Bis(diimino)nickel (NiDI) nanosheet is prepared by two methods of oxidation-promoted interfacial coordination reactions of nickel(II) ions with hexaaminobenzene. The thickness of the large chemically formed NiDI nanosheet can be controlled from atomic layers to thousands of layers. The morphology of electrochemically formed NiDI-modified electrode achievable within a minute suggests longitudinal growth of nanosheets at the surface. Increasing capacity of the NiDI with its amount indicates NiDI as a promising energy storage material.