Preparation method of modified lithium manganese iron phosphate positive electrode material coated and modified by lanthanum oxide and carbon

Abstract

The invention discloses a preparation method of a lanthanum oxide and carbon dual-coated modified lithium manganese iron phosphate positive electrode material, and belongs to the field of lithium ion batteries. A lithium source, a phosphorus source, an iron source, a manganese source and the like are fully mixed and subjected to preliminary sintering to obtain lithium manganese iron phosphate. Lanthanum and a carbon source are introduced, secondary low-temperature short-time heating and sintering are carried out, the lanthanum oxide and carbon dual-modified lithium manganese iron phosphate composite positive electrode material is obtained, a uniform, compact and thin coating layer is achieved, micro doping can be achieved, the double-voltage platform effect can be effectively relieved, and the problem of sudden voltage drop is solved. The preparation process has the advantages of easily available raw materials, simple synthesis steps, low cost, adjustability and the like, and the obtained lithium manganese iron phosphate has wide application prospects in the fields of electrode materials, magnetic materials, catalysis and the like.

Description

Technical Field

[0001] The present invention relates to the field of cathode materials and even lithium-ion batteries, and particularly relates to a preparation method of a modified lithium iron phosphate cathode material coated and modified with lanthanum oxide and carbon. Background Art

[0002] Energy is the driving force for the development of human society. The development of many new energy industries is extremely strong and there is a large demand both at home and abroad. Lithium-ion batteries, as a new type of high-energy green battery, have attracted much attention.

[0003] As an indispensable part of lithium-ion batteries, the cathode material directly determines important properties such as the capacity, life, and safety of the battery. The cathode materials of lithium-ion batteries mainly include layered transition metal oxides LiTMO2 (TM = Co, Ni, Mn, etc.), manganese spinel LiMn2O4, and polyanion compounds Li x TM y PO4 (TM = Fe, Mn, Co, V, etc.). In recent years, with the huge wave of the development of electric vehicles, more and more people need to develop batteries with faster charge and discharge efficiency, higher charge and discharge specific capacity, and most importantly, safety. However, the development of these two materials is restricted by safety aspects such as unstable material structure and side effects between the material and the electrolyte.

[0004] Goodenough first proposed and studied lithium iron phosphate in 1997. LiFePO4 (LFP), as a polyanion cathode material, has a stable discharge platform (3.4V), high discharge capacity (170mAh / g), good cycle stability, thermal stability, inexpensive and easily available raw materials, good safety, and environmental friendliness; it is considered to be one of the most suitable cathode materials for potassium ion batteries. However, the discontinuous FeO6 edge-sharing octahedral network in the LiFePO4 crystal and the PO4 tetrahedra between them affect electron transfer and the insertion and extraction of Li. In addition, the intrinsically low Li + diffusion coefficient and low electronic conductivity of LiFePO4 batteries result in serious capacity attenuation during high-rate charge and discharge processes.

[0005] Lithium iron phosphate (LiFePO4) has attracted much attention due to its relatively low cost, environmental friendliness, and excellent thermal stability. Its low charge and discharge voltage (3.4V) results in low energy density. In order to further improve the battery performance, researchers have turned to phosphate materials of other transition metals, among which lithium iron manganese phosphate (LiMn x Fe 1-xLiMn₂Fe(PO₄)₃ is a highly favored material. Compared with lithium iron phosphate, lithium manganese iron phosphate incorporates two transition metals, manganese (Mn) and iron (Fe), forming a manganese-iron solid solution. This dual-voltage platform material improves the battery's performance, and the introduction of manganese and iron increases the energy density. Lithium manganese iron phosphate has a higher redox potential (4.1V vs. Li / Li + ⁺), and theoretically has a higher energy density (about 20% higher than LFP in terms of specific energy). In addition, this cathode material can match well with electrolytes used in a 4V electrochemical window. However, LiMn x Fe 1-x (PO₄)₃ has low electronic conductivity (<10 -10 ⁻⁵ S / cm), low Li + ⁺ diffusion coefficient, local structure distortion caused by the Jahn-Teller effect of Mn 3+ ³⁺, and disadvantages such as volume change of the battery during charge and discharge of LMFP. To overcome these drawbacks, researchers have adopted different strategies, such as using nanoscale particles, doping, carbon coating, conductive additives, etc. to optimize the performance of LiMn x Fe 1-x (PO₄)₃. CN202311369542.1 discloses a lithium manganese iron phosphate prepared by a coprecipitation method, but it involves the release of a large amount of organic solvents, which is not conducive to environmental protection, and the synthesis process has high energy consumption and a long synthesis cycle. CN115588737A discloses a preparation method for secondary modified lithium manganese iron phosphate, obtaining primary modified lithium manganese iron phosphate and further physically coating to obtain secondary modified lithium manganese iron phosphate. However, the secondary modified lithium manganese iron phosphate provided by this invention has great processing difficulty, and at the same time, carbon coating increases the specific surface area of lithium manganese iron phosphate, resulting in uneven dispersion during processing. CN103762362A discloses a hydrothermal method for preparing nanoscale lithium manganese iron phosphate cathode material, mixing H₃PO₄ solution and LiOH·H₂O according to a molar ratio, adding concentrated ammonia water, controlling the temperature at 170 - 200 °C and holding for 7 - 10 h; then washing and carbon coating to obtain the product. However, the material prepared by this method has an unstable structure, and the capacity decays rapidly after repeated charge and discharge, and the high-temperature and high-pressure reaction is relatively dangerous.

[0006] Currently, lithium manganese iron phosphate, as a relatively important electrode material, has wide applications in the fields of electrochemical energy storage, magnetic materials, and catalysts. However, the existing synthesis methods of lithium manganese iron phosphate have problems such as difficult access to raw materials, complex synthesis steps, and low comprehensive performance, and at the same time, the requirements for purity and crystal structure are limited. Therefore, there is a need to provide a simple, low-cost, highly controllable, and excellent-performance synthesis and modification method for lithium manganese iron phosphate. Summary of the Invention

[0007] In view of this, the purpose of the present invention is to provide a lithium iron manganese phosphate composite material, its preparation method and application. The lithium iron manganese phosphate material provided by the present invention has good charge and discharge performance and cycling performance, and also has excellent electrical conductivity and structural stability, effectively alleviating the double voltage platform effect of the lithium iron manganese phosphate material and solving the problem of voltage sudden drop. The mass percentage of La2O3 in the lithium iron manganese phosphate is 4.845%, and the mass percentage of C in the lithium iron manganese phosphate is 2%-8%. La2O3 and carbon form a double protective layer. First, a protective layer of La2O3 is formed, and then a carbon coating layer is formed on the outer layer. And there will be partial La micro-doping in the final product.

[0008] The present invention provides a lithium iron manganese phosphate composite material with the general formula of formula (I):

[0009] LiMn a Fe b PO4@La2O3 / C c Formula (I);

[0010] 0.5 ≤ a ≤ 0.9, 0.1 ≤ b ≤ 0.5, 0.02 ≤ c ≤ 0.08, and a + b = 1. The mass percentage of La2O3 in the lithium iron manganese phosphate is 3-5%, such as 4.845%.

[0011] Preferably, in formula (I), 0.5 ≤ a ≤ 0.85, 0.15 ≤ b ≤ 0.5, 0.02 ≤ c ≤ 0.04

[0012] The above-mentioned low-cost and high-cycle-stability preparation method of an olivine phosphate material involves dissolving compounds of an iron source and a manganese source in absolute ethanol respectively, and mixing them under stirring conditions to form a precursor mixed slurry; subsequently, dissolving compounds of a lithium source and a phosphorus source in absolute ethanol respectively, and mixing them under stirring to form a mixed slurry containing only the lithium source and the phosphorus source; slowly mixing the above two batches of slurries under magnetic stirring to form a mixed liquid; introducing the mixed liquid into the feed tank of a sand mill through a glass rod, fully stirring and sand milling; after sand milling, collecting the slurry to obtain the final precursor slurry, centrifuging and separating it, placing it in a vacuum oven for drying, and then sintering it under an inert atmosphere, and obtaining pure lithium iron manganese phosphate after cooling with the furnace. Disperse the lithium iron manganese phosphate material in deionized water, and disperse lanthanum oxide powder and carbon source powder particles in deionized water with an ultrasonic device under stirring, and then pump them into the above solution. The suspension is continuously stirred at 80 °C until the solvent evaporates, and the obtained product is annealed briefly to form a lithium iron manganese phosphate cathode material modified by lanthanum oxide and carbon. The obtained cathode material has a very dense and thin coating layer of lanthanum oxide and carbon on its surface, and realizes micro-doping and improves the crystallinity to form an LMFP@La2O3 / C composite cathode material; the above process not only realizes the preparation of products by a one-step solid-phase method in a low-temperature environment, but also constructs a uniform coating layer of lanthanum oxide and carbon, improves the electronic conductivity and ion migration ability of the raw materials, and at the same time the particles can be fully refined.

[0013] The above-mentioned low-cost and high-cycle-stability preparation method of lithium iron manganese phosphate olivine includes the following steps:

[0014] (a) Dissolve the iron source and the manganese source in absolute ethanol successively, and mix them under stirring conditions to form a precursor mixed slurry A;

[0015] (b) Dissolve the lithium source and phosphorus source compounds in absolute ethanol successively, and mix them under stirring conditions to form a precursor mixed slurry B;

[0016] (c) Slowly add the mixed slurry A to the mixed slurry B under stirring conditions to form a mixed liquid;

[0017] (d) Introduce the mixed liquid obtained in step (c) into a sand mill, fully stir and sand mill. After sand milling, collect the slurry to obtain the final precursor slurry C; centrifuge and separate the slurry C and place it in a vacuum oven for drying, and calcine the obtained powder material under an inert atmosphere, and obtain pure lithium iron manganese phosphate after cooling with the furnace;

[0018] (e)Disperse the lithium iron manganese phosphate obtained in step (d) in deionized water. Under stirring, disperse lanthanum oxide powder and carbon source powder in deionized water by ultrasonic wave, and then pump them into the above lithium iron manganese phosphate dispersion; continuously stir at 80 °C until the solvent evaporates; anneal the obtained powder material to obtain a lithium iron manganese phosphate cathode material double-modified by lanthanum oxide and carbon. The surface of the obtained cathode material has a very dense and thin coating layer of lanthanum oxide and carbon, and micro-doping and improvement of crystallinity are achieved.

[0019] Subsequently, assemble the obtained composite cathode material into a battery, which effectively alleviates the double voltage platform effect of the lithium iron manganese phosphate material and solves the problem of voltage sudden drop.

[0020] Raw materials such as lithium source, phosphorus source, iron source, and manganese source can be purchased from commercial chemical suppliers and undergo necessary purification treatments.

[0021] The iron source described in step (a) is selected from one or more of ferrous oxalate, ferrous oxide, magnetite, ferrous acetate, and ferrous sulfate. The manganese source is selected from one or more of manganese carbonate, manganese oxalate, manganese oxide, manganese acetate, and manganese carbonate.

[0022] The lithium source described in step (b) is selected from one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, disodium hydrogen phosphate, and lithium phosphate. The phosphorus source is selected from one or more of lithium phosphate, ammonium dihydrogen phosphate, and ammonium hydrogen phosphate.

[0023] In steps (a) and (b), if there are corresponding element intersections among the iron source, manganese source, lithium source, and phosphorus source, different sources can be superimposed and calculated separately.

[0024] The calcination temperature in step (d) is 500 °C to 600 °C, and the time is 5 to 8 h.

[0025] The carbon source described in step (e) is selected from one or more of sucrose, Super P, conductive carbon black, and graphene oxide suspension.

[0026] The calcination temperature in step (e) is 300 °C to 400 °C, the time is 3 to 5 h, and the calcination atmosphere is an inert gas.

[0027] The final product structure is a granular morphology, which is a secondary micron-sized spherical particle aggregated by primary nano-sized particles, and the particle size of the primary nano-sized particles is 40 to 100 nm.

[0028] A lithium-ion battery mainly includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode is mainly the lithium iron manganese phosphate composite material described in claim 1. It is assembled in the order of the negative electrode case, Li sheet, separator, electrolyte, positive electrode sheet, gasket, spring sheet, and positive electrode case. When assembling the coin cell, it should be carried out in a glove box filled with Ar (O2 ≤ 0.1 ppm, H2O ≤ 0.1 ppm). Finally, a packaging machine is used to press it to obtain the assembled coin cell.

[0029] The present invention has the following advantages:

[0030] (1) Alleviates the dual voltage platform effect

[0031] Lanthanum oxide has good thermal stability and chemical stability, which can effectively protect the lithium iron manganese phosphate material, reduce material loss and life attenuation. The surface double coating layer can well block the dissolution of manganese and protect it from the "attack" of HF in the electrolyte; The combination of liquid phase coating and heat treatment enables partial La to be micro-doped and diffused into the internal structure, alleviating the dual voltage platform effect and solving the problem of voltage sudden drop.

[0032] (2) Greatly improves the cycle stability

[0033] This invention is relatively easy to implement through simple mixing and chemical reactions. By using the method of double modification with lanthanum oxide and carbon, the conductivity of the lithium iron manganese phosphate cathode material can be improved, enhancing the material's conductivity. This enables the formation of an excellent conductive network from the interior to the surface of the material particles, improving the charge-discharge capacity and cycle stability of the material.

[0034] (3) High controllability of particle size and strong experimental adjustability

[0035] By adjusting the sanding conditions and raw material ratios, the desired specific lithium iron manganese phosphate material with controllable particle size can be obtained. The synthesis parameters can be optimized according to specific requirements to obtain the characteristics of the desired product.

[0036] (4) Low cost, environmentally friendly, and capable of large-scale processing

[0037] This invention uses easily available raw materials and has simple synthesis steps. The product can be synthesized at 500 - 550 °C without the need for excessively high temperatures, significantly reducing costs and not generating organic solvent pollution to the environment. The synthesis conditions are simple and easy to adjust, enabling batch processing of the material.

[0038] In summary, the synthesis method and modification of lithium iron manganese phosphate provided by the present invention have practicality and economy, and can be widely applied in the field of positive electrodes such as phosphates. Description of the Drawings

[0039] Figure 1 X-ray diffraction patterns of Example 1 and the comparative example;

[0040] Figure 2 It is the transmission electron microscope spectrum of Example 1;

[0041] Figure 3 It is the scanning electron microscope image of Example 2;

[0042] Figure 4 It is the first charge-discharge curve graph of Example 1 and the comparative example;

[0043] Figure 5 It is the cycle performance image (1C current density) of Example 1. Specific embodiments

[0044] The present invention will be further described below in conjunction with specific embodiments and the accompanying drawings. It should be noted that these embodiments are only used to illustrate the present invention and are not used to limit the present invention. In addition, it should be understood that those skilled in the art within the scope of the technical concept of the present invention can make changes, modifications, and substitutions to the present invention, and these simple variations and combinations should also be regarded as the content disclosed by the present invention and all fall within the protection scope of the present invention.

[0045] Example 1

[0046] (1) According to the stoichiometric ratio Li:Mn:Fe:P = 1:0.5:0.5:1, the compounds of the iron source and the manganese source are respectively dissolved in absolute ethanol and mixed under stirring conditions to form a precursor mixed slurry.

[0047] (2) Subsequently, the compounds of the lithium source and the phosphorus source are respectively dissolved in absolute ethanol and mixed under stirring to form a mixed slurry containing only the lithium source and the phosphorus source; the above lithium source, iron source, and phosphorus source can be used interchangeably.

[0048] (3) The TM precursor mixed slurry in step (1) is uniformly added to step (2) under the condition of magnetic stirring to form a mixed slurry, and continuous stirring is carried out during this process to form a mixed slurry;

[0049] (4) Start the sand mill. After setting the cooling value to 10.0 and fully condensing, the mixed liquid obtained in step (3) is slowly introduced into the inlet tank of the sand mill. Set the main shaft speed of the sand mill to 2000 rpm, set the stirring speed at the feed port to 6 - 8 rpm, continuously stir to prevent the material from becoming viscous and carry out sufficient sanding for 10H. After the sanding is completed, collect the slurry to obtain the final precursor slurry;

[0050] (5) Centrifuge the precursor slurry and vacuum dry it for 5H. The dried powder is placed in a tube furnace under an Ar atmosphere for heat treatment, heated to 550°C at a heating rate of 5°C / min, and held for 8h. Then it is cooled to room temperature with the furnace, and pure LMFP is obtained;

[0051] (6) Disperse 5 g of the pure LMFP product in deionized water, and under stirring, disperse 0.243 g of rare earth lanthanum oxide powder and 0.6 g of sucrose in deionized water. The mass fraction of sucrose is 11.989% of the pure LMFP, and then pump it into the above-mentioned LMFP dispersion. The suspension is continuously stirred at 80 °C until the solvent evaporates. The obtained powder is annealed briefly for 4 h in an inert atmosphere at 400 °C. A dense and thin layer of lanthanum oxide forms on the surface of the obtained cathode material, and there is a carbon coating layer outside the lanthanum oxide (the total content of lanthanum oxide is 4.845%, and the final residual carbon content is 4%). Moreover, the crystallinity is improved to achieve partial La micro-doping into the metal sites.

[0052] The lithium manganese iron phosphate cathode material LiMn 0.5 Fe 0.5 PO4@La2O3 / C;

[0053] Figure 1 is the charge-discharge curve of the lithium manganese iron phosphate prepared in Example 1 at a current of 0.1C. Its initial charge specific capacity reaches 160.06 mAh / g, and the discharge specific capacity reaches 153.46 mAh / g. The efficiency is 96.9%

[0054] Comparative example

[0055] (1) According to the stoichiometric ratio Li:Mn:Fe:P = 1:0.5:0.5:1, dissolve the compounds of the iron source and the manganese source in absolute ethanol respectively, and mix them under stirring conditions to form a precursor mixed slurry.

[0056] (2) Subsequently, dissolve the compounds of the lithium source and the phosphorus source in absolute ethanol respectively, and mix them under stirring to form a mixed slurry containing only the lithium source and the phosphorus source; the above lithium source, iron source, and phosphorus source can be used interchangeably.

[0057] (3) Slowly add the precursor mixed slurry in step (1) to step (2) under the condition of magnetic stirring, and continuously stir during this process to form a mixed slurry;

[0058] (4) Start the sand mill. After setting the cooling value to 10.0 and fully condensing, slowly introduce the mixed liquid obtained in step (3) into the inlet tank of the sand mill. Set the main shaft rotation speed of the sand mill to 2000 rpm, set the stirring speed at the feed port to 6 - 8 rpm, continuously stir to prevent the material from becoming viscous, and perform sufficient sanding for 10 h. After sanding is completed, collect the slurry to obtain the final precursor slurry;

[0059] (5) Centrifuge the precursor slurry and dry it under vacuum for 5 h. Place the dried powder in a tube furnace and perform heat treatment under an Ar atmosphere. Heat it to 550 °C at a heating rate of 5 °C / min and hold for 8 h. Then cool it to room temperature with the furnace, and the pure LMFP cathode material is obtained;

[0060] (6) Disperse the pure LMFP product in deionized water, and disperse 0.6 g of sucrose in deionized water with an ultrasonic device under stirring. The mass fraction of sucrose is 11.989% of the pure LMFP. Then pump the above solution into it. The suspension is continuously stirred at 80 °C until the solvent evaporates. Anneal the obtained powder briefly at 400 °C for 4 h. Only a carbon coating layer exists on the surface of the obtained cathode material, and the lithium manganese 0.5 Fe 0.5 PO4 / C is obtained. (The content of lanthanum oxide is 0, and the carbon content is 4%). Use an X-ray powder diffractometer to characterize the materials prepared in Example 1 and the comparative example. The results are as Figure 1 shown. The characteristic peaks of lithium manganese iron phosphate appear in the spectrum, and there are no impurity peaks. It shows that the carbon coating layer on the surface of the cathode material exists in an amorphous form.

[0061] Example 2

[0062] (1) According to the stoichiometric ratio Li:Mn:Fe:P = 1:0.6:0.4:1, dissolve the compounds of iron source and manganese source in absolute ethanol respectively, and mix them under stirring conditions to form a precursor mixed slurry.

[0063] (2) Subsequently, dissolve the compounds of lithium source and phosphorus source in absolute ethanol respectively, and mix them under stirring to form a mixed slurry containing only lithium source and phosphorus source; the above lithium source, iron source and phosphorus source can be used interchangeably.

[0064] Control the stoichiometric molar ratio of manganese and iron sources to be 1.5 in this step.

[0065] (3) Slowly add the TM precursor mixed slurry in step (1) to step (2) under magnetic stirring conditions to form a mixed slurry, and continuously stir during this process to form a mixed slurry;

[0066] (4) Start the sand mill. After setting the cooling value to 10.0 and fully condensing, slowly introduce the mixed liquid obtained in step (3) into the inlet tank of the sand mill. Set the main shaft speed of the sand mill to 2000 rpm, set the stirring speed at the feed port to 6 - 8 rpm, continuously stir to prevent the material from becoming viscous and perform sufficient sanding for 10 h. After sanding is completed, collect the slurry to obtain the final precursor slurry;

[0067] (5) Centrifuge the precursor slurry and vacuum dry it for 5 h. Place the dried powder in a tube furnace and perform heat treatment in an Ar atmosphere. Heat it to 550 °C at a heating rate of 5 °C / min and hold for 8 h. Cool it to room temperature with the furnace, and then the pure LMFP cathode material is obtained.

[0068] (6) Disperse 5 g of the pure LMFP product in deionized water, and disperse 0.243 g of rare earth lanthanum oxide powder and 0.6 g of sucrose in deionized water with stirring using an ultrasonic device. The mass fraction of sucrose is controlled to be 11.989%, and then pump it into the above solution. Continuously stir the suspension at 80 °C until the solvent evaporates. Anneal the obtained powder briefly at 400 °C for 4 h. There is a dense and thin layer of lanthanum oxide and a carbon coating layer on the surface of the obtained cathode material, which improves the crystallinity and realizes partial La micro-doping into the metal sites, and then the lithium manganese iron phosphate cathode material LiMn 0.6 Fe 0.4 PO4@La2O3 / C (where the content of lanthanum oxide is 4.845% and the carbon content is 4%); Use an emission scanning electron microscope to characterize the lithium manganese iron phosphate prepared in Example 2, and the results are as Figure 2 shown. It can be seen that the product of lithium manganese iron phosphate is spherical in morphology, the average particle size of the particles is 40 nm, and the particle size distribution is uniform. Table 1 shows the data statistics of the lithium manganese iron phosphate cathode material prepared in Example 2 at a current density of 0.1C. Its initial charge specific capacity reaches 157.8 mAh / g, and the discharge specific capacity reaches 151.65 mAh / g. The efficiency is 96.1%.

[0069] Example 3

[0070] This example is used to illustrate another type of the present invention with the same treatment method as Example 1, but the stoichiometric molar ratio of the manganese source and the iron source is 3. Different stoichiometric ratios of manganese carbonate and iron phosphate are introduced during the synthesis of the lithium manganese iron phosphate cathode.

[0071] According to the method of Example 1, the difference is that: in step (1), the mass of the added iron phosphate powder is 7.903 g, and the mass of the manganese carbonate powder weighed is 6.348 g. The chemical formula of the final product lithium manganese iron phosphate cathode is LiMn 0.75 Fe 0.25 PO4@La2O3 / C, where the added amount of sucrose is 2.292 g. The test data are shown in Table 1. The charge-discharge curve of the lithium manganese iron phosphate prepared in Example 3 at a current density of 0.1C shows that its initial charge specific capacity reaches 153.25 mAh / g, and the discharge specific capacity reaches 145.35 mAh / g. The efficiency is 94.85%.

[0072] Example 4

[0073] This example is used to illustrate another type of the present invention, which has the same processing method as Example 1, but the stoichiometric molar ratio of the manganese source and the iron source is 5.66. During the synthesis of the lithium manganese iron phosphate cathode, manganese carbonate and ferrous phosphate with different stoichiometric ratios are introduced.

[0074] According to the method of Example 1, the difference is that: in step (1), the mass of the added ferrous phosphate powder is 7.669 g, and the mass of the manganese carbonate powder weighed is 6.582 g. The chemical formula of the final product lithium manganese iron phosphate cathode is LiMn 0.85 Fe 0.15 PO4@La2O3 / C, where the added mass of sucrose is 2.292 g.

[0075] The test data are shown in Table 1. The charge-discharge curve of the lithium manganese iron phosphate prepared in Example 4 at a current of 0.1C shows that its initial charge specific capacity reaches 150.02 mAh / g, the discharge specific capacity reaches 140.29 mAh / g, and the efficiency is 93.51%. From the analysis of the test results, as the stoichiometric molar ratio of the manganese and iron sources increases, the capacity of the product shows a gradually decreasing trend.

[0076] Example 5

[0077] This example is used to illustrate another type of the present invention, which has the same processing method as Example 1, but the mass of the used sucrose is different, and lithium manganese iron phosphate with different carbon coating amounts is synthesized after solid-phase sintering.

[0078] According to the method of Example 1, the difference is that: in step (3), the mass of the added sucrose is 1.965 g, and the mass fraction is controlled to be 6.03%. The chemical formula of the product lithium manganese iron phosphate cathode is LiMn 0.85 Fe 0.15 PO4@La2O3 / C.

[0079] Example 6

[0080] This example is used to illustrate another type of the present invention, which has the same processing method as Example 1, but the carbon coating amount is different, the mass of the used sucrose powder is different, and lithium manganese iron phosphate with different carbon contents is synthesized after solid-phase sintering.

[0081] According to the method of Example 1, the difference is that: in step (2), the mass of the added sucrose powder is 4.077 g, and the mass fraction is controlled to be 17.98%. The final chemical formula of lithium manganese iron phosphate is LiMn 0.5 Fe 0.5 PO4@La2O3 / C.

[0082] Example 7

[0083] This example is used to illustrate another type of the present invention, which has the same processing method as Example 1, but different carbon content, different mass of sucrose powder used, and different lithium iron phosphate manganese with different carbon contents are synthesized after solid-phase sintering is completed.

[0084] According to the method of Example 1, the difference is that: the mass of sucrose powder added in step (2) is 4.093 g, and the mass fraction is controlled to be 23.276%. The final chemical formula of lithium iron phosphate manganese is LiMn 0.5 Fe 0.5 PO4@La2O3 / C.

[0085] Table 1 General formula and performance comparison data of lithium iron phosphate manganese prepared in Examples 1-8 and Comparative Examples (where the mass percentage content of La2O3 in the examples is fixed at 4.845%, and the content of lanthanum oxide in the comparative example is 0)

[0086]

Claims

1. A modified lithium iron phosphate cathode material modified by double coating of lanthanum oxide and carbon, characterized in that, Having the general formula of formula (Ⅰ): LiMn a Fe b PO4@La2O3 / C c Formula (Ⅰ); 0.5 ≤ a ≤ 0.9, 0.1 ≤ b ≤ 0.5, 0.02 ≤ c ≤ 0.08, and a + b = 1; The mass percentage of La2O3 in lithium iron manganese phosphate is 3 - 5%. La2O3 and carbon form a double protective layer. First, a protective layer of La2O3 is formed, and then a carbon coating layer is formed on the outer layer.

2. The modified lithium iron phosphate cathode material modified by double coating of lanthanum oxide and carbon according to claim 1, characterized in that In formula (I), 0.5 ≤ a ≤ 0.85, 0.15 ≤ b ≤ 0.5, 0.02 ≤ c ≤ 0.

04.

3. The preparation method of a modified lithium iron phosphate manganese oxide cathode material modified by double coating of lanthanum oxide and carbon according to claim 1 or 2, characterized in that, It includes the following steps: (a) Dissolve the iron source and manganese source in anhydrous ethanol successively and mix them under stirring conditions to form a precursor mixed slurry A; (b) Dissolve the lithium source and phosphorus source compound in anhydrous ethanol successively and mix them under stirring conditions to form a precursor mixed slurry B; (c) Slowly add the mixed slurry A to the mixed slurry B under stirring conditions to form a mixture; (d) Introduce the mixture obtained in step (c) into a sand mill, fully stir and sand mill. After sand milling, collect the slurry to obtain the final precursor slurry C; The slurry C is centrifuged and separated, placed in a vacuum oven to dry, and the obtained powder material is calcined in an inert atmosphere and cooled with the furnace to obtain pure lithium iron manganese phosphate; (e) Disperse the lithium iron manganese phosphate obtained in step (d) in deionized water, disperse lanthanum oxide powder and carbon source powder in deionized water under ultrasonic waves in a stirring state, and then pump them into the above lithium iron manganese phosphate dispersion; Stir continuously at 80 °C until the solvent evaporates; Anneal the obtained powder material to obtain a lithium iron manganese phosphate cathode material modified with lanthanum oxide and carbon. The surface of the obtained cathode material has a very dense and thin lanthanum oxide and carbon coating layer, and micro-doping and improved crystallinity are achieved.

4. The method according to claim 3, characterized in that In step (a), the iron source is selected from one or more of ferrous oxalate, ferrous oxide, magnetite, ferrous acetate, and ferrous sulfate; The manganese source is selected from one or more of manganese carbonate, manganese oxalate, manganese oxide, manganese acetate, and manganese carbonate; In step (b), the lithium source is selected from one or more of lithium carbonate, lithium hydroxide, lithium dihydrogen phosphate, lithium hydrogen phosphate, and lithium phosphate; The phosphorus source is selected from one or more of lithium phosphate, monoammonium phosphate, and diammonium hydrogen phosphate; In steps (a) and (b), if there are corresponding element intersections among the iron source, manganese source, lithium source, and phosphorus source, different sources can be superimposed and calculated respectively.

5. The method according to claim 3, characterized in that, In step (d), the temperature of the calcination is 500 °C to 600 °C, and the time is 5 to 8 h.

6. The method according to claim 3, characterized in that, In step (e), the carbon source is selected from one or more of sucrose, Super P, conductive carbon black, and graphene oxide suspension.

7. The method according to claim 3, characterized in that, In step (e), the temperature of the calcination is 300 °C to 400 °C, the time is 3 to 5 h, and the calcination atmosphere is an inert gas.

8. The method according to claim 3, characterized in that, The structure of the final product is a granular morphology, which is a secondary micron-sized spherical particle aggregated by primary nano-sized particles, and the particle size of the primary nano-sized particles is 40 - 100 nm.

9. A lithium-ion battery mainly includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode includes the lithium iron manganese phosphate composite material described in claim 1 or 2. It is assembled in the order of negative electrode case, Li sheet, separator, electrolyte, positive electrode sheet, gasket, spring sheet, and positive electrode case. When assembling the button battery, it should be carried out in a glove box filled with Ar (O2 ≤ 0.1 ppm, H2O ≤ 0.1 ppm), and finally pressed with a packaging machine to obtain the assembled button battery.

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