Stability of Solid-Electrolyte Interphase (SEI) on the Lithium Metal Surface in Lithium Metal Batteries (LMBs)

The structural character and mechanical stability of solid-electrolyte interphase (SEI) play a critical role in the formation of dendrites in lithium metal batteries (LMBs). However, due to the complex structures of SEI, the mechanisms by which dendrites nucleate at the interface of Li metal and SEI layer are not well understood. In this work, we employed first-principles calculations using density functional theory (DFT) to study the stability of the innermost layer of the grain-structured SEI on the Li metal surface. The grain structures of LiF/LiF, Li2O/Li2O, and LiF/Li2O and their interaction with the Li surface are considered. The stability of different SEI grain structures is analyzed based on the overpotential due to Li addition. Also, the excess energies required for SEI components to crack along their grain boundary (GB) defects are investigated. The initial observation of these structures suggests that there is a significant rearrangement in the interfacial layers (first few layers) of Li. The system reaches a metastable state when more Li is added to the GB and/or to the triple-phase boundary (TPB) between the GB and Li surface. The energetics from the DFT calculations vary significantly depending upon the grain structures, with LiF/LiF grain structures being the most stable and LiF/Li2O being the least stable. The calculated energies, with and without vacancies, at the TPB region of Li/SEI interfaces, show that the interface between Li2O/Li2O on Li is the least favorable energetically and more susceptible to stress accumulation and cracking followed by LiF/Li2O on Li and LiF/LiF on Li, respectively.