Electrolyte Solvation Structure at Solid-Liquid Interface Probed by Nanogap Surface-Enhanced Raman Spectroscopy

Understanding the fundamental factors that drive ion solvation structure and transport is key to design high-performance, stable battery electrolytes. Reversible ion solvation and desolvation are critical to the interfacial charge-transfer process across the solid-liquid interface as well as the resulting stability of the solid electrolyte interphase. Herein, we report the study of Li⁺ salt solvation structure in aprotic solution in the immediate vicinity (∼20 nm) of the solid electrode-liquid interface using surface-enhanced Raman spectroscopy (SERS) from a gold nanoparticle (Au NP) monolayer. The plasmonic coupling between Au NPs produces strong electromagnetic field enhancement in the gap region, leading to a 5 orders of magnitude increase in Raman intensity for electrolyte components and their mixtures namely, lithium hexafluorophosphate, fluoroethylene carbonate, ethylene carbonate, and diethyl carbonate. Further, we estimate and compare the lithium-ion solvation number derived from SERS, standard Raman spectroscopy, and Fourier transform infrared spectroscopy experiments to monitor and ascertain the changes in the solvation shell diameter in the confined nanogap region where there is maximum enhancement of the electric field. Our findings provide a multimodal spectroscopic approach to gain fundamental insights into the molecular structure of the electrolyte at the solid-liquid interface.