DEVELOPMENT OF HIGH RATE PERFORMANCE PHOSPHATE CATHODE MATERIALS FOR LITHIUM-ION BATTERIES

Abstract

This study mainly focuses on phosphate cathode materials for high power lithium-ion batteries (LIBs), especially for large-scale applications such as electric vehicles (EVs). Phosphate materials of olivine (LiFePO4) and of tavorite (LiVPO4F) crystal structures were selected because of their unique lattice stability and safety features. Olivine LiFePO4 (3.4 V vs. Li+/Li) has already been regarded as the most promising candidate for EVs. However, these phosphate materials lack high power capabilities due to intrinsically low electronic and ionic conductivities. To overcome the problems of poor electronic and ionic transport, electronic structure and fabrication methodology were tailored with cation doping/substitution, conductive carbon coating and nanostructuring. Furthermore, to develop the next generation high voltage LIBs with higher power output, nanostructured 4 V Li(FeMn)PO4 and LiVPO4F composites were also developed using similar technologies. Crystal structure, chemical composition, micro-morphology, electrical and electronic conductivities of these synthesized materials were carefully characterized and analyzed. Electrochemical properties, including specific capacity, charge/discharge rate capability, cyclic performance and Li+ chemical diffusion coefficients, were studied to evaluate the battery performances. The materials developed in this study exhibited amazingly high power performance, fast charge capability and excellent long-term cyclic stability, and thus are very promising for the next generation high power LIBs.