In Situ Electrochemical Surface Plasmon Resonance Study on Lithium Underpotential Deposition and Stripping in Bis(fluorosulfonyl)amide-Based Ionic Liquids

To sensitively detect the electrode surface roughness change in the initial process of the lithium electrodeposition/dissolution processes in ionic liquids (ILs), electrochemical surface plasmon resonance (ESPR) measurements were performed in an in situ manner on a gold electrode in a glyme-Li salt solvate IL, tetraglyme/lithium bis­(fluorosulfonyl)­amide ([Li­(G4)+]­[FSA–]), and an IL, 1-butyl-3-methylimidazolium bis­(fluorosulfonyl)­amide ([C4mim+]­[FSA–]), containing 100 mM Li+[FSA–]. The SPR angle shifts (ΔθSPR) were tracked simultaneously with the repetitively recorded cyclic voltammograms (CVs) of the Li underpotential deposition (UPD)/underpotential stripping (UPS). ΔθSPR increased/decreased in the UPD/UPS processes, sensitively responding to the refractive index change at the IL/electrode interface. The time derivative ΔθSPR curves basically reproduced CVs but were significantly less influenced by residual current, indicating that ESPR was an effective in situ method to track the Li UPD/UPS processes. In [Li­(G4)+]­[FSA–], the shift in ΔθSPR per deposited Li amount did not change as the CV scan was repeated, indicating no change in surface roughness. In contrast, in [C4mim+]­[FSA–], the same parameter increased with an increase in the scan number, reflecting the increase in surface roughness as confirmed by Fresnel reflectivity simulations. The comparison of ESPR results with the simulations suggests that for both ILs, the surface of the deposited Li layer was smoothened during the period after the Li UPD and before the Li UPS in CVs.