Morphological and Crystalline Evolution of Nanostructured MnO<sub>2</sub> and Its Application in Lithium–Air Batteries

Single-crystal α-MnO<sub>2</sub> nanotubes have been successfully synthesized by microwave-assisted hydrothermal of potassium permanganate in the presence of hydrochloric acid. The growth mechanism including the morphological and crystalline evolution has been carefully studied with time-dependent X-ray diffraction, electron microscopy, and controlled synthesis. The as-synthesized MnO<sub>2</sub> nanostructures are incorporated in air cathodes of lithium–air batteries as electrocatalysts for the oxygen reduction and evolution reactions. The characterization reveals that the electrodes made of single-crystalline α-MnO<sub>2</sub> nanotubes exhibit much better stability than those made of α-MnO<sub>2</sub> nanowires and δ-MnO<sub>2</sub> nanosheet-based microflowers in both charge and discharge processes.