Property and Structure of Carbon-coated LiFePO_4

LiFePO4/C composite cathode materials containing different amounts of carbon were fabricated by mechanical activation-high temperature solid state reaction using citric acid as a carbon source,with the aim to study the effect of carbon-coating on LiFePO4 internal structure.The crystal structure,surface morphology and electrochemical properties of the obtained composites coated with various carbon contents were investigated by the techniques of XRD,SEM,BET,HRTEM,SAED,AC Impedance spectra,and constant current charge-discharge testing.And the structural changes originating from the carbon-coating were also analyzed.Under high temperature,citric acid was decomposed into network-like structure amorphous carbon,and coated on the surface of LiFePO4 particles,which could inhibit the grain growth and cause the smaller sizes.The crystal growth direction and micro-structure of LiFePO4/C were significantly influenced by carbon-coating that resulted in growing along direction preferentially.According to AC Impedance spectra measurements,all the impedances of carbon coated LiFePO4 decreased evidently,and the diffusion coefficient of Li ions increased by up to two orders of magnitude compared with uncoated LiFePO4,indicating the ion and electric conductivity,discharge capacity and cycling performance were greatly improved.