Shelby Brantley

Chemistry and Biochemistry

Faculty Advisor: Steven Corcelli

Understanding the Dynamics of Solid-Electrolyte Interphase in Sodium Ion Batteries with Fluoroethylene Additive

A primary limitation of rechargeable batteries is the build-up of the solid electrolyte interface (or interphase, SEI) on the electrode surface. The formation of the SEI causes irreversible capacity, cycle lifetime, and rate capacity restraints for sodium batteries. Additives, such as fluoroethylene (FEC), can reduce or even eliminate the side reactions that cause the SEI, however, they have been used sparingly due to the lack of fundamental understanding of the mechanisms that produce the SEI. In collaboration with the Krummel group at Colorado State University, my goal is to use a combination of molecular dynamic simulations and 2D IR data are used to gain a fundamental understanding of liquid electrolyte solutions.

Liquid electrolyte solutions of dimethyl carbonate, ethyl carbonate, ethylene carbonate, and fluoroethylene carbonate are experimentally studied by adding methyl thiocyanate as a probe and identical solutions are simulated using the polarizable force field, AMOEBA. Using correlation functions, the dynamics of the solutions can be evaluated on a molecular level using molecular dynamics and related to the 2D IR data for validation and interpretation. Electric field correlation functions provide information regarding the whole solution dynamics by calculating the electric field on the nitrogen of the probe and directly relate to the center line slope. Further evaluation using rotational correlation functions for each solvent molecule provides data indicating which solution component is contributing to the changing the solution dynamics measured by the probe. Using cylindrical distribution functions, the solvent shell can be visualized to determine which solution components are in the solvation shell of the probe. Combining all of the information, a better understanding is formed of which components are contributing to the changing dynamics of the electrolyte solution, and therefore, a more fundamental understanding of how the additive, FEC, changes the dynamics of the liquid electrolyte solution is developed. This can be further built on to understand what decomposition reactions are present and how the dynamics of the solution effect these mechanisms.