Revisiting the Electrochemical Response of Alkali Metals in Contact With Liquid Battery Electrolytes

Alkali metal electrodes are of great importance as high energy density anodes and counter electrodes in half‐cells, but their electrochemical response, especially for sodium and potassium metal, is currently not well understood. Herein, electrochemical impedance spectroscopy (EIS) and complementary galvanostatic polarization have been used to study temperature, time and concentration dependence of symmetric cells with alkali metal electrodes and model electrolytes, extract information on porosity of solid electrolyte interphase (SEI) in different cases, activation energy and charge transfer resistance, assess chemical diffusion coefficients, as well as temperature and time‐dependent effective cationic transference numbers upon longer rest. We show how the same chemistry‐agnostic EIS equivalent circuit model can be used to explain the EIS results in symmetric cells. Most importantly, we show that charge transfer contribution is considerable in these systems, that both resistances and activation energies of SEIs should be used to evaluate them, and that effective cationic transference number degrades in both Li and Na cells upon time ageing.