Disordered Structure and Reversible Phase Transformation from K‐Birnessite to Zn‐Buserite Enable High‐Performance Aqueous Zinc‐Ion Batteries

Abstract

The layered δ‐MnO2 (dMO) is an excellent cathode material for rechargeable aqueous zinc‐ion batteries owing to its large interlayer distance (~0.7 nm), high capacity, and low cost; however, such cathodes suffer from structural degradation during the long‐term cycling process, leading to capacity fading. In this study, a Co‐doped dMO composite with reduced graphene oxide (GC‐dMO) is developed using a simple cost‐effective hydrothermal method. The degree of disorderness increases owing to the hetero‐atom doping and graphene oxide composites. It is demonstrated that layered dMO and GC‐dMO undergo a structural transition from K‐birnessite to the Zn‐buserite phase upon the first discharge, which enhances the intercalation of Zn2+ ions, H2O molecules in the layered structure. The GC‐dMO cathode exhibits an excellent capacity of 302 mAh g−1 at a current density of 100 mA g−1 after 100 cycles as compared with the dMO cathode (159 mAh g−1). The excellent electrochemical performance of the GC‐dMO cathode owing to Co‐doping and graphene oxide sheets enhances the interlayer gap and disorderness, and maintains structural stability, which facilitates the easy reverse intercalation and de‐intercalation of Zn2+ ions and H2O molecules. Therefore, GC‐dMO is a promising cathode material for large‐scale aqueous ZIBs.