Exploring Key Descriptors of Solid Electrolyte Interphase Formation in Lithium Ion Batteries through Atomistic Simulations

Abstract

Lithium ion batteries (LIBs) have allowed for a technological revolution in portable electronics, power tools, and electric vehicles. Future improvements, however, require an advancement of existing battery technology to provide for faster charging and larger capacities, while maintaining safety and longevity. This fundamentally represents a knowledge and materials challenge that needs to develop a deeper understanding of electrode and electrolyte materials as well as their interfaces. Electrolyte and electrode materials used in LIBs have been studied separately to a great extent, however the structural and dynamical properties of the electrode/electrolyte interface still remain largely unexplored despite its critical role in governing battery performance. In particular, a key issue restraining the utilization of high-energy density anode materials, such as silicon (Si), is their inability to form a stable interphase during charging which results in continuous capacity fade or even cell failure. The design of better storage devices may be improved through a greater fundamental understanding of the formation of the solid electrolyte interphase (SEI) at the anode/electrolyte interface through atomistic computer simulations. We have utilized molecular dynamics (MD) simulations to correlate the bulk composition of the electrolyte to the interfacial structure and its effect on the transport behavior of key intermediates in the reductive decomposition of this electrolyte at a graphitic carbon electrode. Our study clearly demonstrates that the interfacial structure is sensitive to the molecular geometry and polarity of each solvent molecule as well as the surface structure and charge distribution of the negative electrode. Such quantitative analysis of the molecular arrangements at the electrode/electrolyte interface improves the understanding and description of electrolyte decomposition and SEI formation. Using MD and metadynamics techniques, we have systematically studied how factors such as the bulk composition of mixed carbonate electrolytes, precipitation of lithium fluoride (LiF, a result of the unstable PF6- anion species) onto the anode, and some commonly utilized additives (shown to improve the quality of the SEI layer) influence the transport and accumulation of decomposition products/intermediates. In this talk, we will present some of our recent findings including what we believe to be key descriptors in to predicting the decomposition products arising from the electrochemical reduction of the electrolyte. This improved understanding provides design principles for future investigation into forming a stable SEI on high-energy anodes such as Si.