Date

2015

Document Type

Dissertation

Degree

Doctor of Philosophy

Department

Mechanical Engineering

First Adviser

Webb III, Edmund B.

Other advisers/committee members

Delph, Terry; Harlow, Gary; Harmer, Martin; Rickman, Jeffrey

Abstract

In this dissertation, atomistic simulations of Lithium transport into and through graphite grain boundaries are studied. Graphite is a commonly used negative electrode (anode) material in Lithium-ion batteries yet Li diffusion shows high levels of variability in a material that has been in use for the last 25 years. Researchers have used both experiments and computations to prove such variable diffusion and have proposed numerous hypotheses toward explaining the differences. It is known that intralayer transport is most rapid; the purpose of this study is to address secondary mechanisms for diffusion and therefore, cell charging. Although clues have led to the importance of defects such as grain boundaries in battery anodes, there has not yet been an exhaustive study, either experimentally or computationally, that addresses their role. Grain boundaries have widely been studied in metallic systems, but the covalent nature of graphite creates a two-fold motivation for this study. Not only is transport addressed, but also the underlying GB structure that abets and impedes such motion. The aforementioned studies are performed using Molecular Dynamics with both as-written and modified interatomic potentials. Potential optimizations and modifications were performed on existing models to fit the needs of this work. Carbon-Carbon interactions are well described, but Lithium-Carbon and Lithium-Lithium potentials were optimized using ab initio and experimental data of the lithium-graphite and lithium-graphene systems. From this, the modified potentials better represent the equilibrium structures of LixC6 albeit with limitations.Li diffusion from a free surface and into a graphite grain boundary fosters discussions on how surface structure influences transport rates. While no electrolyte or solid-electrolyte interface (SEI) are modeled here, it is thought that all grain boundaries would be subject to approximately the same level of SEI formation and therefore diffusive flux at the GBs will be similar. Therefore, any differences in intercalation rates that manifest provide additional reasoning to diffusional variability in graphite. While the results may not be absolute, the relativity is what is important here.Lastly, grain boundary diffusion is studied for the systems analyzed during intercalation simulations. While one boundary may have faster surface intercalation than another, there are underlying questions as to whether surface behavior correlates with or against internal behavior. Data is presented addressing collective mechanisms for diffusion as well as the role of inherent GB structure on mass transport. Finally, recommendations are made to connect this dissertation with continuum models in addition to advance into new material systems for energy storage applications.

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