Chemomechanical Interactions Dictate Lithium Surface Diffusion Kinetics in the Solid Electrolyte Interphase

The solid electrolyte interphase (SEI) plays a pivotal role in enabling fast ionic transport and preserving the battery electrodes from parasitic reactions with solvents. However, due to large volume changes of lithium (Li) electrodes, the SEI layer can potentially undergo mechanical failure, resulting in electrolyte degradation. The mechanical stability of the SEI is a critical aspect that needs to be modulated for designing rechargeable metal batteries with optimal performance. In this work, we perform density functional theory calculations to investigate the mechanical properties of lithium fluoride (LiF) and lithium oxide (Li₂O) nanofilms and quantify the Li surface diffusion kinetics over these two SEI materials. Based on our analysis, it is identified that Young's modulus and the ideal strength of the SEI are strong functions of the nanofilm thickness and crystallographic direction. Interestingly, we find that mechanical strain substantially alters the Li surface diffusion behavior on the SEI. For a strain of 4%, while the Li surface diffusion rate decreases by two orders of magnitude on the stretched Li₂O film, it increases two times on the stretched LiF film, indicating critical implications on the morphological stability of the metal anode. A fundamental correlation between inherent SEI properties and Li plating behavior is revealed, suggesting a potential pathway to achieve dendrite-free electrodeposition via SEI modulation.