Mechanistic Investigation of Intercalation Process in Two Specific Aqueous and Non-aqueous Aluminum Ion Batteries

In this thesis, the intercalation mechanism in two specific aluminum ion batteries (AIBs), aqueous and non-aqueous, are investigated. The first thrust is focused on aqueous aluminum ion batteries (AAIBs) employing aqueous aluminum trifluoromethanesulfonate (Al(OTF)3) electrolyte due to potential benefits of stability in ambient environment, low cost, and high ionic conductivity of aqueous electrolyte. In AAIB system, transitional metal oxide-based materials with high theoretical capacity have been explored previously, but the underlying charge storage mechanism remained unexplored. To bridge this knowledge gap, the charge storage mechanism of α-MnO2 in 2 m Al(OTF)3 electrolyte was investigated through a series of electrochemical testing and extensive spectroscopic characterization using X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier-transform infrared spectroscopy (FTIR), time-of-flight secondary ion mass spectroscopy (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). The results of this study indicate that (i) hydronium ions could reversibly intercalate/de-intercalate into/from a-MnO2, which is the dominant reaction happened during cycling; (ii) a scant amount of Al3+ could also intercalate into α-MnO2 and (iii) electrolyte complex formed during discharge consists of Al3+, OH-, and OTF-.

As hydronium ion is the active intercalation species, the achievable cell-level capacity of the as-assembled AAIB is greatly compromised. To take advantage of the high theoretical capacity of Al metal, the focus was put on a non-aqueous AIB system in the second thrust of this thesis. In a non-aqueous AIB, the Al metal is employed as anode and chloroaluminate ionic liquid (IL) is usually adopted as electrolyte. Although appreciable electrochemical performance was attained in AlCl4- intercalation-type graphitic/carbonaceous cathode, the heavy reliance on the electrolyte of low theoretical anolyte capacity, compromises the cell-level energy density of the battery. To shift away from the reliance on the low-capacity IL anolyte and take advantage of the high gravimetric and volumetric capacity of Al metal anode, research attention has been focusing on the cathode materials that enable cation (Al3+, AlCl2+ and AlCl2+) charge storage mechanism, through which the capacity of Al metal anode can be fully or partially utilized. In this study, we investigate the electrochemical performance and the underlying charge storage mechanism of an organic cathode made of benzo[1,2-b:4,5-b′]dithiophene-4,8-dione (BDTD) in an IL electrolyte (AlCl3/trimethylamine hydrochloride (TMAHCl) = 1.8 by mole). Organic electrode materials consist of earth-abundant light elements such as C, H, O, S offer opportunities for developing sustainable and energy-dense storage device. The assembled cell can deliver a capacity of 143 mAh g1 in the initial discharge and possesses a flat discharge voltage plateau at around 1.2 V (vs. Al/Al3+). A Super P modified membrane was employed to enhance the overall electrochemical performance of the cell. The BDTD retains >95% of the previous charging cycle after resting for 24 h, exhibits a good capacity retention. The insertion mechanism was systematically studied using XPS, SEM, and ToF-SIMS, and AlCl2+ was identified as the charge carrier ion, which reversibly interact with the carbonyl (C=O) group and contribute to the reversible capacity.

The knowledge gained throughout this research is expected to contribute to the understanding of the reaction mechanism in aqueous and non-aqueous AIBs, which is critical in terms of assessing the practical cell-level achievable energy density and scalability of the developed AIB technologies.

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