Functionalized ionic liquids based on guanidinium cations with two ether groups as new electrolytes for lithium battery

► New functionalized guanidinium ILs with two ether groups are reported. ► They have low viscosity and good electrochemical stability. ► Li/LiFePO 4 cells using these IL electrolytes have good capacity and cycle property. Two new functionalized ionic liquids (ILs) based on guanidinium cation with two ether groups and TFSA − anion are synthesized and characterized. Their physicochemical and electrochemical properties, including melting point, thermal stability, density, viscosity, conductivity, and electrochemical window are determined. Both the ILs are liquids at room temperature, and the viscosities are about 60 mPa s at 25 °C. Behavior of lithium redox, chemical stability against lithium metal and charge–discharge characteristics of lithium battery, are also examined for the two ILs as electrolyte with 0.6 mol kg −1 LiTFSA. Though the cathodic limiting potentials of the two ILs are higher than 0 V versus Li/Li +, the lithium plating and stripping on Ni electrode can be observed in the two IL electrolytes, indicating their good chemical stability against lithium metal. Li/LiFePO 4 cells using the two IL electrolytes without any additive showed good cycle property at the current rate of 0.2 C at 25 °C and 55 °C.