Year

2016

Degree Name

Doctor of Philosophy

Department

AIIM

Abstract

The global effort to improve the lifetime, power densities and energy efficiency of energy storage and conversion technologies, such as batteries, fuel cells and supercapacitors, has become dramatically more extensive with increased demand from portable electronics and the electrification of transportation. Currently, lithiumoxygen (Li-O2) batteries have been viewed as the most promising next-generation electrochemical energy storage technology to meet the transportation application in the near future. Unlike traditional Li-ion batteries, Li-O2 batteries abandon the intercalation electrodes and Li ions react directly with O2 from the air in a porous electrode. This unique battery chemistry and electrode architecture induce an extremely large theoretical specific energy ~ 3600 W h kg-1, which may be capable of providing enough energy storage capability for electric vehicles to drive more than 500 miles (per charge). Such high specific potential energy density is several times higher than that possible Li-ion battery technology and can even match the 1700 W h kg-1 for a gasoline energy system. To date, the main challenges faced by the Li-O2 batteries are the cycling-instability of electrolytes and the low round–trip efficiency which could be increased by optimizing cathode catalysts and structures. These shortcomings will hamper the commercialization of this technology. In the past ten years, many advances have been achieved, but the challenges remain. In this doctoral work, several promising efficient carbon-supported catalysts for Li-O2 batteries have been synthesized as novel cathode catalysts and their electrical performance have been investigated in detail. These include MnO-carbon nanotube, graphitic C3N4-graphene, and B4C nanowires-carbon nanotubes composites. Meanwhile, onedimensional AgPd-Pd composite porous nanotubes were also prepared by galvanic replacement reaction and applied in rechargeable Li-O2 batteries. These porous nanotubes show favourable rechargeability and excellent energy efficiency, facilitating rapid O2 and electrolyte diffusion through the nanotubes, as well as forming a continuous conductive network throughout the whole energy conversion process. In addition, the lack of stable electrolyte for Li-O2 is another enormous challenge to be overcome. The properties of formulated electrolytes are crucial for the interfacial structure between the electrodes, O2 gas, and non-aqueous electrolytes and accordingly govern the performance of Li-O2 batteries. The most widely used electrolytes for Li-O2 batteries are almost exclusively the electrolytes composed of organic solvents and lithium salts. Most of the conventional electrolytes still suffer from rapid degradation with cycling, however. Herein, we have designed a special flexible lithium oxygen battery device using a gel-solid polymer electrolyte, which can not only avoid electrolyte evaporation, but also protects the lithium metal anode during reaction. In this work, RuOx nanoparticles decorated uniformly on nitrogendoped graphene were employed as cathode materials. The results show that the gelsolid polymer electrolyte has high ionic conductivity and low activation energy with a high round-trip efficiency.

FoR codes (2008)

0912 MATERIALS ENGINEERING

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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.