Operando electrochemical impedance spectroscopy and its application to commercial Li-ion batteries

Electrochemical impedance spectroscopy (EIS) is a non-invasive technique for examining kinetics of electrochemical systems. Applied to energy storage devices, the impedance contains information about the state-of-charge and state-of-health of a battery. Classical EIS measurements, as implemented in many cyclers, are restricted by two important assumptions: linearity and stationarity. However, Li-ion batteries are inherently nonlinear, and are nonstationary while under operation. Classical EIS can thus only be performed on batteries in steady-state, and hence, no information about batteries under operation can be extracted. In this article, operando EIS is introduced as a promising tool for measuring time-varying impedance data of Li-ion batteries. The mathematics behind the impedance extraction are detailed, starting from nonstationary and mildly nonlinear current and voltage data. The technique is then applied to reproducible experiments, while charging and discharging commercial Li-ion batteries, using a commercial potentiostat. The operando impedance is shown to be different from the classical impedance, and the nonlinear behaviour of the battery is studied. Applications of operando EIS are discussed, with the focus on the modelling of the operando impedance data through equivalent circuit models, revealing the evolution of the ECM parameters over time. • Reproducible time-varying impedance measurements of commercial LG M50 batteries. • Compared and contrasted classical versus operando EIS while charging and discharging. • The impedance under operation is consistently lower than while resting. • Response to a multisine separated into linear, nonlinear and noise contributions. • The nonlinearities in the measurements are independent of the excitation power.