Battery electricity storage has been one of the main strategies to reach a sustainable energy network. They are adequate to store energy and release it later, supporting a high volume of variable renewable electricity. In this context, lithium-air batteries (LABs) have the potential to be a high-capacity battery option, with theoretical energy densities higher than currently available lithium-ion ones. However, they are still commercially unfeasible. In the last few decades, there has been immense progress in LABs technology with the development of stable electrolytes, porous cathodes, and catalysts. Nonetheless, minor attention has been given to the protection of the lithium metal electrode, especially against reactive substances present in the atmospheric air, such as water and oxygen. In this work, a protective membrane was synthesized to protect the metallic lithium anode against water. The synthesis was carried out using polytetramethylene glycol (PTMEG), 4,4-diphenylmethane diisocyanate (MDI), and a blend of 1,4 butanediol with glycerine as a chain extender. The synthesized membrane was tested using an aprotic lithium-oxygen (Li-O2) battery assembled with carbon paper as the cathode, metallic lithium as the anode, and 0.1 mol.L-1 lithium perchlorate (LiClO4) in dimethyl sulfoxide (DMSO) with 550 ppm of water concentration as electrolyte. Furthermore, the cyclability of the batteries with the novel polymeric membrane was compared with the standard glass microfiber separator. The results showed a higher cyclability of the batteries assembled with the polymeric separator over the glass microfiber separator.