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

This study innovatively fabricates nanoscale Mg 0.5 Zn 0.5 Mn 2 O 4 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) 2 SO 4 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 Mg 0.5 Zn 0.5 Mn 2 O 4, 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.