Preparation And Electrochemical Performance Of The Phosphate Cathode Materials For Lithium Ion Batteries

In order to cope with two global problems, “Energy Crisis” and “Environment Pollution”, developing and using the new energy sources are playing an increasingly important role, and lithium ion batteries are indispensable as an efficient energy storage devices. After the successful commercialization of the layered LiCoO2 cathode, olivine structured LiFePO4 materials have attracted considerable interest because of its good thermal stability, high reversible theoretical capacity, excellent cycling performance, abundance of natural iron resources, and environmental benignity. However, with the development of the new energy areas, there is an urgent need to develop advanced lithium ion batteries with high performances. Li MnPO4 and LiCoPO4 are considered as attractive cathode materials, on the premise of security, because of their high voltage platform of 4.1 V and 4.8 V(vs. Li+/Li) and high theoretical energy density. Hence, this paper take LiMnPO4 and Li CoPO4 as research objects and the material design and synthesis have been systematically studied. The main obtained progress and achievement are list below:LiFePO4/C submicron fibers were obtained by electrospinning technique. And the ratio of the graphitized carbon in the fibers was increased by the presence of the MWCNTs-COOH. The obtained LiFePO4/CNTs submicron fibers can reach 161.2 mAh/g at 0.1 C and 121.8 mAh/g at 10 C. After 100 cycles it can maintain 157 mA/g with a good capacity retention of 97.4% and coulombic efficiency of 99%. In addition, a new type electrode with self-supported structure was developed using the nonwovens of LiFePO4/CNTs submicron fibers.The electrode displayed good electrochemical performances, and can be developed to flexible cells as well as the application of flexible devices.The LiMnPO4/C cathode material with hierarchical structure was prepared by solvothermal methode using monodisperse hollow sphere Li3PO4 as precursor. The hierarchical structure can take both advantages of the nano scale and micron scale materials.The material showed high initial discharge capacities of 145.6 mA h g-1 at 0.1 C and a good capacity retention(88.6 %) after 100 cycles, as well as the 96.3 mA h g-1 at 10 C. The facile routine reported herein was simple and flexible, and may be extended to synthesize other materials.The fibrous LiCoPO4/C cathode material was prepared by electrospinning technique for the first time. Meanwhile, the calcining conditions have a strong effect on the construction and morphology of the fibers. Detailed investigation identified that the sample pre-treated at 550 °C(LCP-550) showed high initial discharge capacities of 119 mA h g-1 at 0.1 C and a good capacity retention(78.2 %) after 40 cycles at 0.1 C. The fibrous structure coated with carbon could provide good electronic conduction, as well as the short Li ion diffusion distance along b axis direction together with the well-ordered single-crystalline structure of the fibers. In addition, the uniform thin carbon coating could efficiently impede the decomposition of electrolyte and enhance the cycle ability of the electrodes, but would not hind the lithium-ion transportation through the interface between electrolyte and electrode.The size and morphology of the LiCoPO4 particle are effectively controlled by changing the EG /water ratio in mixing solvent using solvothermal process. The cathode material shows high initial discharge capacity of 123.8 mA h g-1, with a good capacity retention(83 %) after 100 cycles at 0.1 C. It is inferred that the antisite defect concentration becomes lower gradually in the LCP crystals with the increase of EG content in the mixed solvent. The facile routine reported herein may be extended to synthesize other LiMPO4 olivine-type materials.