Studies On Cathode Materials Of LiFePO₄ For Li-ion Batteries

The increasing concerns on energy, environment and the rapid development of modern science and technology have more demands on batteries. Lithium ion batteries (LIB) are favorable because of the properties of high vlltage, high energy density, long cycling life, little local action, non-memory effect and pollution-free. Currently, the cathode material that commercialized is mainly LiCoO2. However, because of its pollution, high price, poor thermal stability and security, the development of new cathode materials which are low cost, environment-friendly, and have better electrochimecal properties and better security are becoming the focus of resent research.LiFePO4 is one of the most potential cathode materials which has a high theoretical capacity (170mAh/g) and voltage (about 3.5V versus Li/Li). It also have good stability both at room temperature and high temperature. Besides, it is environmentally benign and inexpensive. However, LiFePO4 have some fatal problems including its low electronic conductivity, low lithium ion diffusivity, and low tap density. 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.Spherical LiFePO4 was obtained by liquid co-deposited-carbonthermal reduction. The LiFePO4 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 co-deposited-carbonthermal reduction sintered at 550℃for 10 hours showed the best electrochemical properties. The first discharge capacity was 141.8mAh/g with a current density of 20mA/g.And we also researched the modification by coating LiFePO4 materials with different carbon sources, such as acetylene black, glucose and sucrose. Studies on carbon sources, mixing accounts and mixing styles indicated that the sample coated with 5% excess glucose sintered at 550℃for 10 hours has the most optimized electrochemical properties with a discharge capacity of 154.6mAh/g under a current density of 20mA/g. The spherical material LiFePO4 was synthesized by liuqid-microwave method with 50~200nm particle diameter. The results of charge-discharge test of the products coated with sucrose showed a lithium insertion plateau around 3.5V vs. Li together with a specific capacity of 129.7mAh/g with a current density of 100mA/g. After 100 cycles, it remains a specific capacity of 125.4mAh/g. And under the temperature of 50℃, it also have a specific capacity of 143.3mAh/g.