Enhanced cycleability of faradic CoNi2S4 electrode by reduced graphene oxide coating for efficient asymmetric supercapacitor

Abstract Poor cycleability is the main shortage of faradic supercapacitors, which can be generally improved by the compositing or coating with flexible and conductive auxiliary components. Herein, reduced graphene oxide (rGO) coated CoNi 2 S 4 grown onto Ni foam substrate was synthesized by a two-step hydrothermal strategy. The rGO coating apparently improved the cycleability of the CoNi 2 S 4 electrode without sacrificing total capacitance, therefore resulted in efficient and long lifetime energy storage device. The rGO coated CoNi 2 S 4 electrode, denoted as CoNi 2 S 4 -rGO electrode, demonstrated a specific capacitance ( C s ) of 1680 F g -1  at 1 A g −1 and considerable rate capability (62% capacitance retaining ratio within 1–20 A g −1 ). More importantly, the cycleability of the CoNi 2 S 4 -rGO electrode enhanced by 46% relative to the mere CoNi 2 S 4 electrode after 5000 charge-discharge cycles, which is mainly due to the inhibition of shedding of faradic CoNi 2 S 4 from electrode by rGO coating. When used as positive electrode of asymmetric supercapacitor (ASC), the CoNi 2 S 4 -rGO//porous carbon supercapacitor delivered an energy density of 51.7 Wh kg −1 at 762 W kg −1 , and 60.5% of its initial capacitance was maintained after 5000 charge-discharge cycles. The balanced energy, power densities and cycleability highlight the potential of the rGO coated faradic electrode in energy storage devices.