The lithium metal–Li+ interface was investigated in N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulphonyl)imide room temperature ionic liquid, using impedance simulation, experimental electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM). The electrolyte contained vinylene carbonate as the solid electrolyte interphase (SEI) forming additive. The Warburg coefficient value obtained from impedance plot deconvolution (A W >10Ωs−1/2) and SEM lithium surface image suggest the SEI layer formation. Therefore, the charge transfer process on lithium reduction and oxidation takes place at the solid Li|SEI interface. Simulation studies show that the typical shape of impedance plots for the Li|Li+ system (reported in the literature) suggests the exchange current density below 10−2 Acm−2. The exchange current density of Li|SEI|Li+ in the ionic liquid electrolyte, obtained from impedance spectra and Tafel plot is between 4.5×10−4 Acm−2 and 3.0×10−4 Acm−2 (Li+ concentration of 0.7moldm−3).