Abstract
Contrary to early motivation, the majority of aluminium ion batteries developed to date do not utilise multivalent ion storage; rather, these batteries rely on monovalent complex ions for their main redox reaction. This limitation is somewhat frustrating because the innate advantages of metallic aluminium such as its low cost and high air stability cannot be fully taken advantage of. Here, we report a tetradiketone macrocycle as an aluminium ion battery cathode material that reversibly reacts with divalent (AlCl2+) ions and consequently achieves a high specific capacity of 350 mAh g−1 along with a lifetime of 8000 cycles. The preferred storage of divalent ions over their competing monovalent counterparts can be explained by the relatively unstable discharge state when using monovalent AlCl2+ ions, which exert a moderate resonance effect to stabilise the structure. This study opens an avenue to realise truly multivalent aluminium ion batteries based on organic active materials, by tuning the relative stability of discharged states with carrier ions of different valence states.
Highlights
Contrary to early motivation, the majority of aluminium ion batteries developed to date do not utilise multivalent ion storage; rather, these batteries rely on monovalent complex ions for their main redox reaction
The core advantage of aluminium ion batteries (AIBs) is its capability to store multivalent carrier ions to increase the specific capacity of electrode materials
Most the cathode materials reported to date operate on the basis of the storage of monovalent complex ions such that the true benefit of AIBs has not been fully taken advantage of
Summary
To elucidate the reaction mechanism of TDK in comparison with PQ, we conducted DFT calculations to estimate the formation energies of both molecules when each molecule binds with a monovalent (AlCl2+) or divalent (AlCl2+) complex ion. From the viewpoint of the chemical structure, TDK has a lower benzene ring-to-carbonyl group ratio of 0.5 compared to 1.0 of PQ, resulting in a less pronounced stabilisation of radicals by the resonance effect This means that upon the formation of a radical, TDK can form a smaller number of resonance structures than PQ. These measurement points were set to enable the states of TDK to be captured after binding with every AlCl2+ ion. The optimised molecular structures of selected TDK-nAlCl2 or TDK-nAlCl combinations are presented. c Atomic charge analysis of PQ and TDK when bonded to a single AlCl2+ or AlCl2+ ion
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