Optimization of Ionic Conduction in Ion-containing Polymers for Li-ion Batteries

Open Access
- Author:
- Shiau, Huai-suen
- Graduate Program:
- Chemical Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 13, 2014
- Committee Members:
- Michael John Janik, Dissertation Advisor/Co-Advisor
Michael John Janik, Committee Chair/Co-Chair
Ralph H Colby, Committee Member
Themis Matsoukas, Committee Member
Enrique Daniel Gomez, Committee Member
James Patrick Runt, Committee Member - Keywords:
- ionomers
electrolytes
ionic conduction
lithium batteries - Abstract:
- In this dissertation, I used computational chemistry methods to explore the properties of ion-conducting polymers (ionomers) for use as electrolytes in more efficient, long-lasting Li-ion batteries. The principle of Quantum Mechanics was applied to establish simulation models for prediction of mobile ion concentration and ion mobility which determine the ion conductivity. Several favorable features for optimal electrolytes will be demonstrated and compared with experimental results for model validation. A four-state model was built to optimize the ionomer composition for maximal mobile ion concentration. A cluster-continuum solvation model (CCM) was developed to incorporate specific solvation in the first shell surrounding the cation, all surrounded by a polarizable continuum. A free Li cation, Li+-anion pair, triple ion and quadrupole was used to represent the states of Li+ within the ionomer in the CCM. Predicted concentrations of Li+-conducting states (free Li+ and positive triple ions) are compared among a series of anions to indicate favorable features for design of an optimal Li+-conducting ionomer; the perfluorotetraphenylborate anion maximizes the conducting positive triple ion population among the series of anions considered. An ion-hopping model was established to investigate the effect of anion-anion spacing on Li+ hopping barriers. An optimal spacing of 1.3 nm to facilitate ion transfer with a minimum hopping barrier is identified, and the existence of an optimal ion content is supported by the notion of polarizability volume overlap. This implies that there is a peak in the ion mobility as a function of ion content, leading to an observed peak in conductivity as seen in many experiments on Li-PEO ionomers. The proposed ion hopping mechanism is corroborated by comparison of predictions with observed experimental ion mobility, facilitated by extracting the distribution of hopping distances from small-angle X-ray scattering data. The unique properties of ionomers arise from self-assembly of ionic groups into various aggregates. A cluster-continuum ab initio solvation model is used to simulate ion aggregates in a poly(ethylene oxide) (PEO) solvating environment. The interplay of specific solvation energy, entropy and ion associations is investigated regarding the transition between “sparse” aggregates (ion chains) and “dense” aggregates (ion sheets). The conversion from ion sheets to ion chains can be solely enthalpically driven by the difference in PEO specific solvation. Enthalpy of sulfonate - Li interactions increase with T because the dielectric constant decreases as 1/T. In contrast, the Li solvation interaction with ether-oxygens is much less sensitive to temperature because such very local specific interactions have dielectric saturation. The stronger ion-ion interactions that increase with temperature combine with the entropy gain from less solvation to drive ions to aggregate as temperature is raised. The implication of the free energy differences of ion chains between contact and separated states for conducting ion fractions in ionomers is discussed in comparison with the experimental results from dielectric relaxation spectroscopy.