The synthesis of ultrathin magnetic nanosheets is significant for the development of next-generation nanodevices but remains to be a big challenge. Herein, we present facile approaches for large-scaled production of cell-unit-thick single-crystalline iron oxide nanostructures. The ultrathin hematite (alpha-Fe2O3) nanosheets with unit-cell thickness of 1.3 nm are firstly synthesized using a metal-ion-intervened hydrothermal method. Then the alpha-Fe2O3 nanosheets are converted to magnetic maghemite (gamma-Fe2O3) nanosheets with unit-cell thickness of 0.8 nm by a simple calcination process. The Li-ion battery studies reveal that unit-cell-thick gamma-Fe2O3 nanosheets exhibit great enhanced electrochemical performances with high specific capacitance approaching to its theoretical value and excellent cycling stability. (C) 2017 Elsevier B. V. All rights reserved.