Studies on the medium-frequency impedance arc for Lithium-ion batteries considering various alternating current amplitudes

An impedance measurement approach with various current amplitudes is proposed to investigate the impedance behavior of power Lithium-ion battery in the frequency domain. Notably, the impedance arc is divided into amplitude-independent response in the high-frequency range and amplitude-dependent response in medium-frequency range. A shrinking of semi-circle diameter of the impedance arc at medium-frequency part of measured spectrum is distinctly observed with rising current amplitude. Thus, a series of particular experiments are performed to facilitate the interpretation of the phenomenon. The possible influencing factors for the battery, including its state of charge, internal pressure, and temperature rise during the high-rate short-time charge/discharge, are illustrated and excluded for analysis, respectively. Based on the analysis of Butler-Volmer equation and Arrhenius empirical equation, a formula, which contains temperature and current known to influence the charge-transfer resistance, is derived logically. It indicates that the charge-transfer resistance reduces as the current amplitude increases at a fixed temperature, which fundamentally dominates the shrinking of the semi-circle at medium frequencies. Therefore, the medium-frequency impedance arc is able to represent the chemical reaction process, even under large current excitation (within a certain range).