On the interpretation of the impedance response of a passivated lithium metal anode

The impedance response of passivated lithium metal anodes has been the subject of numerous studies. However, the exact significance of the main contribution – the mid-frequency arc due to the formation of the solid electrolyte interphase (SEI) - has not been satisfactorily explained. In particular, many studies have pointed to the existence of two closely coupled arcs – instead of one, which further complicates the interpretation. This study systematically investigates the possible underlying processes that determine the impedance characteristics of the SEI using four electrolytes with concentrations ranging from 1 M to 10⁻⁴ M. The experimental results show that features attributed to processes in the electrolyte phase, such as migration and diffusion, scale significantly with concentration. However, the resistance associated with the coupled mid-frequency arc (the “SEI arc”) shows a modest increase, challenging conventional hypotheses. A novel two-dimensional transmission line model is introduced to account for the heterogeneous topology of the SEI and to capture the interplay of liquid and solid phases. The model accurately describes the observed trends over the entire concentration range and reveals the crucial influence of the SEI on the overall impedance. This work provides new insights into the structure-function relationships of the SEI and highlights the need for topology-aware modeling to understand lithium metal anodes.