The effect of Li + addition at the interface of a 1-ethyl-3-methylimidazolium bis(fluorosulfonyl)amide ([C2mim][FSA]) room-temperature ionic liquid (RTIL) and a Pt electrode is investigated by infraredvisible sum-frequency generation (IV-SFG) vibrational spectroscopy. Addition of Li + to the Pt|[C2mim][FSA] system results in the extension of the electrochemical window (EW) by more than 1.0 V at its negative edge. The potential dependence of the SF signal reveals that the [FSA] − anion of neat [C2mim][FSA] is desorbed at −1.5 V while it remains in place even at −2.0 V when Li + is added. The SFG spectra indicate that the [FSA] − anion at the Pt|[C2mim][FSA] interface interacts with Li + at the interface with the negatively-charged Pt electrode. This [FSA] − anion layer anchored through Li + suppresses [C2mim] + cation adsorption on the negatively-charged Pt electrode, resulting in a wider electrochemical window.
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