MODULATING ION AGGREGATION AND MOBILITY IN IONOMERS

Open Access
Author:
Caldwell, David Wesley
Graduate Program:
Chemical Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 02, 2017
Committee Members:
  • Janna K. Maranas, Dissertation Advisor
  • Michael John Janik, Committee Chair
  • Scott Thomas Milner, Committee Member
  • James Patrick Runt, Committee Member
  • James Patrick Runt, Outside Member
Keywords:
  • Polymer Electrolytes
  • Ion Aggregation
  • Conduction
  • Diffusion
  • Simulations
  • Single Ion Conductors
  • Ionomers
  • Percolation
Abstract:
Polymer electrolyte research has the potential to revolutionize battery technology, similar to what was seen during the emergence of Lithium Ion batteries. Polymers have the advantage over volatile organic liquid electrolytes with respect to their superior mechanical properties and electrochemical stability. Before polymer electrolytes can take the place of their liquid counterparts, limitations involving ion transport must be addressed. The room temperature ionic conductivity of solid polymer electrolytes, 10-5 S/cm or lower, is below the industry set goal of 10-3 S/cm, despite decades of research. This dissertation provides new insights into the underlying mechanisms of ion transport in polymer electrolytes and their relations to the polymer host matrix. By taking advantage of these transport mechanisms, new polymer electrolytes can be designed to meet the growing demand for safe portable energy. Ionomers are a particular class of polymer electrolytes, where one of the ionic species is bound to the polymer backbone. Binding the anion to the polymer backbone reduces the formation of anionic concentration gradients and increases the cationic transference number, both desired properties for electrolytes used in batteries. The ionomer in this work was selected because of the availability of experimental and simulation data, as well as previous research indicating its potential to decouple the mechanical properties of the polymer from ion transport. The decoupling of these properties enables the development of a dendrite suppressing electrolyte. By suppressing dendrites, alkali metal can be used as an anode, resulting in increased volumetric and gravimetric energy density. The underlying mechanism which enables this decoupling is not well understood in literature, but is postulated to involve ion aggregates. This dissertation explores the relationships between ion aggregation, ion transport, and mechanical properties in ionomers. Ion aggregation is controlled by varying the concentration of ions using two approaches. The low ion content systems are studied by increasing the fraction of functionalized isophthalate groups along the ionomer chain. The high ion content systems are studied by reducing the number of poly(ethylene oxide) monomers between isophthalate groups. Temperature was varied as a third parameter to construct a more complete description of ion aggregation and transport. This work shows that ion transport is accurately described by jump diffusion, which involves discreet jumps between coordination sites of oxygen atoms. This model is then further decomposed into three main categories; ion transport through poly(ethylene oxide), ion aggregates, or the interface between poly(ethylene oxide) and ion aggregates. At low ion content, the majority of ion transport is through the poly(ethylene oxide) backbone. As ion content is increased, the formation of ion aggregates provides channels through which ions jump without directly interacting with poly(ethylene oxide). The transition from a poly(ethylene oxide) dominate transport to ion aggregate dominate transport explains the observed experimental relationship of conductivity and ion content. Percolation theory is used as a foundation to describe ion aggregation below the aggregate percolation threshold. A model is developed which reproduces the distribution of ion aggregate sizes and is shown to be applicable across a wide range of ion content and temperature. This new framework for describing ion aggregation has the potential to be expanded to cover a larger range of electrolytes. Finally, a series of uniaxial stress-strain simulations demonstrate the impact of ion aggregation on mechanical properties. These results are then used to construct a time-temperature-ion content master curve which provides an estimate of the ion content necessary for dendrite suppression.