Unraveling Ion Diffusion Pathways and Energetics in Polycrystalline SEI of Lithium-Based Batteries: Combined Cryo-HRTEM and DFT Study

The solid-electrolyte interphase (SEI) in lithium-based batteries has been extensively studied regarding its composition, structure, and formation mechanisms. However, an understanding of the ion transport through the SEI remains incomplete. Revealing the underlying ion diffusion processes across the SEI holds great potential for enhancing battery performance and improving safety. In this study, we present the outcomes of first-principles density functional theory (DFT) calculations based on cryogenic high-resolution transmission electron microscopy (cryo-HRTEM) imaging, which elucidate the dominant diffusion pathways, energetics, and diffusion coefficients associated with lithium (Li) diffusion through the polycrystalline SEI. Specifically, we focus on Li diffusion through the grain boundaries (GBs) formed by the three primary inorganic components of the SEI, namely, Li2O, LiF, and Li2CO3. Our findings reveal that Li diffusion primarily occurs through numerous open channels created by the GBs. The energetics and potential barriers reveal significant variations depending upon the structural characteristics of these channels, with a distinguished trend being faster Li diffusion within the GB compared to neighboring crystalline regions within the grain interiors. The analysis of the charge density in GBs revealed that Li dendrite formation occurs in GBs with less Li diffusion kinetics.