Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Dr. Xueliang Sun

Abstract

Lithium-air and sodium-air batteries are promising energy storage systems for future smart grids and electric vehicles due to their extremely high theoretical energy densities. However, electrode material development and architecture design for cathode as well as the battery cycleability are big challenges for these batteries. This research aims at developing various novel nanomaterials with desired morphology and structure as cathode materials for lithium-air and sodium-air batteries.

For lithium-air batteries, various carbon nanostructured cathodes were developed. They include: (1) Carbon black nanoparticles were treated under ammonia and carbon dioxide/hydrogen atmospheres and the surface area, porosity, defects, nitrogen-doping, and functional groups were modulated. These parameters for battery performance were investigated and it was found that the surface area of mesopores rather than others played an important role for the discharge capacity due to the passivation effect of discharge products. (2) One-dimensional (1D) nitrogen-doped carbon nanotubes (N-CNTs) electrode showed 50% higher of discharge capacity and better electrocatalytic activity for discharge product decomposition than pristine carbon nanotubes (CNTs) electrode. (3) Two-dimensional (2D) graphene nanosheets (GNSs) electrode delivered extremely high discharge capacity compared to porous carbon blacks due to the ideal porosity which increased the electrolyte wetting and oxygen diffusion, improving the efficiency of reactions. (4) Nitrogen-doped graphene nanosheets (N-GNSs) exhibited 1.5 times higher of electrocatalytic activity for oxygen reduction reaction than GNSs, further improved 40% of the discharge capacity. In addition, the morphology of discharge products was changed due to the defects and functional groups introduced by nitrogen doping. (5) The correlation between discharge product morphology and battery performance for sulphur-doped GNSs was studied and it was found that the discharge product contained structural defects such as oxygen and/or lithium vacancies resulting in different charge performance.

In terms of exploring catalysts which have potential for improving battery cycleability, a facile rapid microwave-assisted hydrothermal method was developed and it was shown that the morphology and crystallinity of MnO2 were easily controlled by adjusting the reaction parameters.

For sodium-air battery cathode, it was also found that N-GNSs showed higher electrocatalytic activity for oxygen reduction reaction and oxygen evolution reaction, resulting in improving discharge and charge performance.

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