Carbon-coated lithium iron manganese phosphate composite material and preparation method thereof

Abstract

The application provides a carbon-coated lithium manganese iron phosphate composite material and a preparation method thereof. The carbon-coated lithium manganese iron phosphate composite material is composed of a lithium manganese iron phosphate positive electrode material body and an isomerized polypyridine composite carbon nanotube coating layer. The isomerized polypyridine composite carbon nanotube coating layer built by spraying can not only optimize the stability of the surface of the lithium manganese iron phosphate material, but also form a dimer unit through chelation by adjacent C=N groups of the isomerized polypyridine, which can provide Li + Additional active sites, and provide additional Li + Capacity for the material. In addition, the surface layer with high specific capacity and high conductivity also forms a surface buffer zone, which significantly reduces the cycle polarization and effectively improves the large-rate cycle performance of the material.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of lithium ion batteries, and particularly relates to a lithium manganese iron phosphate and a preparation method thereof BACKGROUND

[0002] Lithium ion batteries, as a kind of energy storage devices widely used in energy storage applications, power batteries and portable electronic devices, are known for their high working voltage, low self-discharge and good safety. The positive electrode materials thereof include lithium cobaltate, lithium manganate, a nickel-manganese binary system, a nickel-cobalt-manganese ternary system, a nickel-cobalt-aluminum ternary system, lithium iron phosphate (LiFePO4) and lithium manganese iron phosphate (LMFP). Among them, lithium iron phosphate (LiFePO4) stands out with lower cost, higher safety and excellent cycle life. As a further modification and development of lithium iron phosphate, lithium manganese iron phosphate (LMFP, LiMn 1-x Fe x PO4) is a composite solid solution material mixed with the characteristics of lithium iron phosphate and lithium manganese phosphate. Similar to LiFePO4, LMFP has excellent thermal stability and chemical stability, and superior voltage platform, with a potential of about 4.1V, while the potential range of lithium iron phosphate is 3.4-3.5V. Although the theoretical specific capacity is similar to that of lithium iron phosphate, under the same conditions, the higher the voltage of LMFP, the greater the theoretical energy density is 15-20%. This makes it an important candidate target material for further development and improvement of LiFePO4 positive electrode materials.

[0003] However, the poor inherent conductivity of LMFP poses a challenge, making it almost insulating. Although there have been some improvements, such as carbon coating to enhance conductivity, the conventional carbon coating method is basically through carbon cracking to form a carbon coating layer, which has the problems of uneven coating and affecting the effective capacity of the material. The uniform heterogeneous poly-pyridine composite carbon nanotube coating layer constructed by spraying not only effectively improves the stability and electrical conductivity of the surface of the lithium manganese iron phosphate material, but also the dimer unit formed by the adjacent C=N groups of the heterogeneous poly-pyridine can provide additional active sites for Li + + by chelation, further improving the electrical conductivity of the material while providing additional reversible capacity. In addition, the surface layer with high specific capacity and high electrical conductivity also forms a surface buffer zone, significantly reducing the cycle polarization and effectively improving the large-rate cycle performance of the material. SUMMARY

[0004] To address the inherent poor electrical conductivity of LMFP, the present invention provides a lithium manganese iron phosphate composite material with a heterogeneous polypyridine composite carbon nanotube coating layer and a synthesis method, which is different from conventional carbon coating. By constructing a uniform heterogeneous polypyridine composite carbon nanotube coating layer on the surface of the lithium manganese iron phosphate material, the high electrical conductivity of the heterogeneous polypyridine composite carbon nanotube coating layer is utilized to improve the interfacial stability and cycle performance of the material. In addition, the dimer unit formed by the special adjacent C=N groups can be chelated to Li + Providing additional active sites to further improve the material's Li + Capacity and Li + Diffusion efficiency.

[0005] To achieve this object, the present invention adopts the following technical solutions:

[0006] In a first aspect, the present invention provides a method for preparing a lithium manganese iron phosphate composite material having a carbon nanotube coating layer composited with heterogeneous polypyridine, the preparation method comprising the following steps:

[0007] (1) preparing a manganese salt solution, an iron salt solution, and a phosphate salt solution of a certain concentration, gradually dispersing them uniformly, pouring them into an autoclave for hydrothermal reaction, washing, drying, and calcining in an argon atmosphere to obtain a lithium manganese iron phosphate bulk material;

[0008] (2) gradually mixing carbon nanotubes, Ni(COD)2,1,5-cyclodiene, 2,2'-bipyridine and 5,5'-dibromo-2,2-bipyridine into anhydrous dimethylformamide (DMF), stirring with ultrasonication to fully disperse, heating for reaction, cooling to room temperature, pouring the mixture into a mixture of excess disodium dihydrate ethylenediaminetetraacetic acid and sodium hydroxide, removing excess Ni(COD)2, centrifuging, then washing with an organic solvent detergent and a small amount of water, and drying under vacuum conditions to obtain heterogeneous polypyridine composite carbon nanotubes;

[0009] (3) The heterogeneous polypyridine composite carbon nanotubes are dissolved in an organic solvent to obtain an organic solvent A, and an appropriate amount of lithium source and conductive agent are dissolved in the organic solvent to obtain an organic solution B. The organic solvent A is gradually dripped into the organic solution B, heated and stirred, and after a period of time, a fully dispersed heterogeneous polypyridine composite carbon nanotube dispersion is obtained. The dispersion is injected into a spray coating machine to form a polymer spray, and the lithium iron manganese phosphate bulk material is injected into the spray coating machine at the same time. After the coating is completed, the material is discharged, frozen into a solid with liquid nitrogen, and dried in a vacuum freeze dryer. Finally, the dried product is crushed to obtain a heterogeneous polypyridine composite carbon nanotube-coated lithium iron manganese phosphate composite material.

[0010] Preferably, the lithium source in step (1) includes lithium hydroxide or lithium carbonate or lithium dihydrogen phosphate or lithium acetate, any one or more combinations thereof, and the lithium salt solution concentration is 0.7-3.0 M; the manganese source includes manganese sulfate or manganese nitrate or manganese oxalate or manganese acetate, any one or more combinations thereof, and the manganese salt solution concentration is 0.2-1.0 M; the iron source includes iron sulfate or iron nitrate or ferrous nitrate or iron oxalate or ferrous oxalate, any one or more combinations thereof, and the iron salt solution concentration is 0.2-1.0 M; the phosphorus source includes phosphate or phosphoric acid, any one or two combinations thereof; preferably, the phosphate includes ammonium dihydrogen phosphate or lithium dihydrogen phosphate, any one or two combinations thereof, and the phosphorus salt solution concentration is 1.0-4.0 M; the antioxidant includes ascorbic acid or citric acid, any one or two combinations thereof; the organic dispersant includes ethanol or ethylene glycol or polyethylene glycol or polyvinyl alcohol, any one or more combinations thereof.

[0011] Preferably, the temperature of the hydrothermal reaction in step (1) is 150-250 °C, and the hydrothermal reaction time is 8-16 h.

[0012] Preferably, the molar ratio of the carbon nanotubes, Ni(COD)2, 2,2'-bipyridine, 5,5'-dibromo-2,2-bipyridine, 1,5-cyclooctadiene, and anhydrous dimethylformamide (DMF) in step (2) is (0.5-1.0):1:(0.95-1.05):(0.70-0.90):(4.0-6.0):(60-70).

[0013] Preferably, the temperature of the heating reaction in step (2) is 65-80 °C, and the reaction time is 8-16 h.

[0014] Preferably, the type of the organic solvent washing agent in step (2) includes one or more of methanol, dichloromethane, or acetone.

[0015] Preferably, the organic solvent in step (3) is one or more of xylene, N-methyl pyrrolidone, ethanol, isopropyl alcohol, or ethyl acetate; the lithium source is one or more of lithium hydroxide, lithium carbonate, or lithium nitrate; the conductive agent is one or more of graphene, acetylene, active carbon nanotubes, or porous activated carbon; and the mass ratio of the lithium source, the conductive agent, and the organic solvent is (1.0-2.0):(0.8-1.5):(96.5-98.2).

[0016] Preferably, the solid content of the isomeric polypyridine-complexed carbon nanotube dispersion is 10.0-30.0 wt.%; and the mass ratio of the isomeric polypyridine-complexed carbon nanotube to lithium manganese iron phosphate is 0.1-1.0%.

[0017] Preferably, the vacuum freeze-drying time in step (3) is 10-20 h.

[0018] In a second aspect, the present invention provides a heterogeneous polypyridine-composite carbon nanotube-coated lithium manganese iron phosphate composite material, obtained by the preparation method described in the first aspect:

[0019] Preferably, the lithium manganese iron phosphate material comprises a lithium manganese iron phosphate bulk material and a carbon nanotube coating layer composited with surface isomerized polypyridine;

[0020] Preferably, the particle size of the primary particles of the lithium manganese iron phosphate material is 20 to 900 nm;

[0021] Preferably, the carbon nanotube coating layer of isomeric polypyridine composite on the surface of the lithium manganese iron phosphate material is 0.5 to 10 nm.

[0022] The beneficial effects of the present invention are as follows: the present invention constructs a polypyridine composite carbon nanotube coating layer having adjacent C=N group dimer units, and utilizes the Li + The additional active sites further improve the material's Li + Diffusion efficiency and provide additional Li + Capacity (504mAh g -1 ), the stable interface layer forms an effective Li + The buffer zone further optimizes the high-rate performance of the material and effectively reduces the degree of polarization during the cycle. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 Scanning electron microscope image of the lithium manganese iron phosphate composite material coated with heterogeneous polypyridine composite carbon nanotubes provided in Example 1

[0024] Figure 2 The structural formula of isomeric polypyridine DETAILED DESCRIPTION

[0025] Example 1

[0026] This embodiment provides a lithium manganese iron phosphate composite material coated with heterogeneous polypyridine composite carbon nanotubes, which is prepared by the following method:

[0027] (1) Manganese sulfate, ferric sulfate, ammonium dihydrogen phosphate and lithium hydroxide are used to prepare a certain concentration of manganese salt solution, iron salt solution, phosphate salt solution and lithium salt solution respectively, wherein the concentration of manganese salt solution is 0.5M, the concentration of iron salt solution is 0.5M, the concentration of phosphate salt solution is 1.0M and the concentration of lithium salt solution is 1.0M. The manganese salt solution, iron salt solution, phosphate salt solution, lithium salt solution and ascorbic acid are gradually mixed and ultrasonically dispersed and stirred for 0.5h, wherein the molar ratio of manganese, iron, lithium, phosphorus and ascorbic acid is 3:2:15:5.1:0.04. The mixed and dispersed solution is poured into a polytetrafluoroethylene-lined reactor and reacted at 190℃ for 12h. After the reaction is completed, it is centrifuged and dried in a vacuum oven at 80℃ for 6h. Then, it is calcined at 700℃ for 10h under argon to obtain the lithium manganese iron phosphate bulk material.

[0028] (2) Carbon nanotubes, Ni(COD)2, 1,5-cyclodiene, 2,2'-bipyridine and 5,5'-dibromo-2,2-bipyridine are gradually mixed into anhydrous dimethylformamide (DMF), wherein the molar ratio of carbon nanotubes, Ni(COD)2, 2,2'-bipyridine, 5,5'-dibromo-2,2-bipyridine, 1,5-cyclodiene and anhydrous dimethylformamide (DMF) is 0.7:1.0:0.95:0.90:6.0:60. Ultrasonic stirring is performed to fully disperse the mixture, and the mixture is heated to 70°C for reaction for 12 hours. After cooling to room temperature, the mixture is poured into a mixture of excess dihydrate disodium ethylenediaminetetraacetic acid and sodium hydroxide to remove excess Ni(COD)2. The mixture is centrifuged and then washed with methanol, dichloromethane and water, and vacuum dried at 80°C for 12 hours to obtain heterogeneous polypyridine composite carbon nanotubes;

[0029] (3) Dispersing heterogeneous polypyridine-composite carbon nanotubes in xylene to obtain an organic solvent A, lithium hydroxide and porous activated carbon in xylene to obtain an organic solution B, wherein the mass ratio of lithium hydroxide, porous activated carbon and xylene is 1.5:1.0:97.5, gradually dripping organic solvent A into organic solution B, heating and stirring, to obtain a heterogeneous polypyridine-composite carbon nanotube dispersion with a solid content of 15 wt.%, which is injected into a spray coating machine to form a polymer spray, and simultaneously injecting lithium iron manganese phosphate bulk material into the spray coating machine, wherein the mass ratio of heterogeneous polypyridine-composite carbon nanotube dispersion to lithium iron manganese phosphate is 0.3%. After coating is completed, the material is discharged, frozen into a solid with liquid nitrogen, and dried in a vacuum freeze dryer for 15 hours. Finally, the dried product is pulverized to obtain a heterogeneous polypyridine-composite carbon nanotube-coated lithium iron manganese phosphate material.

[0030] Example 2

[0031] (1) The synthesis process of lithium manganese iron phosphate bulk material is consistent with that in Example 1.

[0032] (2) Carbon nanotubes, Ni(COD)2, 1,5-cyclooctadiene, 2,2'-bipyridine and 5,5'-dibromo-2,2-bipyridine were gradually mixed into anhydrous dimethylformamide (DMF), wherein the molar ratio of Ni(COD)2, 2,2'-bipyridine, 5,5'-dibromo-2,2-bipyridine, 1,5-cyclooctadiene and anhydrous dimethylformamide (DMF) was 0.8:1.0:1.0:0.87:4.0:65. After sufficient dispersion by ultrasonic stirring, the mixture was heated to 80°C for 10h. After cooling to room temperature, the mixture was poured into an excess mixture of disodium salt of ethylenediaminetetraacetic acid dihydrate and sodium hydroxide to remove excess Ni(COD)2. After centrifugal separation, the product was washed with methanol, dichloromethane and water, and then dried in a vacuum oven at 80°C for 12h to obtain isomeric polypyridine-complexed carbon nanotubes;

[0033] (3) Isomeric polypyridine-complexed carbon nanotubes were obtained in xylene to obtain organic solvent A, lithium hydroxide and porous activated carbon were dispersed in xylene to obtain organic solution B, wherein the mass ratio of lithium hydroxide, porous activated carbon and xylene was 2.0:1.5:96.5, organic solvent A was gradually added into organic solution B, and heated and stirred to obtain isomeric polypyridine-complexed carbon nanotube dispersion with a solid content of 10wt.%. The dispersion was injected into a spray coating machine to form a polymer spray, and a manganese iron phosphate lithium bulk material was injected into the spray coating machine at the same time, wherein the mass ratio of isomeric polypyridine-complexed carbon nanotube dispersion to manganese iron phosphate lithium was 0.5%. After coating, the material was discharged, frozen into a solid in liquid nitrogen, and dried in a vacuum freeze dryer for 15h. Finally, the dried product was crushed to obtain isomeric polypyridine-complexed carbon nanotube-coated manganese iron phosphate lithium material.

[0034] Example 3

[0035] (1) The synthesis process of the lithium manganese iron phosphate bulk material was consistent with that of Example 1.

[0036] (2) Carbon nanotubes, Ni(COD)2, 1,5-cyclooctadiene, 2,2'-bipyridine and 5,5'-dibromo-2,2-bipyridine were gradually mixed into anhydrous dimethylformamide (DMF), wherein the molar ratio of Ni(COD)2, 2,2'-bipyridine, 5,5'-dibromo-2,2-bipyridine, 1,5-cyclooctadiene and anhydrous dimethylformamide (DMF) was 0.8:1.0:1.0:0.87:4.0:65. After sufficient dispersion by ultrasonic stirring, the mixture was heated to 80°C for 10h. After cooling to room temperature, the mixture was poured into an excess mixture of disodium salt of ethylenediaminetetraacetic acid dihydrate and sodium hydroxide to remove excess Ni(COD)2. After centrifugal separation, the product was washed with methanol, dichloromethane and water, and then dried in a vacuum oven at 80°C for 12h to obtain isomeric polypyridine-complexed carbon nanotubes;

[0037] (3) isomerization poly-pyridine composite carbon nanotubes in xylene to obtain organic solvent A, lithium hydroxide, porous activated carbon dispersed in xylene to obtain organic solution B, wherein the mass ratio of lithium hydroxide, porous activated carbon and xylene is 2.0:1.5:96.5, gradually drop organic solvent A into organic solution B, heating and stirring, to obtain isomerization poly-pyridine composite carbon nanotube dispersion with solid content of 20wt.%, inject it into the spray coating machine to form a polymer spray, at the same time, inject the lithium manganese iron phosphate bulk material into the spray coating machine, wherein the mass ratio of isomerization poly-pyridine composite carbon nanotube dispersion to lithium manganese iron phosphate is 0.8%. After coating, discharge, freeze in liquid nitrogen to solid, dry in a vacuum freeze dryer for 15h, finally crush the dried product to obtain isomerization poly-pyridine composite carbon nanotube coated lithium manganese iron phosphate material.

Claims

1. A carbon-coated lithium manganese iron phosphate composite material, comprising a lithium manganese iron phosphate material body and a carbon nanotube coating layer composited with heterogeneous polypyridine, characterized in that: The core is lithium manganese iron phosphate material, the chemical formula is LiMn x Fe y PO4, wherein 0.3≤x≤0.7, 0.3≤y≤0.7, x+y=1; the shell layer is a carbon nanotube coating layer composited with isomeric polypyridine; the particle size of the core material is 100-600nm, and the thickness of the shell coating layer is 0.5-10nm.

2. A method for preparing the carbon-coated lithium manganese iron phosphate composite material as claimed in claim 1, characterized in that: The following steps are involved: (1) preparing a manganese salt solution, an iron salt solution, and a phosphate salt solution of a certain concentration, gradually dispersing them in a certain order, pouring them into an autoclave for hydrothermal reaction, washing, drying, and calcining in an argon atmosphere to obtain a lithium manganese iron phosphate bulk material; (2) gradually mixing carbon nanotubes, Ni(COD)2,1,5-cyclodiene, 2,2'-bipyridine and 5,5'-dibromo-2,2-bipyridine into anhydrous dimethylformamide (DMF), stirring with ultrasonication to fully disperse, heating for reaction, cooling to room temperature, pouring the mixture into a mixture of excess disodium dihydrate ethylenediaminetetraacetic acid and sodium hydroxide, removing excess Ni(COD)2, centrifuging, then washing with an organic solvent detergent and a small amount of water, and drying under vacuum conditions to obtain heterogeneous polypyridine composite carbon nanotubes; (3) The carbon nanotubes composited with heterogeneous polypyridine are dissolved in an organic solvent to obtain an organic solution A, and an appropriate amount of lithium source and conductive agent are dissolved in an organic solvent to obtain an organic solution B. The organic solution A is gradually dripped into the organic solution B, heated and stirred, and after a period of time, a fully dispersed heterogeneous polypyridine composite carbon nanotube dispersion is obtained, which is injected into a spray coating machine to form a polymer spray, and at the same time, the lithium iron manganese phosphate body material is injected into the spray coating machine; after the coating is completed, the material is discharged, frozen into a solid by liquid nitrogen, and dried in a vacuum freeze dryer. Finally, the dried product is crushed to obtain a lithium iron manganese phosphate composite material with heterogeneous polypyridine composite carbon nanotubes.

3. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 2, wherein: The raw materials for preparing the lithium manganese iron phosphate include a lithium source, a manganese source, an iron source, a phosphorus source, an antioxidant, and an organic dispersant; the lithium source includes one or more of lithium hydroxide, lithium carbonate, lithium dihydrogen phosphate, or lithium acetate; the manganese source includes one or more of manganese sulfate, manganese nitrate, manganese oxalate, or manganese acetate; the iron source includes one or more of ferric sulfate, ferric nitrate, ferrous nitrate, ferrous oxalate, or ferrous oxalate; the phosphorus source includes one or more of phosphate or phosphoric acid; the phosphate includes one or more of ammonium dihydrogen phosphate or lithium dihydrogen phosphate; the antioxidant includes one or more of ascorbic acid or citric acid; and the organic dispersant includes one or more of ethanol, ethylene glycol, polyethylene glycol, or polyvinyl alcohol.

4. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 2, wherein: In step (1), the concentration of the manganese salt solution is 0.2-1.0 M, the concentration of the iron salt solution is 0.2-1.0 M; the concentration of the phosphate solution is 1.0-4.0 M, and the concentration of the lithium salt solution is 0.7-3.0 M; the molar ratio of the phosphate salt, the lithium salt, the manganese salt and the iron salt is 1:3:x:y, wherein 0.3≤x≤0.7, 0.3≤y≤0.7, and x+y=1; the temperature of the hydrothermal reaction is 150-250° C., and the hydrothermal reaction time is 8-16 h.

5. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 2, wherein: In step (2), the molar ratio of the carbon nanotubes, Ni(COD)2, 2,2'-bipyridine, 5,5'-dibromo-2,2-bipyridine, 1,5-cycloalkadiene and anhydrous dimethylformamide (DMF) is (0.50-1.00):1:(0.95-1.05):(0.70-0.90):(4.0-6.0):(60-70).

6. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 2, characterized in that: In step (2), the temperature of the heating reaction is 65 to 80° C., and the reaction time is 8 to 16 hours.

7. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 2, characterized in that: In step (2), the organic solvent detergent includes one or more of methanol, dichloromethane or acetone.

8. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 2, wherein: In step (2), the vacuum drying temperature is 80-90° C., and the drying time is 10-16 hours.

9. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 2, characterized in that: In step (3), the organic solvent is one or more of xylene, N-methylpyrrolidone, ethanol, isopropanol, and ethyl acetate; the lithium source is one or more of lithium hydroxide, lithium carbonate, and lithium nitrate; the conductive agent is one or more of graphene, acetylene, activated carbon nanotubes, and porous activated carbon; the mass ratio of the lithium source, the conductive agent, and the organic solvent is (1.0-2.0):(0.8-1.5):(96.5-98.2); the solid content of the heterogeneous polypyridine composite carbon nanotube dispersion is 10.0-30.0 wt.%; and the mass ratio of the heterogeneous polypyridine composite carbon nanotube dispersion to lithium iron manganese phosphate is 0.1-1.0%.

10. The method for preparing the carbon-coated lithium manganese iron phosphate composite material according to claim 2, characterized in that: In step (3), the vacuum freeze-drying time is 10 to 20 hours.

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