Ultrathin delta-MnO2 nanosheets were synthesized via in-situ reduction of KMnO4 on graphene oxide using graphene oxide as both reductant and self-sacrificing template. The layered delta-MnO2 nanosheets with a thickness of about 2-4 nm inherit the two-dimensional lamellar morphology of graphene oxide. When used as cathode for aqueous zinc ion batteries, the prepared delta-MnO2 nanosheets exhibit a reversible capacity of 133 mAh/g at a current density of 100 mA/g over 100 cycles and a capacity of 86 mAh/g at 500 mA/g due to the layered structure and ultrathin morphology, which is much better than delta-MnO2 microspheres. The electrochemical and structural investigation indicates a possible mechanism of two-step co-insertion of H+ and Zn2+ into the interlayer of delta-MnO2 during discharge process. It's believed that the unique ultrathin morphology of delta-MnO2 can well facilitate ion diffusion and structure stability during cycling, making ultrathin delta-MnO2 nanosheets a promising cathode in aqueous zinc ion batteries. (C) 2019 Elsevier Ltd. All rights reserved.