Preparation And Electrochemical Performance Study Of LiFePO₄/C Core/Shell Nanocomposites As Cathode Materials For Lithium Ion Batteries

Olivine-structured lithium iron phosphate (LiFePO4) has been extensively studied and developed as the cathode-active material for lithium-ion batteries because of low-cost, non-toxicity, high stability, excellent cycling performance and environment friendliness. However, due to its specific crystal structure, LiFePO4 is poor electronic conductivity and ionic difrusivity. In effort to improve electronic conductivity and ionic diffusivity, most researchers adopt strategies of surface coating with conductive materials or decreasing the size of the LiFePO4 particles. In this work, a simple two-step solid state procedure, which involves the synthesis of FePO4/polyaniline (PANI) and subsequent thermal calcination in the presence of carbon sources, has been applied to prepare nano-sized LiFePO4/C with a core/shell structure. The influence of different carbon sources and various contents of carbon on the performances of the material have been investigated. The main related research as follows:1) Nanosized LiFePO4/C composites have been synthesized by a simple two-step procedure. It is found that an initial coating of PANI on FePO4 can effectively inhibit particle growth during the high temperature calcinations process, facilitating the fabrication of the nanosized FePO4/PANI (<100 nm).2) Carbon sources, such as sucrose, polyethylene glycol or conductive carbon black, are found to have great effects on size, shape and electrochemical performance of LiFePO4/C nanocomposites. The LiFePO4/C nanocomposites fabricated with sucrose or polyethylene glycol (PEG) as carbon source have relatively uniform morphology and smaller particles. Among them, the LiFePO4/C nanocomposites prepared with sucrose as carbon source have higher contents of carbon and exhibit the superior electrochemical performance. The LiFePO4/C prepared with polyethylene glycol (PEG) as carbon source have poor contents of carbon and electrochemical performance. Owing to conductive carbon black only mixed LiFePO4 with physics, the LiFePO4/C nanoparticles have nonuniform particle size distribution and agglomerated seriously. The LiFePO4/C prepared with conductive carbon black as carbon source have poor electronic conductivity and electrochemical performance.3) The contents of carbon and thickness of carbon layer have been investigated the effects on the electrochemical performance of the LiFePO4/C nanocomposites when used in the LIBs. It is found that the LiFePO4 nanoparticles with a low content of carbon exhibit poor electrochemical performance, presumably due to presence of the structural disorders and defects in the LiFePO4 particles and the surface layers, which leads to the lithiation/delithiation of Li+ions via a single-phase process. An increase of carbon contents in the LiFePO4 particles can remove the structural disorders and defects, which results in a significant improvement in the electrochemical performance of the LiFePO4/C nanocomposites. However, excess carbon would hinder the penetration of the electrolyte solution into the carbon layers and the inward/outward diffusion of Li+ ions through the carbon layers, and leads to a decrease in the charge/discharge capacity of the LiFePO4/C nanocomposites.In summary, a two-step procedure has been used for the preparation of the nano-sized LiFePO4/C with a core/shell structure. The LiFePO4/C exhibit the superior electrochemical performance, which might be attributed to effectively inhibit particle growth during the high temperature calcinations process. It is significant that several other nano-sized materials and superior performance materials have been successfully prepared by this method.