A high-performance MnO2 cathode doped with group Ⅷ metal for aqueous Zn-ion batteries: In-situ X-Ray diffraction study on Zn2+ storage mechanism

MnO2-based cathodes for aqueous Zn-ion batteries (AZIBs) suffer from the sluggish kinetics of Zn2+ migration and chemical instability during cycling, severely impeding its practical application. The group Ⅷ metal (Fe, Co and Ni) doping in MnO2 is an intriguing idea to improve the electrochemical performance of MnO2-based electrodes for AZIBs due to its special electronic structure of d shell. According to density functional theory calculation results, the doping-element facilitates the increase in kinetics of Zn2+ migration and increase chemical stability during cycling. When tested in AZIBs, a Fe-doping MnO2 (FMO) electrode exhibits a superior cycling stability (a high specific capacity of 338.2 mAh g−1 at 1 A g−1 with a capacity retention of 86.3% after 200 cycles), as same as that of Co and Ni-doping MnO2 electrode. In combination of in-situ X-ray diffraction and ex-situ transmission electron microscope studies reveal that layered-FMO undergoes a structural transition to ZnxMnO2, Mn3O4 and ZnMn2O4, accompanying with the formation of ZnSO4(OH)6·5H2O during discharging process, while those phases revert back to the parent phase during the subsequent Zn extraction. This study might build a solid foundation for developing powerful cathode in AZIBs.