Abstract: The formation process and main composition of solid electrolyte interface（SEI） of the cathode material LiCoO2 of lithium-ion battery were investigated by different measurement methods.It was confirmed by cycle voltammetry curves and EIS that the SEI on the surface of LiCoO2 was formed at the first cycle and with the increase of cycle numbers，SEI became denser and better for Li+ to pass through，but it thickness had no more increase.SEM analysis also proved the SEI formed at the first cycle and its thickness was under 10 nm.XPS confirmed the components of SEI were LiF and organolithium compound.

Key words: LiCoO2, solid electrolyte interface, XPS