Impact of Relaxation Time on Electrochemical Impedance Spectroscopy Characterization of the Most Common Lithium Battery Technologies – Experimental Study and Chemistry-Neutral Modeling

In the electrified vehicle application, understanding of the battery characteristics is very important as it is the state-of-art principal energy source. The key battery parameters can be identified by one of the robust and non-destructive characterization techniques as Electrochemical Impedance Spectroscopy (EIS). However, relaxing the battery cell before performing the EIS method, is crucial for characterization results that should be standardized.In this study, the three most common and commercially available lithium-ion technologies (NMC/graphite, LFP/graphite, NCA/LTO) are investigated at 15-45oC temperature, in the range of 20-80% state of charges (SoCs) and with fresh and aged state of health (SoH) conditions. The analysis shows that the duration of the relaxation time spell before impedance measurement has an impact on the battery's non-linear behavior. A rest time of 2h can be proposed irrespective of battery health condition considering neutral technology-based impedance measurement. Impedance growth in ohmic and charge transfer characteristics is found due to aging and the effect of rest periods are analyzed as well, from an electrochemical standpoint. This experimental data is fitted to develop an empirical model, which can predict the non-linear dynamics of lithium technologies with a 4-8% relative error for longer rest time.