Year

2021

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

The rising demand for energy storage is spurring the need for batteries with high energy density. Metal-oxygen batteries, as promising candidates for next-generation batteries, have attracted attention due to their high theoretical energy density compared with the conventional lithium-ion batteries (3458 Wh kg-1 for Li-O2 batteries, and 1605 Wh kg-1 or 1105 Wh kg-1 for Na-O2 batteries based on the discharge products Na2O2/NaO2), which have been considered as suitable power sources for electric vehicles (EV). Before the metal-oxygen batteries can achieve practical application, however, numerous issues for both Li-O2 batteries and Na-O2 batteries need to be dealt with. There are some issues that Li-O2 and Na-O2 both share: lithium/sodium anode dendrite formation, contamination from H2O and CO2, instability of electrolytes toward O2- species, etc., but they also have special issues that need to be considered. For example, Li-O2 batteries are facing a serious problem with sluggish kinetics due to the insulating nature of Li2O2, while Na-O2 batteries with NaO2 as discharge product does not have this issue because NaO2 is easy to decompose during charging. In this case, for Na-O2 batteries, it is essential to improve their oxygen reduction reaction (ORR) efficiency to increase their discharge capability and avoid premature death. To solve these issues, understanding the reaction chemistry, which involves the oxygen electrocatalyst (for the oxygen reduction reaction and the oxygen evolution reaction) and component properties, is vital. In this thesis, the catalytic mechanisms on the cathode side involving different materials are investigated in metal-oxygen batteries. The mechanism for molybdenum carbide/dioxide heterostructures as cathode materials is explored in Li-O2 batteries to explain how they improve electrochemical performance. The relationship between the oxygen adsorption capability of cathode materials and the morphology of discharge products is elucidated. Based on this relationship, carbon paper, which has been used for support in metal-oxygen batteries, has been modified to control the morphology evolution of discharge products in Li-O2 batteries to further optimize their electrochemical performance. What is more, it is proved that the modified carbon paper can also improve the electrochemical performance of Na-O2 batteries. Notably, the modified carbon paper only changes the size of discharge products, not their morphology. Herein, to explore the morphological evolution of discharge products and the electrochemical performance variation in Na-O2 batteries, different ratios of solvents have been used in the electrolyte. The mechanism behind the morphology evolution of discharge products in Na-O2 batteries is demonstrated by introducing the rate of solvation (Rsolvation) and the rate of desolvation (Rdesolvation).

FoR codes (2008)

100708 Nanomaterials, 100707 Nanomanufacturing, 0303 MACROMOLECULAR AND MATERIALS CHEMISTRY, 0904 CHEMICAL 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.