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

Abstract

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 LiTFSIbased 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(H 2 O) x ] + ([TFSI] − ) y ion pairs, while thinner layers (2.8 and 3.3 Å) are found at negative potentials and associated with [Li(H 2 O) x ] + alone. Vibrational spectroscopy shows that the composition of the double layer also changes with potential, where [TFSI] − is enriched at positive and [Li(H 2 O) x ] + enriched at negative potentials.

Physicochemical and Electrochemical Properties of Water‐in‐Salt Electrolytes

“…IR studies were found to be in agreement with these theoretical predictions [74] . In a more detailed vibrational spectroscopy investigation, it was found that the interface structure in 21 m LiTFSI water‐in‐salt electrolyte depends on applied potential, [75] as illustrated schematically in Figure 6b. Furthermore, AFM measurements at various potentials revealed the formation of a thick layer (in the range of 6.5 Å) at positive potentials due to the presence of large TFSI anions that evolved to a thinner layer consisting of water molecules and hydrated Li + .…”

“…Hence, the structure of the ions and solvent in water‐in‐salt electrolyte, as presented above, is likely to have an impact on their structure at the electrode surface. Insight to the structure of the electrified interface is obtained by modeling, [72–74] vibrational spectroscopy, [74,75] and atomic force microscopy (AFM) [75] …”

“…(b) Potential‐dependent double layer of a WIS LiTFSI electrolyte on an Au charged electrode surface. Reproduced with permission [75] . Copyright 2020, American Chemical Society.…”

“…Finally, the electron transfer properties of redox probes could be influenced by the nature of water‐in‐salt and the structure of the electrified interfaces. Preliminary investigations with model redox systems such as ferricyanide/ferrocyanide in water‐in‐salt electrolytes have been recently reported [75,95] and can serve as basis for further studies involving electron transfer processes within the electrochemical stability window. In summary, despite interest of water‐in‐salt electrolytes for electrochemical energy storage, detailed fundamental studies are needed to get a better understanding of this new class of materials.…”

Gelation, Clustering and Crowding in the Electrical Double Layer of Ionic Liquids

“…salt-in-ILs (SiILs) [7][8][9][10][11][12][13], which increases the energy that can be stored in the device. The promise of IL-based technologies has caused significant interest in understanding ILs, and other concentrated electrolytes, such as water-in-salt electrolytes (WiSEs) [14][15][16][17][18][19][20][21][22][23][24]. Indeed, owing to the high concentration and lack of high dielectric solvent, the energy of electrostatic interactions between nearest neighbors is much larger than thermal energy [3].…”