Zwitterion as Additives to Boost Dielectric Constant and Increase Conductivity for Single-ion Conducting Ionomers

Mei, Wenwen

Zwitterion as Additives to Boost Dielectric Constant and Increase Conductivity for Single-ion Conducting Ionomers

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Author:: Mei, Wenwen
Graduate Program:: Materials Science and Engineering
Degree:: Doctor of Philosophy
Document Type:: Dissertation
Date of Defense:: April 26, 2022
Committee Members:: Robert Hickey, Chair & Dissertation Advisor
Scott Milner, Outside Unit & Field Member
Ismaila Dabo, Major Field Member
Ralph Colby, Major Field Member
John Mauro, Program Head/Chair
Keywords:: ion-containing polymer
zwitterion
dielectric relaxation spectroscopy
Abstract:: Polymeric materials have potential in many energy-related devices such as capacitors, actuators, and ionic or electronic conductors. Their inherent advantages, such as easy processing, lightweight, and mechanical integrity, are desirable for building better devices. However, their low ionic conductivity and low dielectric constant are well recognized in many energy-related applications. For example, the ionic conductivity of polymer electrolytes is two orders of magnitude lower than that of commercial liquid electrolytes. The actuation of dielectric elastomers requires a voltage of kV because the dielectric constant for typical dielectric elastomers is low (i.e., < 10). Improving the dielectric constant and conductivity remain a critical challenge for their practical implementations, which could have large implications for the global goal of sustainability and carbon reduction. This thesis provides molecular understanding and design strategies to enhance dielectric constant and ionic conductivity for single-ion conducting ionomers, which could also apply to other soft materials. This study focuses on how the chemical composition (structure) impacts the dielectric properties and ion conduction to unravel the design strategies to maximize dielectric constant and ion conduction for polymers. Firstly, we developed understandings of the dielectric properties of zwitterionic liquids, which exhibit the highest dielectric constant among organic molecules. The huge dielectric constant is attributed to their large molecular dipole by covalently linking the cation and anion. We further provide insights on tuning the dielectric constant of zwitterionic liquids via suitable cation substituents. We discovered that the ethylene oxide-based cation substitutes could frustrate crystallization and enhance molecular mobility, resulting in supercooled zwitterionic liquids at ambient temperature in contrast to the mostly reported crystalline solids. Our results suggest that zwitterionic liquids can be useful additives to raise the dielectric constant for soft materials by proper chemical structure design. Secondly, we discovered the significant impacts of anion chemical composition on the ion conduction for poly(ethylene oxide)-based Li-ion single-ion conducting ionomers. Those ionomers are potential candidates as polymer electrolytes for electric vehicles, yet their conductivities cannot meet the current requirements for device operation. It is commonly believed that low Tg can always raise ion mobility and, consequently, conductivity. Therefore, the direct approach developed is lowering the Tg of ionomers by incorporating low-Tg segments or adding plasticizers. Our study shows that in addition to Tg, anion chemical compositions impact ion conduction. A strongly charged localized anion (i.e., sulfonate) results in significant ion aggregation that drastically reduces ion mobility and ion number density. A weakly charged localized anion (i.e., sulfonylimide) results in less aggregated morphology, raising ion mobility and ion number density. Our results highlight the necessity of simultaneously lowering Tg and reducing ion aggregation to promote ion transport in polymer electrolytes, suggesting that tailoring anion chemical composition can be the next step. Lastly, we explored the zwitterion addition approach to promote the dielectric constant and conductivity of single-ion conducting ionomers. We design the ionomer/zwitterionic liquids blends for anion-conducting and cation-conducting ionomers based on the accumulated knowledge of zwitterionic liquids and ionomers. Our results show that for strongly aggregated ionomers (typically with small counterions), the addition of zwitterionic liquids effectively breaks up the ion aggregates. The addition of zwitterionic liquids promotes dielectric response and ion transport based on the raised conductivity and dielectric constant. However, the relatively high Tg of zwitterionic liquids counteracts the benefits of breaking up ion aggregates. The conductivity increase is within two orders of magnitude and is observed at elevated temperatures (> 80 °C). Nevertheless, our study validates the hypothesis that breaking up ion aggregates is beneficial for ion transport despite raised Tg. Our study also sheds light on new understandings of the ion-conducting behavior of concentrated electrolytes. It remains to be understood how the chemical composition-specific local interactions over the molecular length scale (~ nm) impact the macroscopic properties since the classical theories of dilute solutions and dielectric continuum cannot explain the whole experimental phenomena.

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