Degree

Doctor of Philosophy (PhD)

Department

The Gordon A. and Mary Cain Department of Chemical Engineering

Document Type

Dissertation

Abstract

Secondary lithium-bromine (Li-Br2) batteries have theoretical potentials near 4.1 V vs Li/Li+ and capacities more than 2 times greater than conventional Li-ion batteries. Herein, secondary, non-aqueous Li-Br2 half-cell batteries are reported using a Li metal anode, carbon-coated glass fiber separator, non-aqueous Li-based electrolytes with and without the addition of lithium bromine (LiBr) salt, and positive electrodes consisting of either chemically brominated non-graphitic carbon or carbon derived from the carbonization of metal-organic frameworks (MOFs) with LiBr embedded into the micro- and mesopores of the carbon matrix. The separator is effective in mitigating the transport of Br2 to the negative electrode. “Pre-bromination” of the positive electrode with a LiBr electrolyte results in chemisorption of Br2 and facilitates improved coulombic efficiencies and capacity retention. Charging and discharging profiles at a 1C charge rate over 100 cycles at 3.8 V show that cells with brominated carbon positive electrodes exhibit a capacity retention two times greater than untreated electrodes. Likewise, Coulombic efficiencies increase to approximately 94% for brominated positive electrodes. While charge-transfer resistances decreased with “pre-bromination” due to the improved electron transfer with C-Br bonds, open circuit self-discharge and increased ohmic resistances result in lower overall capacities of 137 mAh/g-LiBr. Carbonization of MOFs produces carbon with structured pore sizes that confines Br-species into the micro- and mesopores of the carbon matrix. Material and electrochemical performance comparisons were completed using brominated carbonized ZIF-8, MIL-53(Al), and HKUST-1. Between these materials, the brominated carbonized ZIF-8 positive electrodes showed superior electrochemical performance with specific capacities of 273 mAh/g-LiBr, 98% Coulombic efficiency, and 88% capacity retention over 100 cycles at a 1C charge rate, corresponding to a practical energy density of 219 Wh/kg based on the mass of carbonized MOF and LiBr. Coupled with XPS, BET, and SEM-EDS analysis, it is theorized that the improved electrochemical performances is caused by greater confinement of Br-species into micropores of the carbon matrix and the added synergistic benefits of heteroatom doping from carbon-nitrogen bonding. These initial results show promise for Li-Br2 batteries as future Li-ion battery alternatives.

Date

6-28-2020

Committee Chair

Flake, John

DOI

10.31390/gradschool_dissertations.5303

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