Parameterization of linear equivalent circuit models over wide temperature and SOC spans for automotive lithium-ion cells using electrochemical impedance spectroscopy

Abstract

Three different types of automotive lithium-ion pouch cells are analyzed with electrochemical impedance spectroscopy over a wide range of operating points in SOC (10–90%) and temperature (−10 to +40°C) with the goal of establishing parameters for an accurate linear equivalent circuit model. The impedance vs frequency is analyzed and attributed to physical behaviors from current collectors, electrode and electrolyte, charge transfer and diffusion. The change in resistance is statistically analyzed as a function of SOC and temperature in an effort to quantify if there are any monotonic patterns. Results show that the charge transfer resistance versus temperature represent the largest relative change of resistance, followed by diffusion resistance versus temperature and the electrolyte resistance over temperature. Only weak correlations with resistance versus SOC are found irregardless of frequency range. Frequency-domain fitted equivalent circuit model parameters are evaluated by comparison of physical measurements in long dynamic load cycles using root-mean-squared of the voltage error between model and physical measurements as performance index. A simple R+2RC model capturing diffusion dynamics performs best at RMSE 15.46mV, better than most similar studies and better than the more complex R+6RC also evaluated.