Towards an Atomic-Level Understanding of Solid Electrolyte Interphase Formation through Molecular Simulation

Abstract

Lithium ion batteries (LIBs) have been one of the greatest technological achievements in recent history, allowing for leaps forward in consumer technologies. However, growing demand for electric vehicles worldwide is fueling demand for LIBs with larger capacity, faster charging, and longer cycle lives without compromising on safety. This has prompted a resurgence in the study of lithium metal as a next-generation anode, despite challenges with stabilization of the anode-electrolyte interface and the formation of dendrites. In order to realize the large theoretical capacity offered by lithium metal as an anode material, the interface must be engineered to allow for stable cycling through the in situ formation of an ionic conducting and electrically insulating surface layer known as the solid electrolyte interphase (SEI) by reductive decomposition of the electrolyte. While formation of a stable SEI remains an issue in the advancement of next-generation batteries, a fundamental understanding of the process at the graphite electrode/electrolyte interface remains elusive despite its ubiquity in commercial LIBs. The time- and size-scales at which the SEI is formed makes its study experimentally difficult at the appropriate resolution for the underlying phenomena which are critical to determining its composition, morphology, and properties. In order to build an atomic-level understanding of the SEI formation process, we have utilized molecular dynamics (MD) simulations with both classical force fields and density functional theory (DFT) to explore the reaction and transport processes which occur at the anode/electrolyte interface including reduction of ethylene carbonate (EC), intermediate diffusion near the electrode surface, and subsequent chemical reaction of the intermediate species. We have explored the effects of acyclic carbonates on reactions kinetics and interfacial structure as well the effects of some common additives. In this talk, we will present some of our recent findings on how local structure of the electrolyte influences reaction kinetics and transport processes which lead to the aggregation and precipitation of reduced carbonate solvents to form the SEI. This improved understanding of the underlying processes which contribute to the formation of the SEI on graphite electrodes may not only improve engineering of such interfaces in commercial applications, but also generate new ideas in the pursuit of a stable lithium/electrolyte interface.