Highly active mixed-valent MnOx spheres constructed by nanocrystals as efficient catalysts for long-cycle Li-O-2 batteries

Developing highly active and cost-effective catalysts for Li-O-2 batteries with high capacity, long cycle life and good rate performance is still challenging. In this work, a simple and efficient strategy is developed for the synthesis of MnOx spheres with a core-multishell structure. The spheres are assembled by ultrafine manganese oxide nanocrystals containing mixed-valent Mn-II, Mn-III, and Mn-IV species. The Li-O-2 batteries with MnOx sphere electrodes can deliver high discharge capacity, long-life cycling and excellent rate performance, namely, a specific capacity as large as 9709 mA h g(carbon)(-1) is delivered at 100 mA g(carbon)(-1) and maintained over 320 cycles at a limited capacity of 1000 mA h g(carbon)(-1). Even at a high capacity (2000 mA h g(carbon)(-1)) with a high current density of 200 mA g(carbon)(-1), the batteries can still endure more than 120 cycles. During the long cycling, the batteries show ultra-stable electrochemical performance with a high discharge potential (2.8-2.9 V) and low charge voltage (3.7-3.8 V). High-resolution transmission electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy analyses for cycled mixed-valent MnOx electrodes are used to reveal the electrochemical mechanism.