Preparation method of lithium manganese iron phosphate positive electrode material uniformly coated with small organic molecules and lithium manganese iron phosphate positive electrode material

Abstract

The invention discloses a preparation method of a lithium manganese iron phosphate positive electrode material uniformly coated with small organic molecules, which comprises the following steps: dissolving p-phenylenediacetylene dicarboxylic acid in a solvent, mixing with lithium manganese iron phosphate, uniformly adsorbing the coated material on the surface of the material, and stabilizing the coating layer through heat treatment. According to the invention, p-phenylenediacetylene dicarboxylic acid is synthesized, carboxyl of small organic molecules and metal ions on the surface of lithium manganese iron phosphate form coordinate bonds, the coordinate bonds are uniformly and stably modified on the surface of lithium manganese iron phosphate, and conjugated benzene rings and alkyne structures improve the electronic conductivity of the material; after coating, a conductive network is formed on the surface of the material, so that electron transmission is improved, and the electrochemical performance of the electrode material is improved. The conductivity of the lithium manganese iron phosphate material modified by the method is increased, the specific capacity and the rate capability are remarkably improved, the first charge-discharge specific capacity of the lithium manganese iron phosphate material can reach 155.8-156.8 mAh / g, and the specific capacity of the lithium manganese iron phosphate material under 2C rate can reach 140.2-141.2 mAh / g.

Description

Technical Field

[0001] The present invention relates to a preparation method of a lithium iron manganese phosphate positive electrode material uniformly coated with organic small molecules. The modified lithium iron manganese phosphate can be used as a lithium ion battery positive electrode material for manufacturing various types of power batteries. Background Art

[0002] Lithium manganese iron phosphate (LiMnFePO4, LMFP), as a cathode material, holds broad application prospects in lithium-ion batteries due to its high safety, low cost, and environmental friendliness. However, LMFP's low electronic conductivity and ion diffusion rate limit its performance under high-rate charge and discharge conditions. To overcome these shortcomings, researchers have proposed various modification methods, among which surface coating is an effective strategy. Surface coating can improve the material's electronic conductivity, inhibit side reactions, and enhance structural stability, thereby enhancing its electrochemical performance.

[0003] At present, surface coating mainly includes inorganic coating materials, such as carbon, metal oxides, etc. Although they can improve conductivity, the coating uniformity is poor and impurities may be introduced. The second type of polymer coating: such as polypyrrole, polyaniline, etc., although they can improve conductivity, have poor high-temperature stability and a thicker coating layer. The third type of organic small molecule coating, organic small molecules have good chemical stability and adjustable physical and chemical properties, and can be uniformly coated on the surface of lithium manganese iron phosphate by chemical bonding or physical adsorption. Compared with traditional coating materials, the organic small molecule coating layer can cover the material surface more evenly, reduce particle agglomeration, and increase the specific surface area and electrochemical activity of the material. In addition, organic small molecules can introduce functional groups through molecular design to further optimize the electrochemical properties of the material. Therefore, the development of a preparation method for uniformly coating lithium manganese iron phosphate positive electrode materials with organic small molecules is of great significance for improving material performance and promoting its commercial application. Summary of the Invention

[0004] This invention provides a method for modifying lithium manganese iron phosphate. This method generates small molecules of terephthalic acid through coupling and oxidation reactions. These molecules are then chemically bonded to the surface of lithium manganese iron phosphate nanoparticles, improving both the material's conductivity and its electrochemical stability. The specific technical solution is as follows: A method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules comprises the following steps: S1: Mix lithium manganese iron phosphate powder and terephthalic acid in a certain mass ratio and ball mill for 5-10 hours; S2: Take out the sample, sieve it, and heat and keep it in an inert atmosphere to stabilize the coating layer, thereby obtaining a lithium manganese iron phosphate positive electrode material uniformly coated with small molecules.

[0005] The preparation method of terephthalic acid comprises the following steps: (1): Dissolve p-iodobenzoic acid, phenylacetylene, Pd(PPh3)2Cl2, CuI and triethylamine in anhydrous DMF and stir at room temperature to react; (2): After the reaction is completed, the reaction mixture is neutralized with dilute hydrochloric acid, the product is extracted with ethyl acetate, the organic phase is dried and concentrated, and purified by column chromatography or recrystallization to obtain terephthalic acid; (3): Dissolve terephthalic acid in NaOH solution, add excess potassium permanganate, and heat under reflux for 4-6 hours; (4): After the reaction is completed, the insoluble matter is removed by filtration, the product is acidified with dilute hydrochloric acid, filtered, washed and dried to obtain terephthalic acid.

[0006] The ratio of p-iodobenzoic acid, phenylacetylene, Pd(PPh3)2Cl2, CuI and triethylamine is 1:1.2~1.5:0.01:0.02~0.1:2~3.

[0007] (Pd(PPh3)2Cl2 is the core component of the palladium catalyst, providing active Pd(0) species; CuI is a co-catalyst, forming a copper-alkyne complex (Cu-alkyne) with the terminal alkyne, reducing the pK of the alkyne CH a , accelerating its deprotonation and promoting the palladium catalytic cycle; triethylamine is a base: neutralizing the protons of terminal alkynes (such as acetylenedicarboxylate) to generate alkyne anions (CuI assists this process), and the second is a reducing agent: reducing Pd(II) (such as Pd(PPh3)2Cl2) in situ to active Pd(0) species.

[0008] The concentration of the dilute hydrochloric acid is 1-5% (mass fraction), ensuring that the final pH is close to neutral (5-7).

[0009] The ratio of terephthalic acid: sodium hydroxide: potassium permanganate is 1:2-2.5:3-3.5, and the mass concentration of the sodium hydroxide solution is 10-20%.

[0010] The concentration of the dilute hydrochloric acid is about 5-10% (mass fraction), and the final acidification is to pH=2-3.

[0011] The ratio of lithium manganese iron phosphate to terephthalic acid is 1: (1-5).

[0012] The heating and holding temperature is 150-300° C., and the holding time is 2-5 hours.

[0013] The principle of the invention is to react p-iodobenzoic acid with phenylacetylene through a Sonogashira coupling reaction to form p-terephthalyne benzoic acid. The phenylacetylene group is oxidized to a carboxyl group, resulting in a small molecule of p-terephthalyne dicarboxylic acid. The small molecule is then mixed with a lithium iron manganese phosphate cathode material, uniformly coating the material surface with a coating material, and then stabilizing the coating layer through heat treatment. The conjugated benzene ring and alkyne structure of the modified lithium iron manganese phosphate improve the material's electronic conductivity, and the formation of chemical bonds has better electrical properties than physical coating.

[0014] The beneficial effects of the present invention are as follows: 1. By introducing conjugated benzene rings and alkyne structures, the carboxyl group (-COOH) forms a coordination bond with the metal ions (Mn / Fe) on the surface of lithium manganese iron phosphate, forming a uniform organic coating layer on the surface of the material, significantly improving the electronic conductivity of the lithium manganese iron phosphate positive electrode material, thereby improving the battery's rate performance.

[0015] 2. The introduction of the conjugated structure enhances the overall structural stability of the material, reduces the interfacial charge transfer impedance, inhibits the structural collapse or phase change of the material during the charge and discharge process, and improves the safety and reliability of the battery.

[0016] 3. Through heat treatment, the organic small molecule coating layer forms a chemical bond with the surface of lithium manganese iron phosphate, which has higher stability than physical coating and effectively prevents the coating layer from falling off or decomposing during the charging and discharging process. DETAILED DESCRIPTION

[0017] The present invention is not limited to the scope of the present invention, and those skilled in the art can make some modifications and improvements based on the content of the above invention.

[0018] Example 1 A method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules, comprising the following steps: (1) Weigh 1 g of p-iodobenzoic acid, 1.2 g of phenylacetylene, 0.01 g of Pd(PPh3)2Cl2, 0.02 g of CuI, and 3 g of triethylamine and dissolve them in anhydrous DMF. Stir at room temperature for 24 h. (2) After the reaction is completed, the reaction mixture S1 is neutralized with 1% dilute hydrochloric acid to a pH of 5, the product is extracted with ethyl acetate, the organic phase is dried and concentrated, and purified by column chromatography or recrystallization to obtain terephthalic acid; (3) Dissolve 1 g of the S2 product terephthalic acid in a 10% alkaline aqueous solution of NaOH, add 3 g of potassium permanganate, and heat under reflux for 4-6 hours; (4) Filter and remove insoluble matter in S3, acidify with 10% dilute hydrochloric acid, filter, wash and dry to obtain terephthalic acid; (5) Weigh 1 g of lithium manganese iron phosphate powder and 1 g of terephthalic acid, place them in a ball mill, and ball mill for 8 hours; (6) The sample was taken out and sieved to remove the grinding medium. The sample was heated in an inert atmosphere for 4 hours at a temperature of 150°C to obtain a lithium manganese iron phosphate positive electrode material uniformly coated with small molecules.

[0019] Example 2 A method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules, comprising the following steps: (1) Weigh 1 g of p-iodobenzoic acid, 1.5 g of phenylacetylene, 0.01 g of Pd(PPh3)2Cl2, 0.1 g of CuI, and 3 g of triethylamine and dissolve them in anhydrous DMF. Stir at room temperature for 24 h. (2) After the reaction is completed, the reaction mixture S1 is neutralized with 1% dilute hydrochloric acid to a pH of 5, the product is extracted with ethyl acetate, the organic phase is dried and concentrated, and purified by column chromatography or recrystallization to obtain terephthalic acid; (3) Dissolve 1 g of the S2 product terephthalic acid in a 10% alkaline aqueous solution of NaOH, add 3 g of potassium permanganate, and heat under reflux for 4-6 hours; (4) Filter and remove insoluble matter in S3, acidify with 10% dilute hydrochloric acid, filter, wash and dry to obtain terephthalic acid; (5) Weigh 1 g of lithium manganese iron phosphate powder and 1 g of terephthalic acid, place them in a ball mill, and ball mill for 8 hours; (6) The sample was taken out and sieved to remove the grinding medium. The sample was heated in an inert atmosphere for 4 hours at a temperature of 150°C to obtain a lithium manganese iron phosphate positive electrode material uniformly coated with small molecules.

[0020] Example 3 A method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules, comprising the following steps: (1) Weigh 1 g of p-iodobenzoic acid, 1.5 g of phenylacetylene, 0.01 g of Pd(PPh3)2Cl2, 0.02 g of CuI, and 2 g of triethylamine and dissolve them in anhydrous DMF. Stir at room temperature for 24 h. (2) After the reaction is completed, the reaction mixture S1 is neutralized with 3% dilute hydrochloric acid to a pH of 5, the product is extracted with ethyl acetate, the organic phase is dried and concentrated, and purified by column chromatography or recrystallization to obtain terephthalic acid; (3) Dissolve 1 g of the S2 product terephthalic acid in a 10% alkaline aqueous solution of NaOH, add 3 g of potassium permanganate, and heat under reflux for 4-6 hours; (4) Filter and remove insoluble matter in S3, acidify with 10% dilute hydrochloric acid, filter, wash and dry to obtain terephthalic acid; (5) Weigh 1 g of lithium manganese iron phosphate powder and 2 g of terephthalic acid, place them in a ball mill, and ball mill for 8 hours; (6) The sample was taken out and sieved to remove the grinding medium. The sample was heated in an inert atmosphere for 4 hours at a temperature of 150°C to obtain a lithium manganese iron phosphate positive electrode material uniformly coated with small molecules.

[0021] Example 4 A method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules, comprising the following steps: (1) Weigh 1 g of p-iodobenzoic acid, 1.5 g of phenylacetylene, 0.01 g of Pd(PPh3)2Cl2, 0.02 g of CuI, and 2 g of triethylamine and dissolve them in anhydrous DMF. Stir at room temperature for 24 h. (2) After the reaction is completed, the reaction mixture S1 is neutralized with 3% dilute hydrochloric acid to a pH of 5, the product is extracted with ethyl acetate, the organic phase is dried and concentrated, and purified by column chromatography or recrystallization to obtain terephthalic acid; (3) Dissolve 1 g of the S2 product terephthalic acid in a 10% alkaline aqueous solution of NaOH, add 3 g of potassium permanganate, and heat under reflux for 4-6 hours; (4) Filter and remove insoluble matter in S3, acidify with 10% dilute hydrochloric acid, filter, wash and dry to obtain terephthalic acid; (5) Weigh 1 g of lithium manganese iron phosphate powder and 5 g of terephthalic acid, place them in a ball mill, and ball mill for 8 hours; (6) The sample was taken out and sieved to remove the grinding medium. The sample was heated in an inert atmosphere for 4 hours at a temperature of 300°C to obtain a lithium manganese iron phosphate positive electrode material uniformly coated with small molecules.

[0022] Example 5 A method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules, comprising the following steps: (1) Weigh 1 g of p-iodobenzoic acid, 1.4 g of phenylacetylene, 0.01 g of Pd(PPh3)2Cl2, 0.5 g of CuI and 3 g of triethylamine and dissolve them in anhydrous DMF. Stir at room temperature for 24 h. (2) After the reaction is completed, the reaction mixture S1 is neutralized with 5% dilute hydrochloric acid to a pH of 5, the product is extracted with ethyl acetate, the organic phase is dried and concentrated, and purified by column chromatography or recrystallization to obtain terephthalic acid; (3) Dissolve 1 g of the S2 product terephthalic acid in a 10% alkaline aqueous solution of NaOH, add 2 g of potassium permanganate, and heat under reflux for 4-6 hours; (4) Filter and remove insoluble matter in S3, acidify with 10% dilute hydrochloric acid, filter, wash and dry to obtain terephthalic acid; (5) Weigh 1 g of lithium manganese iron phosphate powder and 4 g of terephthalic acid, place them in a ball mill, and ball mill for 8 hours; (6) The sample was taken out and sieved to remove the grinding medium. The sample was heated in an inert atmosphere for 4 hours at a temperature of 300°C to obtain a lithium manganese iron phosphate positive electrode material uniformly coated with small molecules.

[0023] Comparative Example 1 The steps of this embodiment are basically the same as those of embodiment 1, except that 1 g of potassium permanganate is added in step (3) and the target temperature in step (6) is 400°C.

[0024] Comparative Example 2 The steps of this embodiment are basically the same as those of embodiment 1, except that: in step (5), 2 g of lithium manganese iron phosphate powder and 1 g of terephthalic acid are used.

[0025] Comparative Example 3 A method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules, comprising the following steps: (1) Weigh 1 g of p-iodobenzoic acid, 1.2 g of phenylacetylene, 0.01 g of Pd(PPh3)2Cl2, 0.02 g of CuI, and 3 g of triethylamine and dissolve them in anhydrous DMF. Stir at room temperature for 24 h. (2) After the reaction is completed, the reaction mixture S1 is neutralized with 1% dilute hydrochloric acid to a pH of 5, the product is extracted with ethyl acetate, the organic phase is dried and concentrated, and purified by column chromatography or recrystallization to obtain terephthalic acid; (3) Dissolve 1 g of the S2 product terephthalic acid in a 10% alkaline aqueous solution of NaOH, add 3 g of potassium permanganate, and heat under reflux for 4-6 hours; (4) Filter and remove insoluble matter in S3, acidify with 10% dilute hydrochloric acid, filter, wash and dry to obtain terephthalic acid; (5) Weigh 1 g of lithium manganese iron phosphate powder and 1 g of terephthalic acid, place them in a ball mill, and ball mill for 8 hours.

[0026] Comparative Example 4 A method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules, comprising the following steps: (1) Weigh 1 g of p-iodobenzoic acid, 1.2 g of phenylacetylene, 0.01 g of Pd(PPh3)2Cl2, 0.02 g of CuI, and 3 g of triethylamine, dissolve them in anhydrous DMF, and stir at room temperature for 24 h. (2) After the reaction is completed, the reaction mixture S1 is neutralized with 1% dilute hydrochloric acid to a pH of 5, the product is extracted with ethyl acetate, the organic phase is dried and concentrated, and purified by column chromatography or recrystallization to obtain terephthalic acid; (3) Weigh 1 g of lithium manganese iron phosphate powder and 1 g of terephthalic acid, place them in a ball mill, and ball mill for 8 hours; (6) The sample was taken out and sieved to remove the grinding medium. The sample was heated in an inert atmosphere for 4 hours at a temperature of 150°C to obtain a lithium manganese iron phosphate positive electrode material uniformly coated with small molecules.

[0027] The resulting small molecule-coated lithium manganese iron phosphate was used as the positive electrode material and assembled into button-type batteries for electrochemical performance testing. The active material, conductive carbon black, and binder (such as PVDF) were mixed in appropriate proportions, then ground into a uniform slurry with the addition of a solvent (NMP). The slurry was then evenly coated onto the current collector (positive electrode aluminum foil / negative electrode copper foil). The slurry was then vacuum-dried at 80-120°C for 12 hours to remove the solvent. The electrode sheets were punched into circular shapes (typically 12-14 mm in diameter, slightly smaller than the inner diameter of the battery case) using a punching machine. The negative electrode case, nickel foam, and lithium sheet were then placed separately. After the electrolyte was added, the separator was placed. A small amount of electrolyte was then added, and the positive electrode material was placed, and the battery case was assembled. The test results are shown in Table 1.

[0028] Table 1:

[0029] As shown in Table 1, the method of coating lithium manganese iron phosphate with a small molecule of terephthalic acid provided by the present invention can effectively improve the conductivity of the material. Examples 1 to 5 all have good specific discharge capacities. At a rate of 0.1C, the specific discharge capacity can reach 155.8 to 156.8 mAh / g, with an initial efficiency of up to 99.4%. The electrochemical performance of Comparative Example 3 decreased, indicating that the ratio of lithium manganese iron phosphate powder to terephthalic acid should be appropriate, and the best effect is achieved when the ratio of lithium manganese iron phosphate powder to terephthalic acid is 1:2. In addition, the electrical performance of the physical mixing method of lithium manganese iron phosphate powder and terephthalic acid in Comparative Example 3 is lower than that of Examples 1 to 5 with heat treatment, indicating that the formation of a uniform carbon coating layer by heat treatment significantly improves the conductivity of the material. In this experiment, by synthesizing a small molecule of terephthalic acid and mixing it with lithium manganese iron phosphate, the coating material is uniformly adsorbed on the surface of the material, and then the coating layer is stabilized by heat treatment, effectively improving the electrochemical performance of lithium manganese iron phosphate.

[0030] The above description is merely a preferred embodiment of the present invention, but the present invention is not limited thereto. Various modifications or improvements may be made within the technical scope of the present invention, and such modifications and improvements fall within the scope of protection of the present invention. Therefore, the scope of protection claimed in the present invention shall be subject to the appended claims.

Claims

1. A method for preparing a positive electrode material of lithium manganese iron phosphate uniformly coated with organic small molecules, characterized in that: The steps include: S1: Mix lithium manganese iron phosphate powder and terephthalic acid in a certain mass ratio and ball mill for 5-10 hours; S2: Take out the sample, sieve it, and heat and keep it in an inert atmosphere to stabilize the coating layer, thereby obtaining a lithium manganese iron phosphate positive electrode material uniformly coated with small molecules.

2. The method for preparing a positive electrode material uniformly coated with organic small molecules according to claim 1, characterized in that: The preparation method of terephthalic acid comprises the following steps: (1): Dissolve p-iodobenzoic acid, phenylacetylene, Pd(PPh3)2Cl2, CuI and triethylamine in anhydrous DMF and stir at room temperature to react; (2): After the reaction is completed, the reaction mixture is neutralized with dilute hydrochloric acid, the product is extracted with ethyl acetate, the organic phase is dried and concentrated, and purified by column chromatography or recrystallization to obtain terephthalic acid; (3): Dissolve terephthalic acid in NaOH solution, add excess potassium permanganate, and heat under reflux for 4-6 hours; (4): After the reaction is completed, the insoluble matter is removed by filtration, the product is acidified with dilute hydrochloric acid, filtered, washed and dried to obtain terephthalic acid.

3. The method for preparing a positive electrode material uniformly coated with organic small molecules according to claim 2, characterized in that: In the step (1), the mass ratio of p-iodobenzoic acid, phenylacetylene, Pd(PPh3)2Cl2, CuI and triethylamine is 1:1.2~1.5:0.01:0.02~0.1:2~3.

4. The method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules according to claim 2, characterized in that: The mass concentration of dilute hydrochloric acid in step (2) is 1-5%, and the pH value is neutralized to 5-7.

5. The method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules according to claim 2, characterized in that: The ratio of terephthalic acid: sodium hydroxide: potassium permanganate in (3) is 1:2-2.5:3-3.5, and the mass concentration of the sodium hydroxide solution is 10-20%.

6. The method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules according to claim 1, characterized in that: The mass concentration of the dilute hydrochloric acid in (4) is 5-10%, and the final acidification is to pH = 2-3.

7. The method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules according to claim 1, characterized in that: The mass ratio of lithium manganese iron phosphate to terephthalic acid in S1 is 1:1-5.

8. The method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules according to claim 1, characterized in that: During the ball milling process, the ball milling is carried out at 200-500 r / min for 5-10 hours.

9. The method for preparing a lithium manganese iron phosphate positive electrode material uniformly coated with organic small molecules according to claim 1, characterized in that: The heating and holding temperature in S2 is 150-300° C., and the holding time is 2-5 hours.

10. A lithium manganese iron phosphate positive electrode material, characterized in that: The organic small molecules prepared by the method according to any one of claims 1 to 9 uniformly coat the lithium manganese iron phosphate positive electrode material.

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