Ion transport and growth behavior of solid electrolyte interphases on Li and Na with liquid electrolytes based on impedance analysis

Electrochemical impedance spectroscopy (EIS) is an irreplaceable tool for investigating ion transport mechanism and growth of solid electrolyte interphases (SEIs) on Li or Na electrodes. Nevertheless, studies providing understanding of the EIS response and establishing a proper equivalent circuit models for SEIs are rare. In this study, using already published and new data, we develop a most simplified, but suitable EIS model by focusing not only on the measured resistances but also on the corresponding activation energies of ion transport and the respective capacitances. The equivalent circuit model takes account of channels filled with liquid electrolyte in parallel with the more resistive bulk parts of the SEI. The growth of initially porous SEIs proceeds at the liquid/solid interface in the channel, where an additional thin passivation layer (for example native film) may exist. The channels (in reality pores of solid SEI) are either eventually filled, or, if the products from the reaction between liquid electrolytes and alkali metals possess a very different molar volume compared to the already-existing material (Li or Na), another internally porous SEI structure forms. The first scenario seems rather typical for Li, in particular at longer growth times. The latter potentially self-similar-growth behavior seems to apply for contacts of glyme-based liquid electrolyte with Na. Inclusion of the lateral compaction effects in an extended model allows for remarkably good description for the Na/glyme scenario.