Water-Mediated Highly Reversible Mg-O2 Batteries
Magnesium-oxygen (Mg-O2) batteries offer high theoretical energy density and low-cost earth-abundant magnesium, yet practical deployment has been impeded by poor cycling stability and low energy efficiency, primarily due to the sluggish decomposition of conventional MgOx discharge products. Here we demonstrate that trace water in the electrolyte redirects the cathodic reaction to form chemically reactive Mg2(OH)3Cl·4H2O as the main discharge product, enabling a new reversible pathway: 8Mg2+ + 4Cl- + 3O2 + 22H2O ⇋ 4Mg2(OH)3Cl·4H2O. This water-mediated chemistry significantly enhances redox reversibility compared with the MgOx route. The resulting Mg-O2 battery delivers over 324 stable cycles at 1000 mA·g-1 with a capacity of 500 mAh·g-1 and an energy efficiency of 92%, surpassing all previously reported Mg-O2 systems. The electrolyte comprises 0.25 M magnesium bis(trifluoromethanesulfonyl)imide (Mg(TFSI)2) and 0.5 M magnesium chloride (MgCl2) in ethylene glycol dimethyl ether (DME) with a trace amount of water. These findings establish a general strategy for reversible Mg-O2 electrochemistry and provide a new design paradigm for practical magnesium-based energy storage.