Abstract
Rechargeable aluminium ion batteries (AIBs) are one of the most promising battery technologies for future large-scale energy storage, due to the high theoretical volumetric capacity, low cost, and high safety. However, the low capacity of intercalation-type cathode materials and poor reversible ability of conversion-type cathode materials reduce the competitiveness of AIBs in practical applications. In this research work, a nanoscale FeF3@expaned graphite (EG) composite has been synthesised as a novel conversion-type cathode material for AIBs. Moreover, a single-wall carbon nanotube-modified separator (SWCNT-G/F) has been introduced into the system, which significantly restricts the shuttle effect of the intermediate product of FeF3. The assembled AIB exhibits a satisfactory reversible specific capacity of 266 mAh g-1 at a current density of 60 mA g-1 after 200 cycles, and a good Coulombic efficiency approaching 100% after 400 cycles at a current density of 100 mA g-1. Ex-situ X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have been applied to explore the energy storage mechanism of FeF3 in AIBs for the first time. On the other hand, the safety issue of ionic liquid remains another factor that limits the development of AIBs. Herein, a gel polymer electrolyte (GPE) has been successfully synthesised via an innovative method where no solvent or initiator was utilised in the polymerisation process. The application of GPE significantly reduces the corrosivity and enhances the moisture sensitivity of EMIC ionic liquid, as well as improving the reversible ability of the AIBs. The FeF3@EG-based AIB with 0.8g-EMIC-gel electrolyte exhibits a reversible capacity of 204.5 mAh g-1 after 1000 cycles at a current density of 100 mA g-1 and stable rate performance for 600 cycles with a Coulombic efficiency of approximately 95%. In addition, CuF2, another transition metal fluoride with similar physical and chemical properties to FeF3, has also been studied as a candidate cathode for AIBs. In comparison to FeF3, CuF2-based AIBs exhibit severe capacity decay and low reversible capacity due to the absence of the intermediate phase (i.e., AlFeF5 in FeF3-based AIBs) during the redox process and high solubility of the intermediate product (CuCl) in the ionic liquid electrolyte. In fact, the findings in this thesis are conceived to serve as guidance for the successful design of low cost and high discharge capacity AIBs for large-scale energy storage and are also meaningful for the fundamental understanding of the metal fluorides cathodes for AIBs. Additionally, the study provides unprecedented insight into novel conversion type cathode materials for AIBs.