In-Situ Vibrational Spectroscopic Analysis of Solid Layer Formation at a Superconcentrated Electrolyte/Electrode Interface

Superconcentrated electrolytes have been attracted considerable attention for battery applications for their useful properties such as low flammability, high oxidation stability, etc. Reversible charge/discharge processes were reported, suggesting the successful formation of appropriate solid electrolyte interphases (SEI) in superconcentrated electrolytes, which prevents co-intercalation of solvent molecules. The origin of SEI was reported to be anion species,¹ while that for conventional dilute electrolytes consist of decomposition products of the organic solvents. This feature leads to wide variety of applicable solvents for superconcentrated electrolytes. However, details of the origin of SEI in superconcentrated electrolytes are still under debate. Therefore, we performed in-situ chemical and mechanical analysis of the superconcentrated electrolyte/electrode interface by using surface-enhanced infrared absorption spectroscopy (SEIRAS) to elucidate the origin and formation process of SEI.<br/>We employed 4.2 M LiTFSA/acetonitrile (AN) solution on an Au electrode as a model system for interface analysis. For a negative-going potential scan, SEIRA spectra showed that both TFSA anions and Li⁺ coordinated by AN move toward the electrode when the surface charge of the electrode is switched from positive to negative. It means increased density of the solution on the surface, which can be explained as follows; Li⁺ is drawn to the negatively charged electrode, and then AN and TFSA anions directly interacting with Li⁺ are drawn to the electrode together. Thus, characteristic coordination structure in superconcentrated electrolytes (anion-Li⁺ bond which is absent in dilute concentration) is responsible for such a phenomenon.<br/>At more negative potentials (&lt; 2.0 V), the local concentration of AN decreased and instead that of TFSA forming aggregated network structure with Li⁺ started to increase. Such a structure appears at oversaturated concentration in the bulk. It means that local ionic concentration increased at this potential range, and finally LiTFSA was deposited on the electrode together with small amount of AN by exceeding the saturation concentration. This model is supported by our in-situ AFM analysis of the viscoelasticity of the deposits.<br/><br/>[1] Y. Yamada, et al.,<i> J. Am. Chem. Soc.</i>, <b>136</b>, 5039, (2014)