Year

2020

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

Scientific and technological developments have the potential to affect every part of society. This is particularly the case with rechargeable batteries, which play a significant role in the daily lives of most people. Lithium batteries have helped to make wearable and portable electronics, electric vehicles, and stationary energy storage possible. Furthermore, they allow the prompt exchange of information without the limitations (such as blackouts) inherent to distributed electricity supply systems (i.e., national power grids). From the perspective of environmental protection, these batteries enable the exploitation of intermittent clean, renewable energy sources such as solar, wind, tides, and terrestrial heat, thus allowing the use of fossil fuels to be reduced, cutting environmental contamination, and restricting pollution and waste production to controlled regions or levels. Nevertheless, the limited energy density of electrode materials, complicated preparation methods, and unsatisfactory cycle life in current lithium batteries have prevented them from being used in large-scale industries. Therefore, searching for novel electrode materials and innovative battery systems has attracted abundant research interest towards developing advanced energy storage technology during the past several decades.

In terms of the electrodes in lithium ion batteries, although carbonaceous materials offer suitable voltage windows, long term cycling performance, and stable rate capability, their inferior theoretical capacity may still make them incapable of meeting the large demands of high-energy-density applications. As potential candidates, germanium (Ge)-based materials and transition metal oxides are highly promising anode candidates due to their large specific capacities and remarkable rate capabilities. Nevertheless, their high price, low charge transfer kinetics, and unstable structure after cycling are limiting their further practical application. Engineering electrode materials on the nanoscale level with multi-functionalization will, therefore, address these problems, which has been widely recognized in other energy conversion fields (e.g. catalysts). This doctoral work will mainly focus on the rational design and controllable synthesis of anode materials by engineering their microstructures, as well as on their intriguing properties for energy storage applications. The main challenges and perspectives on anode materials modified by nanoscale engineering are also discussed.

For Ge-based materials, a hierarchical micro-nanostructured Ge-C framework has been achieved by a facile and scalable structural engineering strategy through controlling the nucleation, with the products featuring high tap density, reduced Ge content, superb structural stability, and a three-dimensional conductive network. The constructed architecture has demonstrated an outstanding reversible capacity of 1541.1 mAh g−1 after 3000 cycles at 1000 mA g−1 (with 99. 6% capacity retention), markedly exceeding all the reported Ge-C electrodes regarding long cycling stability. Additionally, for transition metal oxides, the strategy of tailoring of the atomic structure has been introduced into layered potassium niobate by dehydration-triggered lattice rearrangement. The engineered potassium niobate shows enhanced electrical and ionic conductivity, which could be attributed to the enlarged interlamellar spacing and subtle distortions in the fine atomic arrangements.

Due to the uneven global distribution of lithium and shortages of its reserves (20 ppm in the Earth’s crust, with most being restricted to South America, Australia, China, and the USA), the alkali metal counterparts of lithium, such as potassium, have attracted more attention, because their batteries can share a similar “rocking-chair” mechanism with lithium. An important challenge facing high-performance potassium ion batteries is identifying advanced electrode materials that can store the large-radius K+ ions, as well as tailoring the various thermodynamic parameters. Taking potassium niobate as an example, a careful study of its composition, atomic structure, and performance has been achieved. The X-ray absorption fine structure spectroscopy and electron paramagnetic resonance results reveal that the interatomic distances for the Nb-O coordination in the engineered potassium niobate are slightly elongated, and their degree of disorder has been considerably increased. Its electrochemical performance further demonstrates that the diffusion coefficient of K+ is one order of magnitude higher than that of Li+, and the engineered potassium niobate presents superior specific capacity and rate capability for K+ ions compared to Li+ ions.

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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.