Balancing Bulk Solution Chemistry and Cation Solvation for Stable Aqueous Aluminum Batteries

The reversibility of aluminum (Al) metal anodes in aqueous batteries is limited by the high reactivity of bulk solution chemistry and the instability of strongly hydrated Al³⁺, yet electrolytes are typically optimized empirically without distinguishing these two effects. Here, we rationalize electrolyte design using a dual-scale optimization framework that independently evaluates bulk solution effects and Al³⁺ solvation behavior through combined electrochemical measurements and molecular dynamics simulations. Trialkyl phosphate cosolvents are shown to disrupt the hydrogen bond network of water while partially replacing coordinated H₂O in the Al³⁺ primary solvation shell, with distinct composition-dependent responses at the bulk and solvation levels. An optimal electrolyte (40TMP) emerges from balancing suppressed water reactivity, moderate desolvation penalty, and fast ion transport, delivering an ionic conductivity of 24.6 mS cm^-1 and stable Al||Al cycling for over 500 h. This work provides a rational framework for electrolyte optimization in aqueous aluminum electrochemistry.