Role of BiCl₃ Additives for Aluminum Electrodeposition in Aluminum Trifluoromethanesulfonate-Based Electrolytes: A Critical Evaluation

Rechargeable Al ion batteries (RAIBs) fabricated using Al metal as the anode material have become increasingly popular owing to the advantages of high-abundance, low-cost, and high-capacity of Al metal. AlCl₃-based electrolytes have been extensively investigated for use in RAIBs. The high reactivity with moisture and strong corrosivity toward battery components of the AlCl₃-based electrolytes have driven researchers to develop active-halide-free (AHF) electrolytes. However, the ability of AHF electrolytes to support reversible Al plating and stripping is yet to be established. Recent studies have suggested that adding BiCl₃ to the aluminum trifluoromethanesulfonate-1,2-dimethoxyethane (Al(OTf)₃-DME) electrolyte enables reversible Al plating and stripping. Therefore, we investigated the feasibility of Al electrodeposition using the Al(OTf)₃-DME electrolyte with and without BiCl₃. Cyclic voltammetry and potentiostatic electrolysis indicated that Al electrodeposition is not achieved using the Al(OTf)₃-DME electrolyte with and without BiCl₃. The addition of BiCl₃ only results in the reversible plating and stripping of metallic Bi but not Al. Our study reveals the importance of achieving direct evidence of Al electrodeposition to evaluate whether AHF electrolytes are suitable for use in RAIBs.