Molecular Dynamics of Lithium Ion Transport in a Model Solid Electrolyte Interphase

Li⁺ transport within a solid electrolyte interphase (SEI) in lithium ion batteries has challenged molecular dynamics (MD) studies due to limited compositional control of that layer. In recent years, experiments and ab initio simulations have identified dilithium ethylene dicarbonate (Li₂EDC) as the dominant component of SEI layers. Here, we adopt a parameterized, non-polarizable MD force field for Li₂EDC to study transport characteristics of Li⁺ in this model SEI layer at moderate temperatures over long times. The observed correlations are consistent with recent MD results using a polarizable force field, suggesting that this non-polarizable model is effective for our purposes of investigating Li⁺ dynamics. Mean-squared displacements distinguish three distinct Li⁺ transport regimes in EDC - ballistic, trapping, and diffusive. Compared to liquid ethylene carbonate (EC), the nanosecond trapping times in EDC are significantly longer and naturally decrease at higher temperatures. New materials developed for fast-charging Li-ion batteries should have a smaller trapping region. The analyses implemented in this paper can be used for testing transport of Li⁺ ion in novel battery materials. Non-Gaussian features of van Hove self -correlation functions for Li⁺ in EDC, along with the mean-squared displacements, are consistent in describing EDC as a glassy material compared with liquid EC. Vibrational modes of Li⁺ ion, identified by MD, characterize the trapping and are further validated by electronic structure calculations. Some of this work appeared in an extended abstract and has been reproduced with permission from ECS Transactions, 77, 1155-1162 (2017).