Operando odd random phase electrochemical impedance spectroscopy as a promising tool for monitoring lithium-ion batteries during fast charging

Electrochemical impedance spectroscopy (EIS) is a powerful technique for characterization of lithium-ion batteries (LIBs). It has extensively been used in laboratory research, but seldom in industrial applications, e.g., electrical vehicles (EVs). This is because the latter needs EIS measurements during operation, but achieving valid EIS measurements in dynamic conditions has been a long-lasting challenge. In this work, LiNi_0.6Mn_0.2Co_0.2O₂ (NMC622)/graphite batteries are characterized by operando odd random phase EIS (ORP-EIS) under practical usage conditions. The validity of the EIS data is quantitatively assessed based on the robust theoretical framework of ORP-EIS. The measured impedance data reveals a continuous evolution of charge-transfer phenomena along changes of state-of-charge (SoC). The results are consistent with conventional stationary EIS data. Moreover, charge-transfer related parameters can be extracted through equivalent electric circuit models or graphical analysis methods from operando ORP-EIS. The parameters exhibit consistent behavior under various charging conditions including different temperatures, C-rates, and state-of-health (SoH). As a proof of concept, operando ORP-EIS provides unique physical insights on top of conventional current/voltage-based monitoring methods, and could be powerful technique to monitor the SoC of the batteries during fast charging.