Revealing the electrochemical impedance characteristics of lithium-ion battery (nickel-cobalt-aluminum vs. graphite) under various alternating current amplitudes

To reveal the impact of alternating current (AC) amplitude on impedance, this study investigates the electrochemical impedance with different AC amplitudes for a lithium-ion battery (NCA vs. graphite) and half cells under different states of charge (SOCs), at room and low temperatures. To determine the relationship of different polarization processes between the full cell and half cells, the symmetric cells and the distribution of relaxation times (DRT) are utilized for electrochemical impedance spectroscopy (EIS) analysis. The experimental results indicate that the medium- and low-frequency impedance arcs gradually shrink with the increase of the AC amplitude at low temperatures. DRT focuses on the anode solid electrolyte interphase (SEI), cathode electrolyte interphase (CEI), and charge transfer processes. It is proved that the impedance arc shrinkage is determined by the nonlinear relationship that can be described by the Butler-Volmer equation between current and resistances of the SEI and the charge transfer processes. When the AC amplitude increases to a certain extent, lithium plating also causes impedance arc shrinkage. Moreover, the impedance arc shrinkage of the full cell is mainly affected by the NCA cathode under low SOC. At medium and high SOCs, it is determined jointly by the NCA cathode and graphite anode.