Origin of Solvent Stabilization at Superconcentrated Electrolyte/Electrode Interfaces Revealed by Heterodyne-Detected Vibrational Sum Frequency Generation Spectroscopy

Superconcentratedelectrolyte solutions have attractedmuch attentionbecause of their ability to stabilize solvent molecules for high-voltagerechargeable batteries. However, the stabilization mechanism has notbeen fully understood because of the lack of an in situ observationof the relevant interface. In the present study, we investigated theinterfaces of platinum electrodes and acetonitrile solutions containingdilute and superconcentrated lithium bis-(trifluoromethanesulfonyl)-imide(LiTFSI) using in situ heterodyne-detected vibrational sum frequencygeneration as well as ex situ vibrational sum frequency generationspectroscopies. The results showed that the structures of the electrodeinterfaces are remarkably different with these two solutions and indicatethat acetonitrile decomposes in the dilute solution, whereas the TFSI- anion decomposes in the superconcentrated solution.