Guidelines for Impedance Analysis of Parent Metal Anodes in Solid-State Batteries and the Role of Current Constriction at Interface Voids, Heterogeneities, and SEI

Impedance spectroscopy is widely used in operando studies of solid-state batteries for characterizing charge transport and correlating it with structural features. A typical impedance spectrum reveals, in addition to transport signals of the solid electrolyte, one or more contributions due to processes taking place at the electrode interfaces. The focus of this study is on reversible (parent) metal anodes and a 3D electric network model is used to analyze the variation of their impedance as a function of pressure, temperature, or aging during cycling. This provides a recipe for experimentalists on how to identify impedance contributions arising from different interface effects, such as, charge transfer, dynamic current constriction, and solid electrolyte interphase formation. Rules are derived for assigning the different interface signals or identifying the dominant contribution in case of similar frequency-dependence and a standard procedure for analysis is proposed. The suggested procedure is applied to experimental data of half cells where lithium metal is in contact with garnet-type Li6.25Al0.25La3Zr2O12. This case study yields unambiguously that geometric current constriction due to morphological instabilities at the metal anode interface during cycling is the rate-limiting step for this type of metal anode, rather than the frequently assumed polarization resistance of the electric charge transfer migration process.