In Situ Atomic Force Microscopic Studies of LiFSI-[Py_1,4]FSI Interfacial Nanostructure on Au(111): Solid Electrolyte Interphase and Lithium Underpotential Deposition

In this paper, the structure of the LiFSI-[Py_1,4]FSI interface on Au(111) was investigated using in situ atomic force microscopy and cyclic voltammetry. Raman and IR vibrational spectroscopy was applied to evaluate the coordination of Li⁺ ions with the FSI^– anion. It was found that [Li(FSI)₃]^2– species are present in the solutions. The solvation layer number and the forces decrease at higher concentration of Li⁺ ions. Therefore, the driving force, which is required for the [Li(FSI)₃]^2– species to pass through the solvation layer to reach the electrode surface, is lowered. This increases the onset potentials of the SEI layer formation and lithium underpotential deposition (Li-UPD) at high Li⁺ concentrations. It was found that the onset potentials are −0.8, −0.7, −0.6, and −0.4 V for the Li-UPD and −2.0, −1.6, −1.4, and −1.3 V for the SEI layer formation, vs the Pt-(quasi)reference electrode, corresponding to Li⁺ concentrations of 0.1, 0.25, 0.5, and 1 M, respectively.