A Challenge for Rechargeable Al-Batteries: AlCl3-Free Deep Eutectic Solvent and Cationic Solvate Based Electrolytes
Authors: Mandai Toshihiko , Johansson Patrik 
Published: February 2020

Abstract:
The rapidly growing demands for electric power sources, especially in the electromobility field, but increasing in importance also for large-scale stationary energy storage applications for renewable energy, requires novel highly efficient electrochemical energy storage systems. Post Li-ion batteries are a collection of various promising battery technologies. While some strategies focus on enabling the use of Li metal anodes, such as Li-sulfur and Li-air batteries, others turn away from Li altogether and focuses on multivalent metal based batteries, e.g. Mg,1 Ca,2 and Al.3 Among these, a rechargeable Al-battery would offer the largest volumetric capacity of the negative electrode due to its relatively high density combined with the three electron transfer possible by Al3+. Safety issues, however, have hampered a materialization of any practical Al-battery, as almost all non-aqueous Al-conducting electrolytes and batteries incorporate the extremely reactive AlCl3.
One of the most straightforward approaches to address this issue is by developing AlCl3-free electrolytes that enable an Al-battery. We have prepared hundreds of various traditional salt and solvent based non-aqueous Al electrolytes, but unfortunately no electrochemical activity has been found, possibly due to poor Al salt dissociation and severe side-reactions. Therefore alternative approaches based on different concepts are urged for; two possible candidates are deep eutectic solvents (DESs) and cationic solvates. DESs are fluids composed of mutually miscible components, often metal salts and various solid/liquid organic compounds, which form eutectics by an entropic effect. The recent successes of reversible electro-plating/stripping of multivalent metals from metal complexes with cationic active species,4–6 have inspired us to also investigate similar aluminum cationic solvates.
For the DES based electrolytes, we propose a series of ternary electrolytes consisting of aluminum trifluoromethanesulfonate (Al[TfO]3), N-methylacetamide (NMA), and urea. The replacement of AlCl3 by Al[TfO]3 can address the safety issues of AlCl3. By selecting appropriate compositions, the Al[TfO]3 salt is highly dissociated and the Al conduction mechanism decoupled from viscosity limitations, and consequently these ternary electrolytes can provide excellent ionic conductivities even at room temperature (Figure 1).7 While the reversible electrochemical properties of the optimized ternary electrolyte, Al[TfO]3/NMA/urea = 0.05/0.76/0.19, opens for conceptual new possibilities to design electrolytes, these were however modest compared to conventional AlCl3-based electrolytes. For the cationic solvate based approach, we have successfully synthesized alkylhaloaluminate-free cationic Al complexes with very varying ligand and anion structures,8,9 and the structures and electrochemical properties of these will be discussed.

Acknowledgement:
The financial support by Chalmers Area of Advance Materials Science and the Swedish Energy Agency programme “Batterifonden” is gratefully acknowledged. The authors are also thankful for the continuous support by Chalmers Areas of Advance Energy and Transport.

References:
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Figure 1