State of Charge-Dependent Polynomial Equivalent Circuit Modeling for Electrochemical Impedance Spectroscopy of Lithium-Ion Batteries

Electrochemical impedance spectroscopy (EIS) is used not only to give a thorough understanding of reaction kinetics and transport mechanisms in lithium-ion batteries (LIBs), but also to provide a promising nondestructive tool for state of charge (SOC) estimation. Although various equivalent circuit models (ECMs) have been proposed to model impedance spectra, the impact of SOC on circuit parameters is often neglected in these models. In this study, the nonlinear relationship between circuit parameters and SOC is explicitly characterized using analytical polynomial functions. The effect of polynomial order is systematically investigated by means of fitting and prediction accuracy, in which the prediction performance is evaluated using leave-one-out cross-validation (LOOCV) method. The EIS measurements of a 20-A·h commercial LIB are performed to demonstrate the effectiveness of the proposed model. The results show that a seventh-order polynomial function is sufficiently high to capture the nonlinear effect of SOC on circuit parameters. Moreover, the LOOCV prediction performance of the polynomial function-based ECM is probably better than that of a common interpolation-based ECM.