Porosity of Solid Electrolyte Interphases on Alkali Metal Electrodes with Liquid Electrolytes

Despite the fact that solid electrolyte interphases (SEIs) on alkali metals (Li and Na) are of great importance in the utilization of batteries with high energy density, growth mechanism of SEIs under an open-circuit potential important for the shelf life and the nature of ionic transport through SEIs are yet poorly understood. In this work, SEIs on Li/Na formed by bringing the electrodes in contact with ether- and carbonate-based electrolyte in symmetric cells were systematically investigated using diverse electrochemical/chemical characterization techniques. Electrochemical impedance spectroscopy (EIS) measurements linked with activation energy determination and cross-section images of Li/Na electrodes measured by ex situ FIB-SEM revealed the liquid/solid composite nature of SEIs, indicating their porosity. SEIs on Na electrodes are shown to be more porous compared to the ones on Li in both carbonate and glyme-based electrolytes. Nonpassivating nature of such SEIs is detrimental for the performance of alkali metal batteries. We laid special emphasis on evaluating time-dependent activation energy using EIS.