Surface modification method of lithium iron manganese phosphate positive electrode material

Abstract

The invention belongs to the field of preparation of a lithium iron manganese phosphate positive electrode material, and relates to a surface modification method of the lithium iron manganese phosphate positive electrode material. The surface of the lithium manganese iron phosphate positive electrode material is modified through the mild acid steam, and the carboxyl functional group reacts with the residual lithium salt on the surface of the positive electrode material, so that side reactions are reduced, and the interface stability is improved. According to traditional acid treatment modification, materials are generally subjected to acid pickling through strong acid, then acid pickling is carried out, and the structure of the materials is extremely prone to being damaged through the traditional method. According to the method, the positive electrode material is modified by utilizing the acid steam, secondary treatment is not needed, the process is simple and efficient, the cost is relatively low, and the method is a method for effectively inhibiting the capacity fading of the lithium iron manganese phosphate positive electrode material in the circulation process.

Description

Technical Field

[0001] The invention belongs to the field of preparation of lithium iron manganese phosphate positive electrode materials and relates to a surface modification method of lithium iron manganese phosphate positive electrode materials. Background Art

[0002] In the current power battery field, the mainstream cathode materials are olivine-structured lithium iron phosphate and layered ternary nickel-cobalt-manganese materials (NCM). As a cathode development material based on lithium iron phosphate, lithium iron manganese phosphate has a higher voltage platform than lithium iron phosphate due to the high voltage characteristics of manganese, which also leads to its higher energy density. However, the disadvantage is that the introduction of manganese significantly reduces the conductivity of the material. Compared with the layered structure of the ternary NCM material, lithium iron manganese phosphate has a more stable structure, better safety, and lower cost during the charging and discharging process.

[0003] However, it still faces many obstacles on the road to commercialization. Lithium manganese iron phosphate has a unique manganese precipitation problem. During the battery charging and discharging process, the manganese element in the positive electrode material will undergo a valence change, thereby triggering the Jahn-Teller effect and causing the dissolution of manganese ions. In this way, the positive electrode will form a manganese-deficient phase, the structure will collapse, and the normal deintercalation process of lithium ions will be hindered, thereby increasing the polarization degree of the battery; in addition, the manganese element dissolved in the electrolyte may be deposited on the surface of the negative electrode, destroying the original solid electrolyte interface (SEI) layer structure. In order to maintain battery performance, the SEI layer needs to be continuously repaired and regenerated. This process consumes a large amount of active lithium, resulting in a decrease in battery capacity and a reduction in cycle life.

[0004] In order to solve these problems and promote the commercialization of manganese-based positive electrode materials, researchers have adopted many modification methods, such as surface coating, ion doping and material particle nano-sizing. Recently, some scholars reported that pre-treatment and modification of positive electrode materials by acid treatment provides a method to reduce or eliminate the initial capacity loss. However, after the sample was treated with high-concentration acid, the electrode surface deteriorated rapidly during the cycle. Summary of the invention

[0005] Based on the technical problems existing in the background, in the present invention, a mild acid treatment method is adopted to treat the positive electrode material with acid vapor for different time periods, which can effectively improve the initial coulombic efficiency and cycle performance of the material. The specific scheme is as follows:

[0006] A surface modification method for a lithium iron manganese phosphate positive electrode material comprises the following steps: placing a synthesized lithium iron manganese phosphate positive electrode material above an acid solution, heating the acid solution to form acid vapor for surface modification, drying the treated material, and then performing a heat treatment to obtain an acid-treated lithium iron manganese phosphate product, wherein the acid solution comprises an acid, and the acid is a non-strong acid.

[0007] The chemical formula of the lithium manganese iron phosphate is LiFe x Mn 1-x PO4,0<x<1.

[0008] The acid is selected from one or more of ascorbic acid, acrylic acid, lactic acid, propionic acid, stearic acid, malic acid, and gluconic acid.

[0009] The acid solution refers to an acid completely dissolved in an organic solvent, and the organic solvent is selected from one or more of ethanol, methanol, glycerol, acetone, ethylene glycol, and NMP.

[0010] The concentration of the acid solution is 0.1-1 mol / L.

[0011] The temperature of the acid solution heating is 40-80°C.

[0012] The acid vapor is used for surface modification for 10-20 hours.

[0013] The heat treatment temperature is 200° C.-400° C., the heat treatment time is 2 h-5 h, and the heat treatment is carried out under a protective atmosphere.

[0014] Beneficial effects of the present invention:

[0015] The present invention modifies the surface of the lithium iron manganese phosphate positive electrode material by mild acid vapor. The carboxyl functional group reacts with the residual lithium salt on the surface of the positive electrode material to reduce the occurrence of side reactions and improve the interface stability. Traditional acid treatment modification generally uses a strong acid to pickle the material and then removes the acid. This traditional method can easily destroy the structure of the material. The present invention uses acid vapor to modify the positive electrode material without the need for secondary treatment. The process is simple, efficient and low in cost. It is a method that effectively inhibits the capacity decay of the lithium iron manganese phosphate positive electrode material during the cycle process. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the technical solution of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application.

[0017] Figure 1 This is a scanning electron microscope image of the modified lithium manganese iron phosphate positive electrode material provided in Example 1;

[0018] Figure 2 This is a scanning electron microscope image of the unmodified lithium manganese iron phosphate positive electrode material of Comparative Example 1. DETAILED DESCRIPTION

[0019] It should be noted that, in the absence of conflict, the embodiments and features in the embodiments of the present application can be combined with each other. The technical solutions in the embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings in the embodiments of the present invention.

[0020] Example 1

[0021] Disperse 0.5 mol of acrylic acid in 1 L of alcohol solution to prepare a 0.5 mol / L acrylic acid solution, and heat it in an oil bath at 50°C; take 10 g of lithium iron manganese phosphate positive electrode material and evenly spread it on the filter paper above the acrylic acid solution, and treat it with acrylic acid vapor for 15 hours. The treated material is placed in a drying oven and dried at 70°C, and then treated at 300°C for 3 hours under high-purity nitrogen to obtain the lithium iron manganese phosphate positive electrode material surface-modified by acrylic acid vapor.

[0022] Example 2

[0023] The difference between this embodiment and embodiment 1 is that acrylic acid is replaced by malic acid.

[0024] Example 3

[0025] The difference between this embodiment and embodiment 1 is that acrylic acid is replaced by gluconic acid.

[0026] Example 4

[0027] The difference between this embodiment and embodiment 1 is that the concentration of the acrylic acid solution is increased from 0.5 mol / L to 1 mol / L.

[0028] Example 5

[0029] The difference between this embodiment and embodiment 1 is that the acrylic acid steam treatment time is adjusted from 15 hours to 20 hours.

[0030] Comparative Example 1

[0031] The difference from Example 1 is that the lithium manganese iron phosphate is not modified.

[0032] Comparative Example 2

[0033] The difference from Example 1 is that the lithium manganese iron phosphate material is treated by immersing it in an acrylic acid solution.

[0034] Weigh the active material, carbon black and polyvinylidene fluoride (PVDF) in a mass ratio of 8:1:1. First, dissolve PVDF in an appropriate amount of NMP and stir magnetically until the solution becomes transparent. Then add the positive active material and conductive carbon black to the above solution and stir for 12 hours. Spread the slurry on aluminum foil and bake it in a vacuum drying oven at 120℃ for 12 hours before taking it out. Cut the positive electrode sheet into discs with a diameter of 12mm, complete the assembly of the lithium-ion half-cell for testing in the glove box, and let it stand at room temperature for 12 hours for electrochemical testing. The results of the electrical performance test are shown in Table 1.

[0035] Table 1

[0036]

[0037]

[0038] pass Figure 1 and Figure 2 Compared with the scanning electron microscope images, the overall structure of the material shows a complete secondary sphere without obvious changes. However, it can be seen on the surface of the sphere that after the acrylic acid vapor treatment, the surface of the material particles is smoother, reducing the side reaction between the material surface and the electrolyte.

[0039] From the data in Table 1, it can be seen that compared with Comparative Example 1, the initial coulombic efficiency and capacity retention rate of the material after acid vapor treatment are improved. This may be because the carboxyl functional group combines with the residual lithium salt on the surface of the positive electrode material to reduce the probability of side reactions. At the same time, the generated product is coated on the surface of the material, which can also better protect the electrode material during the cycle and inhibit the side reaction between the electrode and the electrolyte. Therefore, acid vapor treatment can effectively improve the cycle performance and first coulombic efficiency of the material. However, since Comparative Example 2 adopts the direct immersion method, the damage of the acid solution to the material surface cannot be effectively controlled, and the final material performance is poor.

[0040] The present invention combines the organic weak acid carboxyl functional group with the residual lithium salt on the surface of the positive electrode material, thereby reducing the probability of side reactions. At the same time, the generated product is directly coated on the surface of the material (without deliberate cleaning), which can also better protect the electrode material during the cycle process and inhibit the side reaction between the electrode and the electrolyte.

[0041] The above implementation modes are only for illustrating the technical concept and features of the present invention, and their purpose is to enable people familiar with this technology to understand the content of the present invention and implement it, and they cannot be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the spirit of the present invention should be included in the protection scope of the present invention.

Claims

1. A surface modification method for lithium manganese iron phosphate positive electrode material, characterized in that: The method comprises the following steps: placing a synthesized lithium iron manganese phosphate positive electrode material above an acid solution, heating the acid solution to form acid vapor for surface modification, drying the treated material, and then performing heat treatment to obtain an acid-treated lithium iron manganese phosphate product, wherein the acid solution comprises an acid, and the acid is a non-strong acid.

2. The surface modification method of a lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The acid is selected from one or more of ascorbic acid, acrylic acid, lactic acid, propionic acid, stearic acid, malic acid, and gluconic acid.

3. The surface modification method of a lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The acid solution refers to an acid completely dissolved in an organic solvent, and the organic solvent is selected from one or more of ethanol, methanol, glycerol, acetone, ethylene glycol, and NMP.

4. The surface modification method of a lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The concentration of the acid solution is 0.1-1 mol / L.

5. The surface modification method of a lithium manganese iron phosphate positive electrode material according to claim 1, characterized in that: The temperature of the acid solution heating is 40-80°C.

6. The surface modification method of a lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The acid vapor is used for surface modification for 10-20 hours.

7. The surface modification method of a lithium iron manganese phosphate positive electrode material according to claim 1, characterized in that: The heat treatment temperature is 200° C.-400° C., the heat treatment time is 2 h-5 h, and the heat treatment is carried out under a protective atmosphere.

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