Chemical stability and functionality of Al2O3 artificial solid electrolyte interphases on alkali metals under open circuit voltage conditions

We studied chemical stability of atomic layer deposition-grown Al2O3 artificial solid electrolyte interphases (SEIs) on lithium and sodium upon contact with liquid electrolyte by electrochemical impedance spectroscopy (EIS) and in the case of Li also by x-ray photoelectron spectroscopy. Both methods show that the formed Al2O3 is porous for all nominal thicknesses, and that the natural SEI grows in its pores and cracks. EIS shows that the porosity of the SEI on Na is higher than the one observed on Li, in particular at higher nominal thicknesses of Al2O3. The observed values of activation energies related to the transport through the SEI indicate either a denser natural SEI in the pores of Al2O3 and/or considerable space charge effect between Al2O3 and the SEI phase.