(Invited) Effect of Ion Coordination on the Long-Range Charge Migration Processes in Ionic Liquid-Based Hybrid Al/Mg Batteries

Abstract

New and novel systems for the reversible electrochemical storage of energy are required to sustain the increasing global energy demands. Indeed the high cost, the intrinsic energy limits and the strategic aspects of lithium and lithium technology-related metals (e.g., Co) are prompting the research towards the development of battery systems based on novel chemistries. In this regard, light multivalent metals are considered the holy grail. Magnesium and aluminum, for example, can be considered one of the best choices due to their low reduction potential (-1.66 and -2.37 V vs. SHE for Al3+/Al and Mg2+/Mg, respectively), high volumetric and gravimetric theoretical capacities (2980 Ah∙kg-1 and 8046 Ah∙dm-3 for Al, and 2205 Ah∙kg-1 and 3832 Ah∙dm-3 for Mg), high abundance in the Earth’s crust and low cost (1’925 and 2’700 $∙ton-1 for Al and Mg, respectively) [1, 2].The main roadblock to the development of these technologies is the discovery of efficient, stable and high-performing electrolytes. Ionic liquids have been demonstrated to be a good choice to be used as solvents in multivalent metal electrolytes, thanks to the low volatility and high solvating powers [3, 4]. Despite large efforts, there are no examples in the literature that demonstrate the advantage of the synergic effect of the coupling of Mg2+/Al3+ in the deposition and stripping processes. Moreover, the fundamental understanding of ion coordination in this novel class of electrolytes would be a cornerstone information necessary to reveal the conduction mechanism.In this work we study the properties of a family of hybrid Al/Mg electrolytes with formula [Pyr14Cl/(AlCl3)1.5]/(δ-MgCl2)x (x = 0, 0.056, 0.091 and 0.146), to foster the synergic effect of these two metals for application in multivalent metal secondary batteries. In this study we combine thermal analyses, vibrational, electric and nuclear magnetic spectroscopies to achieve a clear picture of the ionic coordination occurring in the proposed electrolytes and how ions are able to rearrange and form ionic coordination nanodomains. It is demonstrated that AlCl4 - and Al2Cl7 - species are formed, which act as ligands for δ-MgCl2 units. Thus, the Mg2+/Al3+ conduction occurs through the exchange of anionic species between the different ionic clusters. These phenomena are significantly correlated to the host medium dynamics of the IL matrix. The rise of the temperature results in an increased degree of disorder in the nanodomains, which modifies the long-range charge migration process. Finally, the electrochemical activity in the synergic Al/Mg alloy deposition and stripping is demonstrated. Acknowledgements This project has received funding from the program “Budget Integrato per la Ricerca Interdipartimentale - BIRD 2018” of the University of Padova (protocol BIRD187913).