Highly active mixed-valent MnOx spheres constructed by nanocrystals as efficient catalysts for long-cycle Li–O2 batteries

Abstract

Developing highly active and cost-effective catalysts for Li–O₂ 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 MnO_x 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–O₂ batteries with MnO_x 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 MnO_x electrodes are used to reveal the electrochemical mechanism.