NiCoAl layered double hydroxides (NiCoAl-LDHs) with different morphology, structure, size and pseudocapacior properties have been prepared by tuning the ratio of metallic elements via a hydrothermal method. The optimized element proportion of 2D NiCo2Al-LDH consisting of hexagonal nanosheets delivers a significantly enhanced specific capacitance 2369.4 F g(-1) at a current density of 1 A g(-1). A circle-like growth mechanism is proposed to explain the formation of the 2D NiCo2Al-LDH structures. Its 2D structure and the synergistic effect of three metallic elements assure its high electrochemical performance. An asymmetric supercapacitor (ASC) based on NiCo2Al-LDH exhibits an ultra-high energy density of 91.0 Wh kg(-1) at a power density of 758.2 W kg(-1) as well as long-term stability (92% of its initial capacitance retention at 8 A g(-1) over 5000 cycles), outperforming most of LDH and metal oxides ASCs. (C) 2017 Elsevier Ltd. All rights reserved.