Two ionic liquids based on guanidinium cations and TFSA− anion were prepared, and their electrochemical stabilities were investigated. The cathodic limiting potentials of the two ILs were 0.7V versus Li/Li+, and their electrochemical windows were 4.2V. However, the lithium plating and striping on Ni electrode could been observed in the two IL electrolytes containing 0.3molkg−1 of LiTFSA without additive. And Li/LiCoO2 cells using the two IL electrolytes without additive showed good capacity and cycle property at the current rate of 0.2C.