Interrogating the Chemical Processes that Govern Solid-Electrolyte Interphase Growth with Molecular Simulations

Abstract

A major cause of capacity fade in lithium-ion batteries (LIBs) can be attributed to the formation of the solid-electrolyte interphase (SEI), which is a layer that forms at the interface between the electrode surface and liquid electrolyte. Although this layer plays a key role in determining the lifetime of a battery, its growth mechanism and composition are still not well understood. In general, it is known that this layer is formed via the reductive decomposition of the liquid electrolyte solvent directly contacting the electrode, but there is still a lack of knowledge associated with the various reaction mechanisms that drive this process. This dissertation will cover three approaches we have taken to better understand the SEI growth process with molecular simulation techniques.In the first part of this dissertation, I will discuss how we used reaction network exploration and density functional theory (DFT) to elucidate the role of common LIB electrolyte additives (fluoroethylene carbonate, FEC; and vinylene carbonate, VC) in oligomerization reactions during SEI growth. FEC and VC have been shown experimentally to greatly improve a battery’s performance and longevity, but the underlying mechanisms driving this marked improvement is not well known. I will demonstrate how these additives modulate the SEI oligomerization process while explaining how these reactions connect to the observed improvement in battery performance. In the second part of this dissertation, I will shift my focus toward understanding the thermodynamics and kinetics associated with the reduction of ethylene carbonate (EC) at the electrode-electrolyte interface, which is a commonly studied electrolyte solvent. Most studies employ DFT calculations with implicit solvent models to compute the necessary components that comprise the reduction potential, such as the ion solvation free energy. However, I will highlight how we used the potential distribution theorem, a statistical mechanics approach, to directly compute single-ion solvation free energies for the calculation of electrolyte reduction potentials and discuss how this differs from literature. In addition, I will present our use of Marcus theory to explore the kinetics of EC reduction, wherein we see an interesting competition between the thermodynamics and kinetics for EC reduction with and without a neighboring Li+. To conclude the dissertation, I will discuss our ongoing work on using transition path sampling to characterize the role of interfaces on key electrolyte degradation reactions.