Strongly Coordinated Al3+ Complex Formation Prevents Reversible Electrochemical Cycling in Haloaluminate-Free Ionic Liquids

Abstract

Advancements in aluminium-ion batteries require new non-corrosive electrolytes with high oxidation stability. Recently, haloaluminate-free electrolytes based on Al(OTF) 3 and Al(TFSI) 3 salts have attracted interest for aluminum electrochemistry; nevertheless, reversible Al 0/3+ cycling in these systems remains debated. This work reveals that Al 0 electrodeposition universally fails in Al(OTF) 3 and Al(TFSI) 3 based ionic liquid electrolytes because Al 3+ spontaneously extracts F - from anion and OH - from residual water, forming tightly bound tetrahedral complexes. Energy calculations show that deprotonation of coordinated water requires far less energy than Al 3+ desolvation, explaining why hydrogen evolution dominates. Even rigorous vacuum drying and LiAlH 4 addition cannot eliminate residual water from these hygroscopic salts. These findings provide a fresh perspective on aluminium electrolyte design, emphasizing ligand stability as a prerequisite for diversifying reaction towards Al 3+ desolvation and electroplating over parasitic processes.