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 Ni-decorated 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.