Outstanding sodium and lithium storage capability is successfully demonstrated in ultrathin NiO nanosheets (4-5 nm in thickness) synthesized via a facile solvothermal process followed by annealing in air. For sodium storage, the NiO nanosheets deliver a high reversible specific capacity of 299 mA h g(-1) at a current density of 1 A g(-1), and the capacity still remains up to 154 mA h g(-1) at 10 A g(-1). Upon charge/discharge cycling, the specific capacity maintains to be as high as 266 mA h g(-1) during the 100th cycle at 1 A g(-1). Such sodium storage capability of NiO nanosheets is by far one of the best reported for transition metal oxides. For lithium storage, the cell achieves a high reversible specific capacity of 1242 and 250 mA h g(-1), at 0.2 and 15 A g(-1), respectively. The capacity for lithium storage maintains to be 851 mA h g(-1) during the 170th cycle at 2 A g(-1). The present results demonstrate that ultrathin NiO nanosheets are highly attractive for fast sodium/lithium diffusion with high-rate capability for rechargeable sodium-ion batteries (SIBs) and lithium-ion batteries (LIBs). (C) 2014 Elsevier B.V. All rights reserved.