Determination of Electrode/Electrolyte Interface Impedance via Transfer Length Method and Its Application to Solid Polymer Electrolytes

Accurate analysis of the electrode/electrolyte interface (EEI) is essential for the design of electrochemical systems, including batteries and bioelectronics. However, conventional electrochemical impedance spectroscopy (EIS), which often relies on equivalent circuit modeling and ideal assumptions, faces difficulties in separating interfacial and bulk contributions, particularly in nonresistive media such as solid electrolytes or biological tissues. In this study, we present the EIS-based transfer length method (EIS-TLM), a technique that decouples interfacial and bulk impedance by analyzing the dependence of impedance on electrode distance. The approach was first validated using a 0.9% NaCl aqueous solution, where EIS-TLM achieved excellent agreement with conventional analysis, with R-squared values of 0.98 (magnitude) and 1.00 (phase) for stainless steel electrodes, and 0.93 (magnitude) and 0.98 (phase) for gold electrodes. EIS-TLM was then applied to a polyethylene oxide (PEO)—LiClO4 solid polymer electrolyte (SPE), which exhibits both resistive and capacitive characteristics. EIS-TLM successfully extracted frequency-dependent dielectric permittivity of the bulk electrolyte, with results showing strong consistency with reported values. These findings demonstrate the robustness and versatility of EIS-TLM for impedance analysis in complex electrochemical systems, particularly those involving solid-state or polymer-based electrolytes.