Open Challenges on Aluminum Triflate-Based Electrolytes for Aluminum Batteries

Abstract

Among possible “beyond Lithium” candidates, Aluminum is the most abundant one, and it can theoretically provide three times more charge per redox center as compared to Lithium. However, a drawback of Aluminum batteries is the requirement of an acidic electrolyte based on an ionic liquid and Aluminum chloride (AlCl3) salts to enable plating and stripping. This electrolyte is very corrosive and restricts the use of suitable current collectors and all involved parts of the cell. Recently, Aluminum trifluoromethanesulfonate (Al(OTF)3) has been proposed as a non-corrosive alternative to AlCl3. It was suggested that this salt could enable plating and stripping of aluminum in a melt composed of urea and N-Methylacetamide (NMA). However, to assess the real suitability of these electrolytes, it is necessary to evaluate their electrochemical behavior at different working conditions. With this purpose, we present the electrochemical study of two electrolyte compositions based on the non-corrosive Al(OTF)3 salt, urea and two different solvents, NMA and Ethyl-Isopropyl-Sulfone (EiPS). This work highlights important challenges related to the reversibility of the redox reactions when using Al(OTF)3-based electrolytes and reveals an unexpected behavior with substrates other than Pt or Cu. These aspects should be taken into consideration in future research for AlCl3-free electrolytes.