Loose Al3+ Solvation Aggregates Enable Reversible Aluminum Anodes in Chlorine-Free Electrolyte.

Sustained reversible Aluminum plating/stripping with long cycle life is scarcely realized in chlorine-free organic electrolytes. The electrolyte formula exerts the deposition/stripping behavior of metal anodes. Highly polarizing Al3+ cations show strong binding with electrolyte components, resulting in a significantly different ionic solvation structure from lithium. Here we reveal that pervasive compact ion aggregates (CAGs) solvation structures with an Al3+-Al3+ interatomic distance of ∼5 Å were formed within typical ether or ester electrolytes, leading to low ionic conductivity and passivation interphases. On this basis, we developed a chlorine-free ester electrolyte featuring loose solvation aggregates (LAGs) with an Al3+-Al3+ interatomic distance of 7 Å through modulating the balance of Al3+ coordination between the OTf- anion and solvent. Tris(pentafluorophenyl)borane (TPFPB) interacts with anions to suppress anion over-coordination in Al3+ solvation structures, thereby inducing an organic-inorganic hybrid solid electrolyte interphase (SEI). This loosely solvated Al3+ electrolyte delivers nearly twofold enhanced ionic conductivity and a hybrid SEI, which endows the system with a low Al-plating/stripping overpotential of ∼0.4 V and durable cycling exceeding 4000 h.