Atomistic Simulation of 2D Materials and Ionic Liquids for Energy Storage and Conversion

Energy is a crucial part of our everyday life, but traditional generation is unsustainable, necessitating alternative sources. Storing energy, especially from renewable types of energy like solar and wind, ensures supply during disruptions and enables transportation to areas in need. To meet increasing demands and minimize environmental impact, advancing energy storage and conversion technologies is essential. The focus of this dissertation is investigating energy storage and conversion at microscopic level through 2-dimensional materials and battery electrolytes. Recently, two-dimensional (2D) transition metal carbides and nitrides (MXenes) have become prominent in electronics and electrochemical energy conversion and storage systems, especially since heat production greatly affects these devices' safety and efficiency. Within chapter 2, the effect of surface termination (bare, fluorine and oxygen) on the lattice thermal conductivity of Ti3C2Tz has been investigated using density functional theory and linearized solution of Boltzmann transport equation. It was found that thermal conductivity of fluorine-terminated Ti3C2Tz (108 W/m.K) is approximately one order of magnitude higher than its oxygen-terminated counterpart (11 W/m.K). Phonon dispersions, group velocities, specific heats and scattering rates were studied to shed light on difference in thermal conductivities for different surface terminations. Ternary mixtures of organic solvents, ionic liquids (ILs) and Li salts have shown outstanding performance in Li-ion and Li-oxygen batteries, outperforming both binary mixtures of organic solvents or ILs with Li salts electrolytes. In chapters 3, 4 and 5, the phase behavior of ternary mixtures of ionic liquid, organic solvent and lithium salt is investigated using molecular dynamics simulations. It is shown that at room temperature, the electrolyte separates into distinct phases with specific compositions; an ion-rich domain that contains a fraction of solvent molecules and a second domain of pure solvent. Volume fraction of IL/solvent as well as the temperature were shown to be the important factors for separation of mixtures into two domains. The phase separation is shown to be entropy driven and is independent of lithium salt concentration. In chapter 4, using large-scale classical molecular dynamics simulations, we looked into ten different ternary electrolyte mixtures using combinations of [EMIM]+, [BMIM]+ and [OMIM]+ cations with [NO3]-, [BF4]-, [PF6]-, [ClO4]-, [TFO]- and [NTf2]- anions, tetraglyme and Li salt to study the effect of ionic liquid composition on the phase behavior of ternary electrolyte mixtures. We uncovered that in these electrolytes, phase separation is mainly a function of pairwise binding energy of constituents of the mixture. To corroborate this theory, many simulations were performed at various temperatures ranging from 260K to 500K for each mixture, followed by calculating the binding energy of ionic liquid pairs using density functional theory. In chapter 5, the formation of multiple stable ionic domain within a system was investigated. To this extend, first to remove the potential finite size effect, multiple systems larger than previously simulated systems were constructed and simulated. Second, the effect of presence of both [NO3]- and [NTF2]- anions, corresponding to the anions that make the strongest and weakest binding with cation, was investigated. Finally, we looked into a way to engineer to target or avoid formation of ionic domains by adding appropriate anions to the mixture.