Hydrated Eutectic Electrolytes Stabilizing Quasi‐Underpotential Mg Plating/Stripping for High‐Voltage Mg Batteries

Abstract Aqueous rechargeable Mg batteries (ARMBs) usually fail from severe anode passivation, alternatively, executing quasi‐underpotential Mg plating/stripping chemistry (UPMC) on a proper heterogeneous metal substrate is a crucial remedy. Herein, a stable UPMC on Zn substrate is initially achieved in new hydrated eutectic electrolytes (HEEs), delivering an ultralow UPMC overpotential and high energy/voltage plateau of ARMBs. The unique eutectic property remarkably expands the lower limit of electrochemical stability window (ESW) of HEEs and undermines the competition between hydrogen evolution/corrosion reactions and UPMC, enabling a reversible UPMC. The UPMC is carefully revealed by multiple characterizations, which shows a low overpotential of 50 mV at 0.1 mA cm −2 over 550 h. With sulfonic acid‐doped polyaniline (SPANI) cathodes, UPMC‐based full cells show high energy/power densities of 168.6 Wh kg −1 /2.1 kWh kg −1 and voltage plateau of 1.3 V, far overwhelming conventional aqueous systems.