Solvation Structure Regulation for Highly Reversible Aqueous Al Metal Batteries

Metallic Al has been deemed an ideal electrode material for aqueous batteries by virtue of its abundance and high theoretical capacity (8056 mAh cm^-3). However, the development of aqueous Al metal batteries has been hindered by several side reactions, including water decomposition, Al corrosion, and passivation, which arise from the solvation reaction of Al and H₂O in conventional aqueous electrolytes. In this work, we report that water activity in electrolyte can be suppressed by optimizing the Al³⁺ solvation structure through intercalation of polar pyridine-3-carboxylic acid in an aluminum trifluoromethanesulfonate aqueous environment. Furthermore, the pyridine-3-carboxylic acid molecules are inclined to alter the surface energy of Al, thus suppressing the random deposition of Al. As a result, the Al corrosion in the hybrid electrolyte is restrained, and the long-term electrochemical stability of the electrolyte is tremendously improved. These merits bring remarkable reversibility to aqueous Al batteries using Al-preintercalated MnO₂ cathodes, delivering a retaining energy density of >250 Wh kg^-1 at 0.2 A g^-1 after 600 cycles.