Lithium-ion Battery Cathode Material Of Lifepo <sub> 4 </ Sub> Preparation And Electrochemical Performance

Olivine structured cathode material, LiFePO4, has attracted broad attention due to its advantages of low cost, rich source, environmental benign and higher stability which has great potential for application in power and storage batteries. It has proved that micro structure and physical property characterization are the two most important aspects to influence the performance of the cathode material in lithium ion batteries. Therefore, our work focused on these two aspects:1)exploring the relationship between the micro structure and electrochemical behaviour of LiFePO4 material; 2) based on the inherent low electronic and ion conductivity of LiFePO4, improving its cycling ability and rate capability through synthesis process. The main points of this article include:1)Preparation of LiFePO4 with micro-structure controlled by hydrothermal methodBy varying the pH value of the starting solution, a series of products were obtained. Through the characterization of XRD, Raman, it was found that highly crystallized LiFePO4 without any impurities could only be obtained at neutral and slightly basic conditions. Rietveld pattern fitting of XRD data reveal that there exists defect chemistry, such as cation vacancies and Li/Fe inter-site mixing. The degree of this defect chemistry increased along with decreasing of pH value, which responses to the activation energy during the hydrothermal process. Moreover, the series of LiFePO4 obtained at different pH value showed different morphologies. The results obtained by analyzing the Warburg impedance of EIS data obviously show that the defect chemistry effect is disadvantageous. If Fe atoms occupy the sites of Li atoms, the one dimensional curved pathway for lithium ion diffusion will be blocked. It is also worth mentioning that the defect would affect the electronic behavior of LiFePO4.(2) Preparation of nano-sized LiFePO4 and its electrochemical performance;In the presence of surfactant, nano-sized FePO4 precursor could be obtained by precipitation method. The uniform and ordered chain structure of surfactant would effectively tune the particle size and morphology. Preparing nano-sized LiFePO4 with this FePO4 precursor and carbon coating was used to improve the electronic conductivity. The phase composition, particle morphology, and carbon layer were characterized by XRD, SEM and TEM, the results show that the higher caclined temperature would bring impurities, make the particles grow and agglomerated. The tendency of growing bigger particle at higher calcined temperature could destroy the coated carbon layer and result carbon film peeling off the surface of LiFePO4. With proper calicined temperature, LiFePO4/C particles could be connected by carbon, ensure Li+ ions to get electrons from all directions during the diffusion process. Meantime, it shows that the nano-sized particle which could shorten the pathway for lithium diffusion, and thus improve the cycling ability and rate capability could be improved.Understanding the defect chemistry and morphylogy controlling of LiFePO4 during solution synthesis process will greatly help in its preparation with excellent electrochemical performance.