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

Abstract

Superconcentrated electrolyte solutions have attracted much attention because of their ability to stabilize solvent molecules for high-voltage rechargeable batteries. However, the stabilization mechanism has not been fully understood because of the lack of an in situ observation of the relevant interface. In the present study, we investigated the interfaces of platinum electrodes and acetonitrile solutions containing dilute and superconcentrated lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) using in situ heterodyne-detected vibrational sum frequency generation as well as ex situ vibrational sum frequency generation spectroscopies. The results showed that the structures of the electrode interfaces are remarkably different with these two solutions and indicate that acetonitrile decomposes in the dilute solution, whereas the TFSI– anion decomposes in the superconcentrated solution.