Tailoring Solvation and Interfacial Architecture via Low-Dosage Formic Acid/[Bmim]Cl Deep Eutectic Solvent Additive for Ultra-Durable Zn Metal Batteries.

Deep eutectic solvents (DESs) have demonstrated great promise in aqueous Zn metal batteries (AZMBs), yet their applications as bulk electrolytes are severely hindered by high viscosity and excessive costs. Employing DESs as low-dosage additives offers a compelling strategy to preserve their unique regulatory functions cost-effectively. Herein, a bifunctional DES composed of formic acid (FA) and 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) is designed as a low-dosage additive for electrolyte. It is revealed that FA and [Bmim]Cl synergistically anchor free water via strong hydrogen bonds, suppressing hydrogen evolution. FA preferentially enters the Zn2+ solvation sheath and cooperates with Cl- to reconstruct the solvation structure, lowering the desolvation barrier. At the interface, desolvated FA and [Bmim]+ co-adsorb on the Zn anode, inducing uniform nucleation through electrostatic shielding and enabling compact, horizontal Zn deposition. With the optimized electrolyte, the Zn||Zn symmetric cell cycles over 4000 h at 0.5 mA cm-2, and the Zn||Cu half-cell achieves 99.53% average Coulombic efficiency (CE) after 4000 cycles. Furthermore, the Zn||AC@I2 f full cell maintains 84.2% capacity over 30000 cycles at 5 A g-1. This work highlights the potential of low-dosage DESs in synergistically regulating solvation and interfacial chemistry for durable AZMB.