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

The layered d-MnO2 (dMO) is an excellent cathode material for rechargeable aqueous zinc-ion batteries owing to its large interlayer distance (similar to 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.