• • The water-mediated Mg-O2 battery achieves 324 stable cycles at 1000 mA·g-1 with 500 mAh·g-1 capacity and 92% energy efficiency, exceeding all prior Mg-O2 systems that typically fail within tens of cycles and below 60% efficiency. This directly addresses the cycling stability bottleneck that has confined Mg-O2 to primary cells since the 1960s.
• • The discharge product Mg2(OH)3Cl·4H2O forms via the reaction 8Mg2+ + 4Cl- + 3O2 + 22H2O ⇋ 4Mg2(OH)3Cl·4H2O, replacing the kinetically inert MgOx. The chemically reactive nature of this product lowers the charging overpotential, enabling high reversibility and 92% energy efficiency.
• • The electrolyte formulation of 0.25 M Mg(TFSI)2 and 0.5 M MgCl2 in DME with trace water is critical for inducing the alternative reaction pathway. This simple additive strategy avoids noble metal catalysts and complex electrode nanostructuring, offering a cost-effective route to reversible Mg-O2 chemistry.
• • The 92% energy efficiency and 324-cycle stability at a high current density of 1000 mA·g-1 demonstrate practical viability for Mg-O2 batteries in applications requiring high volumetric capacity (3832 mAh·cm-3 for Mg vs. 2061 mAh·cm-3 for Li) and improved safety due to dendrite-suppressing Mg deposition.