Molecular Modeling of Reaction and Diffusion Processes at Electrochemical Interfaces in Lithium Ion Batteries

Abstract

Lithium batteries are well-known as a critical technology, but they are also a fundamentally interesting chemical system do to the presence of complex phenomena such as chemical and electrochemical reactions, phase changes, and ion transport across interfaces, to name a few. Such complexity, however, provides opportunity for computational approaches which are able to resolve size- and time-scales unreachable through experimental methods. Many of the practical hurdles moving forward in battery technologies lie at the interfaces between the electrolyte and the electrodes, making study of these interfaces an important application of computational tools. However, no single computational approach is robust enough to encapsulate the entirety of the interface while also providing the appropriate resolution to study charge transfer and chemical bond breaking/formation. For this reason, it is critical that a multi-scale approach is taken in order to incorporate both accurate models and accurate methods into the characterization of important interfacial phenomena. Our work focuses on the formation of the so-called solid electrolyte interphase (SEI) by the reductive decomposition of ethylene carbonate (EC) at a graphite electrode surface prior to the initial lithiation during its first charge cycle. The SEI is a solid film consisting of decomposed electrolyte which permits the conduction of Li+ ions to and from the electrode, while passivating further electron transfer. Thusly, its formation process consists of electrochemical reduction, subsequent chemical reactions, reactant/intermediate/product transport, and solid precipitation onto the electrode surface. In order to encapsulate all of these phenomena, multiple simulation tools must be used. In particular, we utilize molecular dynamics (both classical force field and ab initio), enhanced sampling techniques, and mixed MD/Monte-Carlo methods to model reaction, transport, and phase transformation phenomena which contribute to the formation of the SEI. In this talk, we will present some of our recent findings from our investigation of the SEI and the methods we use to study it. In particular, we will discuss our framework for modeling reactions and diffusion processes near the electrode and the importance of explicit consideration of the electrolyte in predicting kinetics. Interfaces are critical to electrochemical systems from lithium ion batteries, to electrocatalysts, to sensors and computational methods possess great potential in shaping our understanding of them.