Aluminium Electrodeposition in Emimcl-AlCl₃ ionogels for Improved Aluminium Batteries

Abstract

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