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 (Rion) and solid electrolyte interphase (SEI) resistance (RSEI) significantly complicates the accurate extraction of RSEI, 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 Rion-to-RSEI coupling on the high-frequency impedance. When a quasi-blocking electrode state was induced in LIBs, Rion 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 Rion 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 Rion and RSEI in the high-frequency regime. When Rion was systematically varied, its dominant impact on impedance spectra was quantified, underscoring the necessity of a precise Rion correction for reliable RSEI determination. This work advances high-frequency impedance interpretation and introduces a robust methodology for accurate RSEI quantification in LIBs.