(Invited) Static and Dynamics View of the Solid Electrolyte Interphase during Charge Transfer Reactions -Modeling and Experiments

Abstract

By treating the solid electrolyte interphase as a model layer, the energy landscape of the simplest charge transfer reaction of Li+ + e- --> Li0 was predicted via a new Li/Li2CO3/liquid-EC interface model with density functional theory calculations. The model revealed that if the SEI layer is a perfect crystal, the charge transfer step will occur underneath the SEI layer. However, it takes time for the SEI to build up into a nanometer-thick layer from the molecular level reduction reactions on negative electrodes. Isotope-assisted time-of-flight secondary ion mass spectrometry analyses revealed a new “bottom-up” SEI growth mechanism. It was discovered that the topmost SEI near the electrolyte formed first and the SEI near the electrode formed later. The formation rate of the surface SEI layer can be intertwined with the Li morphology change, which imposes a deformation rate on the surface layer. Even for Li-metal in the trace amount of oxidizing gas (O2), the Li morphology was switched from spheres to wires and to spheres again by the oxygen partial pressure, which corresponds to different oxidation rates. There is a fine balance between the ion current density and a growth rate of a thin lithium-oxide shell on the surface of the metallic Li. While capturing the time evolution of the SEI layer is still very challenging for modeling, the Li-morphology evolution was captured via a multiscale model. This model connected the atomic level charge-transfer physics with the mesoscale morphological evolution based on the phase-field method and transition state theory. It successfully captured the morphological evolution difference between dendritic Li plating and faceted Mg plating and clarified the most efficient method to achieve a smooth plating surface is lowering the exchange current in the charge transfer reactions.