Hybrid supercapacitors (HSCs) with high energy density have caused great interest. However, it is still lack of general principle to optimize the electrochemical performance. Hereby, an insightful study on HSCs has been carried out using devices based on flower-like NiCo layered double hydroxide (NiCo-LDH, 777 C g(-1)) and activated carbon (AC) in KOH. It has been found that the upper potential limit of NiCo-LDH is about 0.5 V (vs Ag/AgCl) while the lower potential limit of AC is about -1.1 V to -1.3 V. The stable potential window of HSC is 1.6 V. The key to boost cell specific capacitance is to enlarge the potential window of AC by adjusting the Zero Voltage through tuning the mass ratio between the two electrodes. The optimized mass ratio is found to be lower than that calculated by traditional method. We have developed a model to disclose the effect of materials performance, cell potential window and mass ratio on the cell performance. A parameter of match-coefficient (0.8-0.85, in our case) is proposed to estimate the optimized mass ratio. Following the optimized cell parameters, a demo of all-solid-state HSC (optimized mass ratio of 2) has been assembled using solid polymer electrolyte. It exhibits a high specific energy of 69.5 Wh kg(-1) at 450 Wkg(-1) and good cycling stability with 95.7% capacitance retention after 5000 cycles within 1.8 V. This work sheds light on comprehensive understanding towards design and fabrication high performance HSCs. (C) 2019 Elsevier Ltd. All rights reserved.