Document Type

Dissertation

Degree

Doctor of Philosophy (PhD)

Major/Program

Materials Science and Engineering

First Advisor's Name

Bilal El-Zahab

First Advisor's Committee Title

Committee Chair

Second Advisor's Name

William Kinzy Jones

Second Advisor's Committee Title

Committee Member

Third Advisor's Name

Chunlei Wang

Third Advisor's Committee Title

Committee Member

Fourth Advisor's Name

Wenzhi Li

Fourth Advisor's Committee Title

Committee Member

Fifth Advisor's Name

Zhe Cheng

Fifth Advisor's Committee Title

Committee Member

Keywords

Li-O2 battery, Polymer Electrolyte, Hybrid Electrolyte, Composite Gel Polymer Electrolyte, Electrolyte Decomposition, Cyclability, Lithium Transference Number, Glass Microfillers, Ion Complex Formation, Electrochemical Impedance Spectroscopy, Raman Spectroscopy

Date of Defense

10-2-2017

Abstract

The transition from fossil fuels to renewable resources has created more demand for energy storage devices. Lithium-oxygen (Li-O2) batteries have attracted much attention due to their high theoretical energy densities. They, however, are still in their infancy and several fundamental challenges remain to be addressed. Advanced analytical techniques have revealed that all components of a Li-O2 battery undergo undesirable degradation during discharge/charge cycling, contributing to reduced cyclability. Despite many attempts to minimize the anode and cathode degradation, the electrolyte remains as the leading cause for rapid capacity fading and poor cyclability in Li-O2 batteries. In this dissertation, composite gel polymer electrolytes (cGPEs) consisting of a UV-curable polymer, tetragylme based electrolyte, and glass microfibers with a diameter of ~1 µm and an aspect ratio of >100 have been developed for their use in Li-O2 battery application. The Li-O2 batteries containing cGPEs showed superior charge/discharge cycling for 500 mAh.g-1 cycle capacity with as high as 400% increase in cycles for cGPE over gel polymer electrolytes (GPEs). Results using in-situ electrochemical impedance spectroscopy (EIS), Raman spectroscopy, and scanning electron microscopy revealed that the source of the improvement was the reduction of the rate of lithium carbonates formation on the surface of the cathode. This decrease in formation rate afforded by cGPE-containing batteries was possible due to the decrease of the rate of electrolyte decomposition. The increase in solvated to the paired Li+ ratio at the cathode, afforded by increased lithium transference number, helped lessen the probability of superoxide radicals reacting with the tetraglyme solvent. This stabilization during cycling helped prolong the cycling life of the batteries. The effect of ion complexes on the stability of liquid glyme based electrolytes with various lithium salt concentrations has also been investigated for Li-O2 batteries. Charge/discharge cycling with a cycle capacity of 500 mAh·g-1 showed an improvement as high as 300% for electrolytes containing higher lithium salt concentrations. Analysis of the Raman spectroscopy data of the electrolytes suggested that the increase in lithium salt concentration afforded the formation of cation-solvent complexes, which in turn, mitigated the tetragylme degradation.

Identifier

FIDC004004

ORCID

orcid.org/0000-0002-4086-5362

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