Additive-Mediated Disruption of Al3+ Hydration Shells Stabilizes Reversible Aqueous Aluminum Batteries

Abstract

Aqueous aluminum-ion batteries (AIBs) promise safe and low-cost energy storage but are hindered by pervasive hydrogen evolution and aluminum anode corrosion due to strong Al3+ hydration. We introduce hexamethylphosphoramide (HMPA), a high-donor-number additive, to reconstruct this solvation structure and enable reversible Al electrochemistry. Vibrational and NMR spectroscopies show that HMPA coordinates directly to Al3+, displacing inner shell water ligands and disrupting the extended hydrogen bond network, thereby lowering water activity at the electrode interface. Increasing the HMPA concentration markedly enhances electrolyte wetting on aluminum surfaces, with contact angles decreasing from 80.1° in pure aqueous electrolyte to 45.2° at 60 vol % HMPA. Immersion studies reveal robust corrosion suppression with 40 vol % HMPA. In full cells pairing aluminum anodes with polyaniline (PANI) cathodes, the optimized 40 vol % HMPA electrolyte yields an initial specific capacity of 194.9 mAh/g at 100 mA/g, an initial Coulombic efficiency of 81.4%, and a capacity retention of 61.3 mAh/g after 200 cycles with 99.1% efficiency. Kinetic studies via cyclic voltammetry indicate predominantly surface-controlled pseudocapacitive behavior, contributing 81.0% of the charge storage at 0.9 mV/s. This additive-driven solvation engineering strategy provides a scalable, effective pathway toward practical, high-performance aqueous multivalent-ion batteries.