Date of Award

Spring 1-1-2014

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Conrad Stoldt

Second Advisor

Yifu Ding

Third Advisor

John Daily

Fourth Advisor

Todd Murray

Fifth Advisor

Kurt Maute

Abstract

Too often, we check the battery status of our favorite gadget and wonder: "Is it going to make it?" At some point in the not-so-distant future, I believe we will not have to ask this question. Until then, it is the incremental movement of the boundaries of our understandings of energy storage sciences that bring us closer to the next breakthrough technology.

In this thesis, we investigate a material which may one day play a key role in enabling fast-charging, long-lasting energy storage devices: the solid-state electrolyte. While this material has begun to replace liquid electrolytes in batteries, we envisioned a potential use in supercapacitors as well. Our work in solid-state supercapacitor development involved the incorporation of a sulfide- based electrolyte system as both the separator and source of double-layer ions. We demonstrated supercapacitance through a nanostructuring of the electrode layers. While a working device is certainly a meaningful contribution to the field of solid-state energy storage, we desired a deeper understanding of the physics that enables the demonstrated supercapacitance.

To this end, we next undertook study of ion mobility in a solid-state electrolyte using an air-stable system based on lithium germanium phosphate. We synthesized materials with varying chemistries in order to obtain a better understanding of how structural substitutions affect lithium-ion movement. We discover that a common synthetic approach to "improving" electrolyte performance can have the unintended consequences of trapping lithium ions, leading to decreases in conductivity.

Going one step deeper, we ultimately undertook a detailed study of the thermodynamic parameters that govern ion conduction. Through a thorough analysis of structure, we were able to provide physical explanations for observed trends in activation energy and conductivity. Finally, we provide strong evidence for a physical interpretation of entropy parameters based on the site occupancies of lithium ions.

It is hoped that the work conducted in the creation of this thesis can be used to guide forth- coming developments in the battery and supercapacitor communities. To control device operation, it is necessary to understand material function. To understand material function, it is necessary to understand governing parameters from the bottom up. This thesis contributes to a foundation from which to design the power source of tomorrow's favorite gadget.

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