Date of Award

Fall 2011

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Micro and Nanoscale Systems

First Advisor

Collin D. Wick

Abstract

Improving ionic conductivity and lithium mobility in polymer electrolytes is important for their practical use for battery electrolytes. In this study, a combination of molecular dynamics and Monte Carlo simulations was used to bring insight into lithium ion transport in poly(ethylene oxide) (PEO) with plasticizers and also next to alumina solid surface doped with lithium salt. The simulations were performed using a moderately high molecular weight polymer (Mn = 10,000 g/mol) at an EO:Li ratio of 15. For the plasticized system, the PEO with LiN(CF3SO 2)2 (LiTFSI) was mixed with 10 wt% plasticizers that included either cyclic ethylene carbonate (EC) or propylene carbonate (PC). Comparisons with an array of experiments showed a slight underestimation of the compared ionic conductivity, but within a factor of two, at most. With the addition of EC and PC plasticizers, the ionic conductivity increased a moderate degree with most of the increase due to faster TFSI anion motion, but not lithium cation. It was found that propylene carbonate formed complexes with the TFSI anion, in which lithium was an intermediary, creating moderate sized clusters. This formation allowed enhanced diffusion of lithium ions bound with TFSI ions, but this formation was offset by slower diffusion for lithium ions bound with ethylene oxide oxygens. Ethylene carbonate, on the other hand, showed no significant complexing with TFSI anion. The formation of this cluster, therefore, may be an avenue for increasing lithium diffusion but would likely require a plasticizer with stronger interactions with lithium than the carbonates studied. We also examined the influence of both acidic and basic alumina surfaces on the structure and lithium mobility in PEO with LiClO4 salts. The results showed the surface interacted with lithium salt anion in the acidic case via hydrogen bonding, which essentially freezes the lithium salt anion movement at the surface, yet a modest enhancement in lithium ion mobility was observed at low temperature.

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