At present, it is a significant challenge to design electrode materials with both desirable electrochemical performance and high mass loading for supercapacitors by using a simple, efficient and safe method. With the introduction of Ni2+ ions, the hollow Ni-decorated Co9S8 nanospheres meeting the above requirements have been successfully synthesized by one-step electrodeposition method under a two-electrode system. As electrodes for supercapacitors, such Ni-decorated Co9S8 electrodes deliver an ultrahigh areal and volumetric specific capacitance of 5.64 F cm(-2) and 171.85 F cm(-3) at a current density of 1 mA cm(-2), superior rate capability (88.9% retention at a current density of 20 mA cm(-2)) and remarkable cycling stability with a capacitance retention of 89% after 8500 cycles. Also, the as-fabricated flexible quasi-solid-state asymmetric supercapacitors based on Nidecorated Co9S8 nanospheres and active carbon electrodes present high energy density. The practicability and operability of the device were successfully demonstrated by lighting up the LEDs with five different colors.