of Fe 2+Occupies more transition metal sites, resulting in a decrease in the average ionic radius of the transition metal sites, a contraction of the lattice as a whole, and a decrease in the interplanar spacing. According to the Bragg equation: 2dsinθ=λ, a decrease in d value will lead to an increase in the diffraction angle, that is, the XRD peak shifts to the right. And in the three LMFPs with different manganese-iron ratios in the figure, as the Fe content increases, the LM 0.23 F 0.77 The smaller the peak deviation of the standard PDF card of P, the three substances can be determined as the target products, namely, LMFP positive electrode materials in different proportions.

[0079] The discharge capacity cycle performance diagrams of the lithium iron manganese phosphate positive electrode materials prepared in Examples 1-3 assembled into lithium ion half-cells at 0.1C (a), 1C (b) and 5C (c) rates are shown in the following figure: Figure 2 .Depend on Figure 2 It can be seen that the discharge specific capacities of the three groups of samples at 0.1C rate are 118.2 mAh g -1 , 109.3mAh g -1 , 110.7mAh g -1 The discharge capacity at 1C rate is 109.8 mAh g -1 , 100.0mAh g -1 , 100.4mAh g -1 ; The discharge capacity at 5C rate is 81.6 mAh g -1 , 74.6mAh g -1 , 70.1mAh g -1 From the above data, it can be seen that under three different magnification conditions, LM 0.4 F 0.6 P all showed better first-round discharge performance.

[0080] LM prepared in Example 1 0.4 F 0.6 SEM images of P materials are shown in Figure 3 It can be seen from the figure that the lithium manganese iron phosphate material has an irregular spherical structure.

[0081] The discharge capacity cycle performance diagrams of the lithium iron manganese phosphate positive electrode materials prepared in Example 4-6 assembled into lithium ion half-cells at 0.1C (a), 1C (b) and 5C (c) rates are shown in the following diagrams: Figure 4 .Depend on Figure 4It can be seen that: as the amount of ethylene glycol added in the calcination step (5) increases, the electrochemical performance of the lithium manganese iron phosphate material gradually improves, and all show good rate performance and cycle stability. The rate performance of the material was tested at 0.1C, 1C and 5C rates. The first-round discharge specific capacities of LMFP-3, LMFP-5 and LMFP-7 at 0.1C rate were 136.0mAh / g, 145.3mAh / g and 155.2mAh / g respectively; the first-round discharge specific capacities at 1C rate were 120.4mAh / g, 128.5mAh / g and 135.6mAh / g respectively; the first-round discharge specific capacities at 5C rate were 92.4mAh / g, 99.6mAh / g and 108.7mAh / g respectively.

[0082] LM prepared in Example 6 0.4 F 0.6 SEM images of P-7 material at different magnifications are shown in Figure 5 As can be seen from the figure, the lithium manganese iron phosphate material has a rod-like structure. The nanorod-like structure can provide a direct lithium ion diffusion channel, shorten the ion migration path, reduce the polarization effect, and thus improve the electrochemical performance of the material.

[0083] LM prepared in Example 6 0.4 F 0.6 TEM images of P-7 material at different magnifications are shown in Figure 6 It can be seen from the figure that a carbon coating is formed on the surface of the lithium manganese iron phosphate material.

Claims

1. A method for preparing a high specific energy lithium manganese iron phosphate positive electrode material, characterized in that: The steps include: (1) preparing a lithium manganese iron phosphate precursor; (2) Ethylene glycol is added to the lithium iron manganese phosphate precursor in (1) to fully immerse it, and the resulting mixed solution is placed in a tubular furnace, heated to 180-210° C. under an inert atmosphere, and calcined for 2.5-3 hours, then heated to 600-700° C. and calcined for 5-6 hours to obtain the high specific energy lithium iron manganese phosphate positive electrode material.

2. The preparation method according to claim 1, characterized in that: In step (1), a lithium manganese iron phosphate precursor is prepared by a hydrothermal solvothermal method.

3. The preparation method according to claim 2, characterized in that: The solvent used in the hydrothermal solvothermal method is ethylene glycol aqueous solution.

4. The preparation method according to claim 3, characterized in that: The hydrothermal solvothermal method for preparing lithium manganese iron phosphate precursor includes the following steps: S1: dissolving a lithium source, a phosphorus source, a divalent iron source, a divalent manganese source and a reducing agent in a solvent, wherein the solvent is a mixed solution of water and ethylene glycol in a volume ratio of 2:(5-10), and stirring evenly; S2: subjecting the obtained uniform solution to a hydrothermal solvent thermal reaction at 180° C. for 12 hours; after the reaction is completed, washing with alcohol, washing with water, and drying to obtain the lithium manganese iron phosphate precursor.

5. The preparation method according to any one of claims 1 to 4, characterized in that: In step (2), the lithium manganese iron phosphate precursor is immersed in ethylene glycol at a solid-liquid ratio of 1 g: (2-8) mL; preferably, it is immersed in ethylene glycol at a solid-liquid ratio of 1 g: (4-6) mL; more preferably, it is immersed in ethylene glycol at a solid-liquid ratio of 1 g: (4.5-5.5) mL.

6. The preparation method according to any one of claims 1 to 4, characterized in that: In step (2), the lithium manganese iron phosphate is LM x F 1-x P,x=0.4~0.

6.

7. The preparation method according to claim 1, characterized in that: In step (2), under an inert atmosphere, the temperature is raised to 200±5° C., and the mixture is kept warm and calcined for 3 hours, and then the temperature is raised to 680-700° C., and the mixture is kept warm and calcined for 5 hours to obtain the high specific energy lithium manganese iron phosphate positive electrode material.

8. The preparation method according to claim 4, characterized in that: Step S1 includes: dissolving a lithium source and a phosphorus source in an ethylene glycol aqueous solution, stirring evenly to obtain a solution A; dissolving an iron source, a manganese source and a reducing agent in an ethylene glycol aqueous solution, stirring evenly to obtain a solution B; mixing solution A and solution B, and stirring evenly.

9. The preparation method according to claim 4, characterized in that: In step S1, the molar ratio of the lithium source, phosphorus source, iron source and manganese source is 30:10:(4-6):(4-6), preferably 30:10:6:4; and / or The ratio of the amount of the reducing agent to the sum of the amounts of the divalent iron source and the divalent manganese source is 12.5%.

10. The preparation method according to claim 4, characterized in that: In step S1, the lithium source is lithium hydroxide, the phosphorus source is 85 wt% phosphoric acid, the divalent iron source is ferrous sulfate, the divalent manganese source is manganese sulfate, and the reducing agent is ascorbic acid.

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