Research On Modification Of Lithium Iron Phosphate Cathode Material

Morphology and ion transfer rate are important factors affecting the properties of lithium iron phosphate.However,there are still many unclear understanding and technical problems about how to control the morphology and improve the ion transport performance.For this reason,in this thesis,organic small molecules with special structure are used to induce and control the morphology in order to obtain excellent properties,and carbon materials with excellent structure and small molecules are used to change the electrochemical performance of lithium iron phosphate.Aniline with benzene ring structure and dopamine which can coordinate with iron ion are used to change the binding strength of carbonization precursor and iron phosphate.In this thesis,different preparation processes are used,and the results show that:First,iron phosphate precursor was prepared by liquid phase method.The formation conditions of iron phosphate were theoretically analyzed.When the p H was controlled between 0.9and 1.6,the precipitation of Fe PO₄ appeared in the solution and no Fe（OH）₃ was generated.The precursor was prepared by a two-step hydrothermal method to explore the influence of hydrothermal temperature,hydrothermal time and additives on the particle morphology.The results show that when the hydrothermal time is 8h and the hydrothermal temperature is 110℃,the material has the best crystallinity and morphology,and the cationic surfactant CTAB can disperse and control the morphology of particles.Then the Fe PO4 precursor was prepared by coprecipitation method and the lithium iron phosphate cathode material was prepared by one-step hydrothermal method.The experimental results show that the specific capacities of Li Fe PO4 at 0.2C prepared by co-precipitation,two-step hydrothermal and one-step hydrothermal methods are 130.1m Ah·g^-1,132.4m Ah·g^-1 and 122.1m Ah·g^-1,respectively.The material prepared by the co-precipitation method has serious agglomeration and the worst rate performance.The material prepared by two-step hydrothermal method has the highest ion diffusion rate,which is 6.77×10^-14cm²/s.Subsequently,glucose,one-dimensional carbon nanotubes and two-dimensional graphene were selected as carbon sources to explore the effects of high-end carbon materials on lithium ion diffusion,specific capacity and other properties.SDBS was used as ultrasonic dispersant of carbon nanotubes.The composite materials LiFePO₄/C-Glc,LiFePO₄/C-CNTs and LiFePO₄/C-GR were prepared by mechanical ball milling.The initial discharge capacities of the material at 0.2C are130.1m Ahg^-1,144.6m Ahg^-1 and 146.2m Ahg^-1,respectively.Among them,the dispersion of LiFePO₄/C-CNTs particles is poor,and the battery performance is poor at large magnification.LiFePO₄/C-GR not only has a large capacity,but also has excellent rate performance,and the lithium ion transport rate is greatly improved,with a D_LI of 2.48×10^-13cm²/s.Then,the material was positively charged with cationic surfactant CTAB,and the graphene oxide coated LiFePO₄/C-RGO was prepared by electrostatic self-assembly.The specific discharge capacity of the material at 0.2C is up to 151.9m Ahg^-1.The material prepared by the self-assembly method is more uniformly coated,which improves the rate performance of the active material and the conductivity of the material.The lithium ion transmission rate of LiFePO₄/C-RGO reaches 1.12×10^-12cm²/sFinally,polyaniline,dopamine and melamine were selected to prepare carbon-nitrogen co-coated composite materials.Polyaniline uniformly coated LiFePO₄/PANI was prepared by polymerization of aniline catalyzed by trivalent iron in solution.In Tris-HCl buffer solution with p H=8.5,LiFePO₄/PDA was obtained by spontaneous polymerization of dopamine.Melamine-coated LiFePO₄/M was obtained by mechanical ball milling.The first discharge specific capacities of the three groups of materials at 0.2C are 149.1 m Ahg^-1,158.4 m Ahg^-1 and 140 m Ahg^-1,respectively.The rate performance of LiFePO₄/M is unstable,and the continuity between particles is poor.The lithium ion transport rates of LiFePO₄/PANI and LiFePO₄/PDA are 1.69×10^-13 cm²/s and 1.37×10^-13 cm²/s,respectively.