Year

2021

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

The world-wide energy revolution from fossil to renewable energy, such as wind and solar energy, has made greater demand on energy storage systems, which flatten the fluctuations of those energy supplies caused by their intrinsic attributes. The scaled-up implementation of these energy storage systems in power grids requires low cost and high energy density in these systems. The lithium-sulfur (Li-S) battery is one of the most promising systems that can meet the above requirements owing to its high capacity (1672 mA h g-1) and energy density, as well as the low cost of sulfur. Nevertheless, some hindrances are blocking Li-S batteries from practical application in the power grid. The longevity of Li-S batteries is unsatisfactory due to the insulating nature of sulfur, the dissolution of intermediate products, the volume expansion of the sulfur cathode, and the safety issues of Li metal. To solve the above problems, countless conductive matrices (such as carbon) have been explored to increase the sulfur utilization. The cathodes, separator, and anode have been painstakingly designed to mitigate the shuttle effect caused by dissolved intermediates, and various electrolytes have been proposed to either decrease the solubility of intermediates or build a stable solid-electrolyte-interphase (SEI) on the anode. The lithiated sulfur cathode (Li2S) has been investigated to develop a Li-ion sulfur battery with a Li-metal-free anode such as Si or graphite. Based on these concerns, we have proposed different strategies to address different issues related to Li-S batteries. Specifically, the synthesis of CoS2/Li2S aims to develop Li-metal-free Li-ion/sulfur batteries, circumventing the safety concerns related to Li metal. Defect-rich carbon nanotubes (defect-rich CNT) were synthesized to mitigate the shuttling effect and to enhance the reaction kinetics in the sulfur cathode. The exploration of low-concentration salt is a helpful way to develop Li-S batteries with low cost and high stability.

A modified method that was employed to create and manipulate defects in CNTs, has been introduced here to anchor polysulfides along with accelerating electrochemical reactions. As a result, the defect-rich CNTs enabled dramatic improvement of both cycling and rate performance. A specific capacity of 600 mAh g-1 with a current density of 0.5 C was achieved after 400 cycles, and even at very high current density (at 5 C) a specific capacity of 434 mAh g-1 was observed. Cycling stability up to 1000 cycles was also achieved under the conditions of high sulfur loading and lean electrolyte. Theoretical calculations reveal that the improvement is mainly attributable to the electronic structure of the defect-rich carbon, which has a higher binding energy with polysulfide because of the upshift of the p-band centre. Furthermore, rotating disk electrode (RDE) measurements demonstrated that defect-rich carbon led to improved kinetics of the sulfur reduction reaction (SRR), accelerating the polysulfide conversion process and mitigating the shuttling effect. This new design strategy could well be a starting point for a novel method to develop carbon materials with good conductivity and high catalytic activity.

FoR codes (2008)

0204 CONDENSED MATTER PHYSICS

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.