Revisiting High-Frequency Impedance in Li-Ion Batteries: Decoupling Solid Electrolyte Interphase Resistance from Pore Impedance

Electrochemical impedance spectroscopy (EIS) is a cornerstone technique for probing the kinetic behavior of lithium-ion batteries (LIBs). However, in the high-frequency impedance analysis of porous electrodes, the strong coupling between pore-induced ionic diffusion resistance (R_ion) and solid electrolyte interphase (SEI) resistance (R_SEI) significantly complicates the accurate extraction of R_SEI, often introducing substantial estimation errors. In this study, we utilized a LiFePO₄//graphite three-electrode system by integrating experimental measurements with numerical simulations to quantitatively evaluate the influence of R_ion-to-R_SEI coupling on the high-frequency impedance. When a quasi-blocking electrode state was induced in LIBs, R_ion was effectively decoupled and determined via a transmission line model (TLM). A mathematical inverse transformation was then applied to reconstruct an impedance spectrum devoid of R_ion effects. The transformed spectrum exhibited markedly enhanced fitting accuracy and improved adherence to the Arrhenius relationship. Furthermore, TLM-based simulations were performed to elucidate the coupling dynamics between R_ion and R_SEI in the high-frequency regime. When R_ion was systematically varied, its dominant impact on impedance spectra was quantified, underscoring the necessity of a precise R_ion correction for reliable R_SEI determination. This work advances high-frequency impedance interpretation and introduces a robust methodology for accurate R_SEI quantification in LIBs.