Potential-Dependent Layering in the Electrochemical Double Layer of Water-in-Salt Electrolytes

Water-in-salt electrolytes (WiSE) are concentrated aqueous electrolytes recently developed that are of great interest because of their possible relevance for batteries. The origin for their promising application has been ascribed to the formation of percolating nanodomains in the bulk. However, the interfacial structure of WiSE still remains to be understood. In this paper, we characterize the potential-dependent double layer of a LiTFSI-based electrolyte on a charged electrode surface. Ultramicroelectrode (UME) measurements reveal a surface-confinement effect for a ferricyanide redox species at the electrode/WiSE interface. Potential-dependent atomic force microscopy (AFM) shows the presence of layers, the structure of which changes with the applied potential. Thicker layers (6.4 and 6.7 Å) are observed at positive potentials, associated with [Li(H2O)x]+([TFSI]-)y ion pairs, while thinner layers (2.8 and 3.3 Å) are found at negative potentials and associated with [Li(H2O)x]+ alone. Vibrational spectroscopy shows that the composition of the double layer also changes with potential, where [TFSI]- is enriched at positive and [Li(H2O)x]+ enriched at negative potentials.