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

## TL;DR

A new magnesium–oxygen battery design uses moisture to create a reversible discharge product, improving performance and stability.

## Contribution

The discovery of Mg3(OH)5Cl·4H2O as a dominant discharge product enables a new cathodic chemistry with enhanced reversibility.

## Key findings

- The battery achieves 160 stable cycles, the highest reported for Mg–O2 systems.
- It delivers a higher discharge voltage (1.42 V) and capacity (9119 mAh g−1) compared to traditional systems.

## Abstract

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 MgOx 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 MgOx 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−1vs. 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.

A moisture-driven Mg–O2 polymer battery enables a new discharge pathway forming Mg3(OH)5Cl·4H2O as the dominant product, achieving highly reversible redox chemistry and record-long cycling stability.