New functionalized ionic liquids based on pyrrolidinium and piperidinium cations with two ether groups as electrolytes for lithium battery

Four new functionalized ILs based on piperidinium and pyrrolidinium cations with two ether groups and TFSI− anion are synthesized and characterized. Physical and electrochemical properties of these ILs, including melting point, thermal stability, viscosity, conductivity and electrochemical stability, are investigated. All the ILs are liquids at room temperature, and the viscosities of P(2o1)2-TFSI and P(2o1)(2o2)-TFSI are 55 and 53 mPa s at 25 °C, respectively. Behavior of lithium redox, chemical stability against lithium metal and charge–discharge characteristics of lithium batteries, are also investigated for these IL electrolytes with 0.6 mol kg−1 LiTFSI. Though the cathodic limiting potentials of these ILs are 0.4 V versus Li/Li+, the lithium plating and striping on Ni electrode can be observed for these IL electrolytes, and these IL electrolytes show good chemical stability against lithium metal. Li/LiFePO4 cells using these IL electrolytes without additives have good capacity and cycle property at the current rate of 0.1 C, and the cell using the P(2o1)(2o2)-TFSI electrolyte owns good rate property.

► Four functionalized ILs based on piperidinium and pyrrolidinium cations with two ether groups are reported for the first time.
► They have low viscosity and good electrochemical stability.
► Li/LiFeO4 cells using these IL electrolytes without additives have good capacity and cycle property at the rate of 0.1 C, and one electrolyte owns good rate property.