| Description |
Ionic transport is ubiquitous in the world. Energy-storage systems, such as lithiumion batteries (LIBs), alkaline fuel cells (AFCs), and electric double layer capacitors (EDLCs), rely upon an efficient built-in ion transport within the devices. In this dissertation, three typical systems related to energy storage/conversion applications are systematically investigated via molecular dynamics (MD) simulation: alkaline fuel cell membranes, solvate ionic liquids, and solid polymer electrolytes for next-generation of LIBs. Using combined reactive and nonreactive MD simulation approach the chemical structure of polymer and the morphology of hydrated AFCs membranes are correlated with the ion transport mechanisms. Specifically, it is demonstrated that the Grotthuss mechanism plays a vital role in facilitating the transport of OH- through the bottlenecks in water channels inside the membrane. The Grotthuss mechanism also contributes significantly to the overall diffusion in the water-rich domain. The degradation mechanisms of ammonium-based functional groups are investigated as a function of size and/or structure of alkyl groups attached to the cationic groups of polymer chains. The long alkyl chains can significantly stabilize the functional groups, reducing the reaction rates, but lead to increased heterogeneity in water domain size distribution. The asymmetrically modified cationic groups, i.e., those modified with long and short alkyl chains, can promote the formation of larger water channels with uniform size. Therefore, the asymmetrical modification patterns are expected to yield AFC membranes with high performance. iv In battery systems, solvate ionic liquids (SILs) and solid polymer electrolytes (SPE) are considered as next-generation electrolytes. However, with the transport mechanism of Li+ and the origin of low Li+ transference number in SILs, the molecular mechanisms needed to tune the ionic conductivity in SPE remain poorly understood. Based on the joint MD simulation and electrochemical experiment study, the molecular origin of those limits for application of SILs and SPEs are explored. The anticorrelated motion of cations and anions in SILs leads to a low transference number of Li+ due to the necessity of satisfying the conservation of momentum. Adding excessive free solvents or/and using solvents with weaker binding energy between Li+ and solvent are needed to increase the transference number of Li+. MD simulations conducted on supramolecular polyrotaxanes selfassembled from poly(ethylene oxide) (PEO), cyclodextrin (CD), and lithium bistriflimide salts showed that methylation of the hydroxy groups on CD generated higher ionic conductivity. Analysis of structural properties indicated that 70%-80% of Li+ are located outside of the channels formed by the CD rings threaded on PEO chains, providing a new insight into mechanisms of Li+ transport in these structurally complex environments. The spatial distribution of Li+ inspired further modification of CD by grafting polycaprolactone. Both experimental measurements and simulations demonstrated a remarkable increase in ionic conductivity. |
