Electrode Nanomaterials and Electrolytes for Aluminium-ion Batteries

Energy is one of the biggest challenges of the 21st century. Factors such as the decline in availability of non-renewable power sources, the alarming levels of atmospheric CO₂, and the steady increase of the worldwide demand for energy make the worldwide transition to a fully renewable source-based production an extremely urgent necessity. Because of the intermittent nature of most renewable energy sources, battery-based energy storage systems could be a useful tool for such transition. However, current battery technologies such as lithium-ion often lack the cost-effectiveness and safety requirements necessary for large-scale grid energy storage applications; it is therefore important to search for alternative technologies which are more suitable for this purpose. Aluminium-ion batteries have recently emerged as a very promising alternative to lithium-based systems, thanks to the low cost, non-flammability, and three-electron redox chemistry of aluminium. Al-ion batteries could, in principle, offer better cost-effectiveness, higer capacity and improved safety, which would lead to a substantial advance in energy storage technology. This PhD project deals with the investigation of novel electrode nanomaterials and electrolyte systems for Al-ion batteries. Particular emphasis is put on using the special properties of nanomaterials to improve the performance of batteries and on searching for low-cost compounds to be used as alternative electrolytes. Developing these areas will enhance the cost-effectiveness of the technology, and facilitate its commercial feasibility. Vanadium oxide nanofibres and carbon nanofibres were initially tested as cathode materials. The performance of such cathodes, however, did not meet expectations: V₂O₅ nanofibres showed poor reversibility, short cycling life, and underwhelming specific capacity, while carbon nanofibres displayed a mostly capacitive, adsorption-based energy storage mechanism, with no significant ion intercalation taking place. Nevertheless, the tests performed on the latter material led to the discovery of the phenomenon of solid-electrolyte interphase formation: this process was investigated in depth and found to be mainly caused by the presence of defects on the surface of the nanofibres, favouring the decomposition of the electrolyte into insoluble species during the charging phase. Two composite materials were then tested as cathode candidates: solvothermally-prepared core-sheath C/V₂O₅ nanofibres, and a layered nanostructured electrode. The former material showed an interesting behaviour as a battery cathode, as evidence for a multiple-ion intercalation mechanism was found; this phenomenon is however short-lived, as the cathode tends to disintegrate after the first few charge-discharge cycles. In a similar fashion, the fabrication methods used to create the layered electrode were shown to be unreliable: the poor adhesion of the active material to the underlying current collector resulted in highly unstable performance of the cathode, leading to the premature failure of the battery device. Within alternative electrolytes, mixtures of inorganic and non-ionic organic compounds were studied. Eutectic mixtures of aluminium trichloride with acetamide and other small amide analogues were found to achieve good performance as battery electrolytes. Reduction of viscosity was found to be the key to improve cycling performance: this was achieved either by dilution of the electrolytes with an appropriate solvent, or by using combinations of amides to weaken the inter-molecular interactions present in the melts.