Understanding and development of olivine LiCoPO4 cathode materials for lithium-ion batteries

摘要

Olivine LiCoPO4 is a promising candidate as the cathode material for high-voltage lithium-ion batteries due to its high redox potential of 4.8 V vs. Li/Li+ and a theoretical capacity of 167 mA h g(-1). However, use of LiCoPO4 as a cathode in practical applications has been hindered by its unsatisfactory cycle stability, coulombic efficiency and rate capability, which can be attributed to its low electronic conductivity, poor Li+ ion conductivity, and limited stability of electrolytes at high potentials. It is thus important to develop a simple, time and energy saving, easy to control and industrially scalable synthesis method to prepare LiCoPO4 with high specific capacity, good cycle stability and rate capability. Various synthetic routes such as solid-state reactions, hydrothermal/solvothermal synthesis and sol-gel processes have been proposed and various strategies have been applied to improve the electrochemical performance. Carbon coating or the use of carbon network supports enhances the overall electronic conductivity of the composite electrode. Decreasing the particle size of LiCoPO4 or tailoring its crystal growth orientation along the a-c plane reduces the length of Li-ion migration paths, and facilitates easier Li-ion transfer. However, carbon addition and size reduction for LiCoPO4 cathodes can reduce the volumetric energy density of lithium-ion batteries. Ion doping aims to enhance the intrinsic electronic/ionic conductivity of LiCoPO4 although the mechanism is still in controversy. Strategies to mitigate the problem of the electrolyte decomposition at high voltages have also been explored, such as optimization of the electrolyte formation and use of protective coatings, thus improving the cycle stability of LiCoPO4 cathodes in lithium-ion batteries. Understanding of olivine LiCoPO4 cathode materials development for lithium-ion batteries is crucial for further improvement.