Effect of Cation Structure on Electrochemical Behavior of Lithium in [NTf2]-based Ionic Liquids

Room temperature ionic liquids (RTILs) containing bis(trifluoromethylsulfonyl)imide [NTf2]-anion are attractive electrolytes due to their preferable properties such as extremely wide electrochemical stability window and superior ionic conductivity. We study electrochemical behavior of [NTf2]-based RTILs with four types of cations, 1-butyl-1-methylpyrrolidinium [BMPyr](+), 1-butyl-2,3-dimethylimidazolium [BDMIm](+), 1-ethyl-3-methylimidazolium [EMIm](+), and 1-butyl-3-methylpyridinium [BMPy](+) in neat conditions and in the presence of lithium salts. Cyclic voltammetry shows bulk lithium reduction peaks in all RTILs, and surface characterization confirms the presence of Li metal and LiOH on the electrodeposits. The least stable [BMPy] [NTf2] shows a large cathodic peak at a potential more positive than the bulk lithium reduction peak indicating RTIL decomposition, whereas the other RTILs do not have apparent decomposition peaks.