NiCoAl layered double hydroxide nanosheets (NiCoAl-LDHNs) were prepared by a one-step solvothermal method. The shape and size of the obtained nanosheets are optimized by adjusting the solvothermal time and the molar concentration ratio of Ni2+/Co2+ to obtain the electrode material with the best performance. When the solvothermal time is 9 h and the molar concentration ratio of Ni2+/Co2+ is 1:1, NiCoAl-LDHNs has the best morphology and electrochemical performance. When assembled into a supercapacitor, NiCoAl-LDHN-9 has a high specific capacitance of 1228.5 F g(-1) at 1 A g(-1). As the current density is increased to 20 A g(-1), the specific capacitance is 1001.8 F g(-1), which still has a high capacitance retention of 81.6%. When NiCoAl-LDHN-9 was assembled into an asymmetric supercapacitor, NiCoAl-LDHN-9//AC has a specific capacitance of 102.1 F g(-1) at 0.5 A g(-1). The asymmetric supercapacitor devices also show excellent electrochemical performance in terms of energy density (35.9 Wh kg(-1) at 225.8 W kg(-1)), power density (4.8 kW kg(-1) at 22.2 Wh kg(-1)) and cycle life (capacitance retention rate after 10,000 cycles is 87.1%). Those results indicate that NiCoAl-LDHN have the potential to be promising electrode materials for high performance supercapacitors. (c) 2020 Elsevier Inc. All rights reserved.