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

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.