Preparation and characterisation of an aluminium conductive polymer battery in a chloroaluminate ionic liquid for future energy storage

Abstract

The aluminium battery is one of the very promising alternative battery chemistries for future energy storage, because of its sustainability, its performance due to the three electron redox reaction, and its high specific capacity due to the light weight of aluminium. The current approach uses aluminium anodes and graphite cathodes in non-aqueous electrolytes. Even though scientific progress has been made with this approach, the cathode reaction continues to limit the energy storage performance for future aluminium batteries that demand both high specific energy and high specific power. A new approach is the combination of aluminium anodes with conductive polymer cathodes. The novelty of these cathodes is that they behave both as a battery and as a capacitor, giving it more storage capability than previous aluminium battery systems. In this work, the current role of alternative battery systems, beyond lithium-ion, is discussed followed by a proof-of-concept study of an aluminium conductive polymer battery with ionic liquid electrolyte, wherein the conductive polymer poly(3,4 ethylenedioxythiophene) (PEDOT) was synthesised electrochemically in aqueous solution. This study demonstrated the feasibility of an aluminium-PEDOT battery with preliminary performance (specific energy and power) in the range of state-of-the-art rechargeable batteries but shows significant limitations regarding the conductive polymer electrode stability. Therefore, the key challenge of this work was the synthesis of stable and efficient conductive polymers by electropolymerisation in ionic liquids, which was linked with studies of the polymer’s mechanistic behaviour depending on its state of charge. Thereby, the electrochemical and nanomechanical behaviour, as well as the polymer morphology, were investigated by cyclic voltammetry, electrochemical impedance spectroscopy, quartz crystal microbalance measurements, scanning electron and in-operando atomic force microscopy. A further research pillar was the reproduction of polymer properties from two-dimensional to three dimensional substrates, which increase the active surface area per unit cathode volume, via electropolymerisation. The work was brought to its conclusion by performance testing of the aluminium-conductive polymer with the improved three-dimensional polymer cathodes. The final battery performance reached a substantial improvement in reversibility and stability, showing direct and meaningful progress, and proving the relevance of aluminium batteries for future energy storage.

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