A highly stable δ-MnO2 cathode with superior electrochemical performance for rechargeable aqueous zinc ion batteries

Abstract

Recently, aqueous zinc ion batteries (AZIBs) have attracted significant attention owing to their high safety, low cost, and abundant raw materials. However, finding an affordable and stable cathode, which can reversibly store a substantial amount of Zn²⁺ ions without damaging the original crystal structure, is still a major challenge for the practical application of ZIBs. It has already been demonstrated that δ-MnO₂ is a promising cathode for AZIBs owing to its layered structure and superior electrochemical performance; however, the reported results are still unsatisfactory (especially cyclability). Thus, using an oil bath method, we have fabricated a δ-MnO₂ cathode that exhibits a unique mixed phase morphology of mostly spherical nanoparticles and a few nanorods. It is believed that some of the nanoparticles are agglomerated to form nanorods, which may eventually help to offer numerous active sites for Zn²⁺ diffusion, enhancing the electrolyte osmosis and the contact area between the electrode and electrolyte. The obtained cathode delivers a high reversible capacity of ∼204 mA h g⁻¹ for the 100th cycle and ∼75 mA h g⁻¹ over 1000 cycles at a high current density of 3000 mA g⁻¹ with stable long-range cycling. Ex situ results indicate the mechanism of formation of ZnMn₂O₄ during discharge, followed by the evolution of the layered δ-MnO₂ during charge.