Morphology-Driven Impedance Asymmetry of Lithium Metal During Stripping and Plating

Electrochemical impedance spectroscopy (EIS) is a powerful tool for probing electrochemical systems. A rigorous understanding of the impedance behaviour of lithium metal electrodes is essential for analysing the kinetics of lithium metal batteries. Here, using a three-electrode configuration, we uncover a morphology-driven impedance asymmetry of lithium metal, in which pit-dominated stripping leads to high impedance, while dendrite-dominated processes result in low impedance. By analysing the impedance response and polarization behaviour of lithium metal electrodes in Li||Cu half-cells and Li||LFP full cells, we demonstrate that neglecting the state-dependent impedance of lithium metal can lead to systematic misinterpretation of electrochemical measurements. We further show that this phenomenon is general and persists across different electrolyte systems. Finally, we develop an impedance-based approach to quantitatively extract descriptors of dendritic morphology, providing a statistical and macroscopic complement to microscopic techniques. This work establishes a framework for linking lithium morphology to electrochemical response and offers guidance for more accurate analysis of lithium metal and full-cell systems.