Hollow-porous nanospheres of ZnMn2O4 spinel: A high energy density cathode for rechargeable aqueous battery

Highlights: • Time dependent morphology evolution of spinel ZnMn₂O₄ porous hollow spheres by solvothermal synthesis. • Dominant diffusion controlled charge transfer mechanism leading to outstanding electrochemical performance. • High energy density of ZnMn₂O₄//AC rechargeable aqueous battery (~215.7 W h kg⁻¹ at a power density of ~1184.5 kW kg⁻¹). • Ultra-long cycling stability with high energy efficiency (10,000 cycles with ~106% capacitance retention). Well-controlled hollow-porous nanostructures can provide enhanced charge storage capacity owing to the rapid diffusion of electrolyte ions into their interior. However, interface engineering of stable hollow-porous nanostructures with reversible faradaic reactions for efficient energy conversion/storage devices is still a challenge. Herein, we report solvothermal synthesis of spinel ZnMn₂O₄ with a hollow-porous spherical (ZMO_HS) morphology. The formation mechanism of such a hierarchical nanostructure has been discussed. The ZMO_HS exhibits a specific capacity of ~187 mAh g⁻¹ at a current density of 2 A g⁻¹ when tested as a faradaic electrode (vs Pt) for a rechargeable aqueous battery (RAB) in alkaline electrolyte. Furthermore, a full cell RAB constituting of ZMO_HS//activated carbon (AC) demonstrates energy and power densities of ~215.7 Wh Kg⁻¹ and ~1184.5 W kg⁻¹ respectively, with ultra-long cycling stability (~106% capacitance retention after 10,000 cycles), making it a very promising material for next-generation energy storage device applications.