The growing demand for safe, sustainable and energy-dense energy storage devices has spurred intensive investigations into post-lithium battery technologies. Rechargeable aluminium batteries are promising candidates for future electrochemical energy storage systems due to the high theoretical volumetric capacity of aluminium and its natural abundance in the Earth’s crust, but their practical application is currently hindered by the limitations of presently available electrolytes. In this review, we highlight the key considerations needed to optimise the electrolyte design in relation to the aluminium battery system and critically assess the current state of knowledge and new concepts in liquid and quasi-solid polymer electrolytes, focusing primarily on non-aqueous systems. We then discuss the challenges and approaches in developing polymer electrolytes and finally provide an overview of the opportunities in quasi-solid electrolytes which could pave the way to achieving further improvements in aluminium batteries.

Aluminium batteries with non-aqueous electrolyte have initially focused on Lewis acidic ionic liquid systems with heavy Al2Cl7− anions that limit the specific capacity, energy and power. In order to develop the secondary aluminium batteries further for future energy storage beyond lithium-ion, high performance electrolytes that enable efficient aluminium deposition/dissolution must be developed. This work studied the electrodeposition of aluminium from both 1-ethyl-3-methylimidazolium chloride aluminium chloride (EMImCl-AlCl3) ionic liquids with different Lewis acidities, and their gel form- the ionogel. Thereby, cyclic voltammetry, in-operando atomic force microscopy and scanning electron microscopy coupled with energy dispersive X-ray diffraction measurements were used to determine the characteristics of aluminium deposition in the ionic liquid depending on the ratio of AlCl3 to EMImCl. Based on these insights, Lewis acidic and neutral ionic liquids were gelified with polyethylene oxide. The focus was on the feasibility of aluminium deposition in Lewis neutral ionogels containing only lightweight AlCl4− anions. It was proven for the first time that aluminium can be deposited from a Lewis neutral ionogel without any dendrite growth within a very wide potential stability window of 5 V but at a low coulombic efficiency of ≤60%.

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Aluminium batteries are promising candidates for the next-generation of energy storage devices, given the natural occurrence of aluminium (third most abundant element) and the intrinsic safety characteristics of the electrolytes used. The specific capacity of an aluminium negative electrode (2980 mAh g-1) is approximately 77% that of lithium (3860 mAh g-1), while its volumetric capacity of 8040 mAh cm-3 far exceeds that of lithium (2060 mAh cm-3), due to its ability to transfer three electrons [1]. Most non-aqueous aluminium batteries utilise electrolytes including inorganic molten salts, deep eutectic solvents, organic solvents and ionic liquids, and operate by the reversible deposition/dissolution of aluminium at the negative electrode and the intercalation of the chloroaluminate ions at the positive electrode. This work studied the electrodeposition of aluminium from the 1-ethyl-3-methylimidazolium tetrachloroaluminate (EMImCl-AlCl3) ionic liquid and its gel form- the ionogel. Ionogels have previously been shown to improve thermal [2] and moisture stability [3]; here, the ionogel was introduced to widen the potential stability window and enable aluminium deposition from lightweight AlCl4 - anions in Lewis neutral ionogels. Generally, aluminium is deposited by the reduction of heavy Al2Cl7 - ions present only in Lewis acidic ionic liquids, which is typically not possible in neutral and basic compositions as the reduction of the organic cation and subsequent ionic liquid decomposition occurs at a higher potential than the reduction of AlCl4 -. Lewis acidic and neutral ionogels of varying degrees of gelation were synthesised with polyethylene oxide and EMImCl-AlCl3. The electrochemical behaviour of the electrolytes was investigated by cyclic voltammetry, and aluminium deposits from the Lewis neutral ionogel were confirmed by scanning electron microscopy and energy dispersive X-ray spectroscopy. The deposition of aluminium from a Lewis neutral EMImCl-AlCl3 ionogel [2] was demonstrated for the first time (Fig. 1) [4], serving as a basis for further investigation into the role of polymer additives in developing improved ionogel and semi-solid state electrolytes for aluminium batteries. [1] G. A. Elia, K. Marquardt, K. Hoeppner, S. Fantini, R. Lin, E. Knipping, W. Peters, J. F. Drillet, S. Passerini and R. Hahn, Adv. Mater., 28, 7564-7579 (2016). [2] S. A. M. Noor, P. M. Bayley, M. Forsyth and D. R. MacFarlane, Electrochim. Acta, 91, 219-226 (2013). [3] X. G. Sun, Y. Fang, X. Jiang, K. Yoshii, T. Tsuda and S. Dai, Chem. Commun., 52, 292-295 (2016). [4] T. Schoetz, O. M. Leung, I. Efimov, C. Zaleski, A. M. Ortega, N. García García, P. Tiemblo Magro and C. Ponce de Leon, J. Electrochem. Soc., 167, 040516 (2020). Figure 1