Study On LiFePO₄ As Cathode Materials For Li-ion Batteries

The increasing concerns on energy, environmental and the rapid development of modern science and technology have more demands on batteries. Lithium ion batteries (LIB) are attractive for use in the field of mobile power source with the favorable properties of high voltage, long cycling life, high energy density, non-memory effect and pollution-free. Intensive research is being conducted to further improve the performance of lithium ion batteries and reduce the cost of electrode materials. LiFePO₄ was emerging as the most promising cathode material to replace LiCoO2, because of its high theoretical capacity (170mAh/g), and voltage (about 3.5V versus Li/Li), good stability both at room temperature and high temperature, and being environmentally benign and inexpensive. The main problem of LiFePO₄ is low electronic conductivity and low lithium ion diffusivity. The volumetric energy density is also low, which is resulted from the low gravimetric density of the material. In order to overcome these defaults, the first way is to synthesize small fine particles of LiFePO₄ through improved methods, the second way is to coat LiFePO₄ with electronically conductive materials such as carbon or metal powder, the final way is to substitute Li or Fe by other metal ions to enhance the inherent electronic conductivity.Single-phase LiFePO₄ were obtained by liquid co-deposited and solid-state reaction. The LiFePO₄/C was characterized by XRD, SEM, TEM, CV analysis. We also studied the electrochemical properties of the product at different conditions. The results indicated that the sample synthesized by solid-state reaction sintered at 650℃for 12h showed the best electrochemical properties. The first discharge capacity was 120mAh/g with a current density of 20mA/g.And we also researched the modification by coated with MgO and carbon. The carbon came from activated carbon, sucrose and citric acid, which acted as a new carbon source. Further studies to carbon sources, mixing styles indicated that the sample coated with sucrose at 750℃has the most optimized electrochemical properties with a discharge capacity of 167mAh/g with a current density of 20mA/g. Even when cycled with a current density of 85mA/g, it showed a discharge capacity of 139mAh/g with a good voltage platform.The fine-particle electroactive material LiFePO₄/C was synthesized by microwave heating. The reaction conditions favor offering some control of the product morphology and particle size. The results of charge-discharge test and cyclic voltammetry of file products coated with sucrose showed a specific capacity of 169mAh/g. Excellent electrochemical reversibility and cycling stability have been achieved.