Molecular Understanding of Ligand-Governed Al0 Electrodeposition from Al3+ Complexes in Protic Solvents and Ionic Liquids

Commercial Al(OTf)3 and Al(TFSI)3 salts have attracted significant interest in aluminum-ion batteries (AIBs), with aqueous electrolytes exhibiting typical charge–discharge profiles whose origin remains debated. Meanwhile, these salts have also been explored as alternatives to halide-based ionic liquid (IL) electrolytes, which suffer from low oxidative stability and corrosive character in device applications. Here, we systematically analyze the electrochemical behavior of these salts in both protic and IL electrolytes to elucidate the mechanism governing Al0/3+ electrochemistry. This work reveals that Al0 electrodeposition consistently fails in the examined Al(OTf)3 and Al(TFSI)3-based electrolytes owing to residual water, which cannot be fully removed even after rigorous drying processes and the addition of moisture-scavenging LiAlH4. For Al3+ complexes in these electrolytes, breaking a labile ligand is energetically more favorable than complete Al3+ desolvation during charge-transfer processes at the electrode interface. The calculations further suggest that protic molecules coordinated to a Lewis-acidic cation are more readily deprotonated at the electrode interface than in the bulk electrolyte. The less Lewis-acidic Zn2+ shows the same preference for ligand decomposition over desolvation. Cluster calculations further suggest that [Zn(H2O)6]2+ can release twice as many coordinated water molecules as [Al(H2O)6]3+ before deprotonation becomes favorable. These findings provide an important perspective on aluminum electrolyte design, emphasizing ligand stability as a prerequisite for directing the reaction toward Al3+ desolvation and electroplating over parasitic processes.