Two-dimensional (2D) nanomaterials are one of the most promising types of candidates for energy-storage applications due to confined thicknesses and high surface areas, which would play an essential role in enhanced reaction kinetics. Herein, a universal process that can be extended for scale up is developed to synthesise ultrathin cobalt-/nickel-based hydroxides and oxides. The sodium and lithium storage capabilities of Co3O4 nanosheets are evaluated in detail. For sodium storage, the Co3O4 nanosheets exhibit excellent rate capability (e.g., 179 mA h g¿1 at 7.0 A g¿1 and 150 mA h g¿1 at 10.0 A g¿1) and promising cycling performance (404 mA h g¿1 after 100 cycles at 0.1 A g¿1). Meanwhile, very impressive lithium storage performance is also achieved, which is maintained at 1029 mA h g¿1 after 100 cycles at 0.2 A g¿1. NiO and NiCo2O4 nanosheets are also successfully prepared through the same synthetic approach, and both deliver very encouraging lithium storage performances. In addition to rechargeable batteries, 2D cobalt-/nickel-based hydroxides and oxides are also anticipated to have great potential applications in supercapacitors, electrocatalysis and other energy-storage-/-conversion-related fields.