A Moisture‐ and Oxygen‐Impermeable Separator for Aprotic Li‐O 2 Batteries

Despite the unparalleled theoretical gravimetric energy, Li‐O 2 batteries are still under a research stage because of their insufficient cycle lives. While the reversibility in air‐cathodes has been lately improved significantly by the deepened understanding on the electrode–electrolyte reaction and the integration of diverse catalysts, the stability of the Li metal interface has received relatively much less attention. The destabilization of the Li metal interface by crossover of water and oxygen from the air‐cathode side can indeed cause as fatal degradation for the cycle life as the irreversibility of the air‐cathodes. Here, it is reported that cheap poreless polyurethane separator can effectively suppress this crossover while allowing Li ions to diffuse through selectively. The polyurethane separator also protects Li metal anodes from redox mediators used for enhancing the reversibility of the air‐cathode reaction. Based on the Li metal protection, a persistent capacity of 600 mAh g −1 is preserved for more than 200 cycles. The current approach can be readily applicable to many other rechargeable batteries that suffer from similar interfacial degradation by side products from the other electrode.