Moisture-driven discharge chemistry enables the fabrication of highly reversible magnesium-oxygen polymer batteries

Magnesium-oxygen (Mg-O₂) batteries are attractive due to their high energy density and low cost, but their advancement is hindered by poor rechargeability, arising from the low reversibility of conventional MgO _x discharge products. Here, we uncover that Mg₃(OH)₅Cl·4H₂O, in addition to the well-known MgO and MgO₂, forms as the dominant discharge product through the synergistic effect of moisture and oxygen. This discovery reveals a new cathodic chemistry, 12Mg²⁺ + 4Cl^- + 5O₂ + 26H₂O → 4Mg₃(OH)₅Cl·4H₂O, that greatly enhances redox reversibility relative to the traditional MgO _x route. Meanwhile, a waterproof polymer gel electrolyte serves as a robust protective layer for the Mg anode, effectively mitigating moisture-induced corrosion. As a result, the moisture-driven Mg-O₂ polymer battery delivers a higher discharge voltage (1.42 V vs. 0.94 V) and capacity (9119 mAh g^-1 vs. 2665 mAh g^-1) than its dry-O₂ counterpart and achieves 160 stable cycles-the highest reported for Mg-O₂ systems to date. Moreover, it can operate directly in ambient air by harnessing both oxygen and humidity, and can be fabricated into a flexible fiber configuration, offering a practical and adaptable power source for portable and wearable electronics.