The following dissertation presents several studies aimed at better understanding the
behavior of different materials employed in energy storage and conversion applications with the goal of putting forth design criteria that can enhance the performance of fuel cells and batteries with different chemistries. The third chapter models the ion conduction in the rare earth phosphate DyPO4. This material is investigated for use in intermediate
temperature solid oxide fuel cells, and the results highlight the benefits for proton conduction associated with a higher symmetry structure, relative to previously studied orthophosphate solid electrolyte materials. The fourth and fth chapter combines modeling and experiments to understand why Ti substitution of Co in the cathode material Li(NixMnxCo1-2x)O2 results in enhanced capacity and cyclability when used in lithium ion batteries. The results of these studies show that Ti substitution results in a lower intercalation voltage that allows for higher lithium extraction given a set voltage cut-o. Further results indicate that Ti helps suppress the formation of a secondary rocksalt phase that reduces cell efficiency.
The sixth chapter investigates the intercalation mechanism of Li and Na into alkali titanate compounds that are of interest as anode materials in lithium-ion or sodium-ion batteries. The goal is to elucidate the susceptibility of each kind of titanate to phase transformations during the lithiation or sodiation process in a functioning battery. The results indicate that these materials fail to deliver theoretical capacities due to site limitations, that can be further aggravated by solvent uptake. Analysis of sodium energy barriers reveal a strong dependence on the state of charge, and potentially the rate of charging.
The seventh chapter investigates electrolyte concentration effects in lithium-oxygen batteries. The model electrolyte system of lithium bistrifluoromethanesulfonimidate (LiTFSI) dissolved in 1,2-dimethoxyethane (DME) is investigated in order to identify different contributions to the failure mechanism of this class of battery. Combined experimental and computational results indicated that the TFSI anion was susceptible to decomposition, which contributed to cathode passivation in cells employing saturated electrolyte, and to kinetic limitations in cells using dilute electrolytes.
The combined work of this dissertation serves to demonstrate the capabilities of a combined experimental and computational approach to understanding and solving the challenges revolving energy storage and conversion materials. The ability to provide atomistic insights to experimental results allows the creation of design criteria for next generation materials, that leverage the insights gained from this combined approach.
