Kinetic analysis of Li|Li⁺ interphase in an ionic liquid electrolyte

Abstract

Kinetic analysis of the Li|Li⁺ interphase in an electrolyte based on N-metyl-N-propylpyrrolidinium bis(trifluoromethanesulfon)imide ionic liquid (MPPyrrTFSI) and lithium bis(trifluoromethanesulfon)imide salt (LiTFSI) was performed. Li|electrolyte|Li and LiC₆|electrolyte|Li cells were galvanostatically charged/discharged in order to form solid electrolyte interphase (SEI) protecting layer. SEM images showed that the surface of both Li and LiC₆ anodes was covered with small particles. The fitting procedure of electrochemical impedance data taken at different temperatures gave three resistances (R _el, R _SEI, R _ct) and hence, three lnR = f(T ⁻¹) straight lines of different slopes. Specific conductivity and activation energy of the conduction process of the liquid electrolyte, were ca. σ = 2.5 mS cm⁻¹ (at T = 25.0 °C) and $E_{\text{el}}^{\# }$ = 15 kJ mol⁻¹. Activation energy for the conduction process in the SEI layer was ca. 56 kJ mol⁻¹ in the case of the metallic lithium and 62 kJ mol⁻¹ for the graphite anode. Activation energy of the charge transfer process, $E_{\text{ct}}^{\# }$, for Li and LiC₆ anodes was 71 and 65 kJ mol⁻¹, respectively. Analysis of literature data for different electrolytes suggests that the $E_{\text{ct}}^{\# }$ value for Li⁺ reduction may be approximated by 57 ± 5 kJ mol⁻¹. Activation energy for the diffusion processes in the graphite electrode, detected from the Warburg coefficient, was ca 74 kJ mol⁻¹.