Electrolyte Solvation Structure Regulation Promotes Aluminum-Air Batteries to Approach Theoretical Discharge Capacity

Aluminum-air batteries (AABs) feature high energy density and no limitations for charging facilities but suffer from severe self-corrosion reactions. Suppressing the side reaction between water and aluminum is the fundamental strategy to boost AAB performance. Herein, we first disclose that addition of sodium formate (HCOONa) can largely remold the solvation structure of the classic NaOH electrolyte and greatly reduce the amount of free water, thus constructing AABs close to their theoretical capacity. Ab initio molecular dynamics simulations show that multiple Na+ and H2O form hydrated Na+ ion clusters [Nam(OH2)7m]m+ in the HCOONa-NaOH electrolyte. Meanwhile, HCOO- enters the first solvation layer of Na+ and forms a "stable triangle" with Na+ and H2O. The coordination network formed by Na+, H2O, and HCOO- results in a very low level of free water. Moreover, HCOO- further forms a self-assembled passivation layer on the Al surface. As such, the self-corrosion rate of Al in the HCOONa-based electrolyte is drastically reduced. More encouragingly, the ultrahigh capacity (2767 mAh g-1) delivered by AABs using the hybrid electrolyte reaches 92.9% of its theoretical value, concurrently achieving a high energy density of 3702 Wh kg-1. This work paves the way for constructing durable AABs through an electrolyte regulation strategy.