Hollow MnO2 Nanospheres Containing Mg and Zn for Aqueous NH4+ Storage: Implications for Ammonium-Ion Batteries

This study innovatively fabricates nanoscale Mg0.5Zn0.5Mn2O4 hollow spheres specifically designed for aqueous ammonium-ion batteries (AAIBs), with a focus on enhancing electrochemical energy storage capabilities. The synthesized electrode material exhibits exceptional performance, boasting a high specific capacity of 100 mAh·g–1 at 2 A·g–1 current density, alongside extraordinary cycling stability, retaining 60% capacity after an impressive 600 cycles in an (NH4)2SO4 electrolyte. The distinguished characteristics are attributed to the unique nanoarchitecture: a thin shell for rapid electron/ion transport, an ample hollow interior mitigating structural stress from ion insertion/extraction, and a doubled active surface area facilitating superior charge transfer. Density functional theory (DFT) analysis reveals an optimized electronic structure with an advantageous d-band center positioning in Mg0.5Zn0.5Mn2O4, ensuring heightened redox activity and thus enhanced capacity compared to other compositions. This investigation underscores the potential of tailored nanoengineering in advancing AAIB cathode materials for sustainable, high-performance energy storage solutions.