Three-dimensional ZnFe₂O₄@MnO₂ hierarchical core/shell nanosheet arrays as high-performance battery-type electrode materials

In this paper, a highly ordered three-dimensional ZnFe₂O₄@MnO₂ core/shell nanosheet arrays (ZnFe₂O₄@MnO₂ NSAs) on three-dimensional (3D) Nickel foam (Ni foam) have been prepared via a facile, stepwise hydrothermal approach and further investigated as a battery-type electrode material. It is found that the honeycomb-like ZnFe₂O₄@MnO₂ NSAs have outstanding electrochemical performances using cyclic voltammetry (CV), galvanostatic charge-discharge (CD), and electrochemical impedance spectroscopy (EIS) in a three-electrode system. Based on the unique nanostructure, the ZnFe₂O₄@MnO₂ NSAs exhibit an exceptional capacity of 1084 C g⁻¹ at current density of 2 A g⁻¹ and long-term capacity retention of 96.1% after 5000 cycles at current density of 4 A g⁻¹. Moreover, the ZnFe₂O₄ nanosheet arrays (ZnFe₂O₄ NSAs) also have 714 C g⁻¹ at a current density of 2 A g⁻¹ exhilaratingly and the capacity value is proven to be the highest when compared with other structures of ZnFe₂O₄ taken from the other literatures. Some synergistic effects are identified to be responsible for the observations: Firstly, high crystalline quality of the ZnFe₂O₄ NSAs core which is directly grown on 3D Nickel foam (a conductive current collector), allowing fast electron transport; secondly, the unique structure of honeycomb-like MnO₂ shell that has an ultralarge surface area for electrochemical reaction; thirdly, smart combination of ZnFe₂O₄ and MnO₂ effectively utilizes their different virtues and also compensate their defects; finally, highly conductive 3D Ni foam substrate, which fully eliminate conductive additives and binders. - Graphical abstract: In this work, we describe the preparation of 3D vertical ZnFe₂O₄ nanosheet arrays and hierarchical honeycomb-like ZnFe₂O₄@MnO₂ nanosheet arrays. Furthermore, we research their applications in battery-type electrodes. - Highlights: • We report the novel honeycomb-like, three-dimensional ZnFe₂O₄@MnO₂ core/shell nanosheet arrays. • Hierarchical ZnFe₂O₄@MnO₂ nanosheets are fabricated by a facile and stepwise hydrothermal approach. • This core/shell structure shows the fascinating synergetic properties. • The application as an excellent material for high-performance battery-type electrode. • ZnFe₂O₄@MnO₂ nanosheet arrays show superior electrochemical performances.