Operando Raman Analysis of Concentration Polarization in Gel Polymer Electrolytes

Abstract Gel polymer electrolytes (GPEs) have attracted considerable attention as promising electrolyte materials because they combine the high ionic transport properties of liquid electrolytes with the safety of solid-state electrolytes. However, electrochemical polarization can cause salt concentration gradients to develop near the electrode interface. These gradients can induce concentration polarization, which increases overpotentials and reaction inhomogeneity. Despite its importance, the spatiotemporal evolution of concentration polarization inside operating GPEs remains insufficiently understood. In this study, spatially resolved operando Raman spectroscopy was used to visualize and quantitatively analyze the temporal evolution of salt concentration distributions and ionic association states (free/bound) in a GPE. The GPE was composed of G4 (tetraglyme), poly(ethylene oxide-co-propylene oxide) (P(EO/PO)), and LiN(SO2CF3)2. In the Raman spectrum obtained during potentiostatic measurements on a [Li|GPE|Li] symmetric cell, under polarization, the band assigned to the N(SO2CF3)2 anion shifted toward higher wavenumbers and the fraction of the bound component increased on the dissolution-electrode side. These results indicate a local increase in salt concentration, which reversibly relaxed toward the initial state upon switching to open-circuit voltage conditions. The development of concentration polarization was found to depend strongly on the applied voltage. Spatially resolved measurements revealed that the salt concentration increased toward the electrode on the dissolution-electrode side and decreased on the deposition-electrode side. These observations demonstrate that concentration polarization was not confined to the electrode interface but instead extended throughout the electrolyte. The operando Raman spectroscopy approach established in this study enables direct visualization of concentration polarization and ionic association dynamics in operating GPEs with both spatial and temporal resolution. This methodology provides valuable insights into ionic transport behavior under operating conditions and offers a useful framework for the evaluation and rational design of advanced electrolyte systems.