Synergistic effect of cobalt and nickel on the superior electrochemical performances of rGO anchored nickel cobalt binary sulfides

Reduced graphene oxide (rGO) anchored spinel nickel cobalt binary sulfides nanosheets (NixCo3-xS4/rGO) with sandwich-like structure are synthesized via a facile hydrothermal process. The synergistic effect of nickel and cobalt on the superior electrochemical properties of the NixCo3-xS4/rGO nanocomposites is firstly illustrated in detail through the comparison of the phase structures, metal valent states and electrochemical behaviors of the nickel doped cobalt sulfide (Ni0.3Co2.7S4/rGO), cobalt doped nickel sulfide (Ni2.7Co0.3S4/rGO) and nickel cobalt binary sulfide (Ni1.5Co1.5S4/rGO). Results indicate that the fast valence change of nickel species mainly contributes the faradaic reaction of the active materials, while the cobalt species offer the high electronic conductivity and assist the charge transfer process. Due to the synergistic effect of cobalt and nickel, the Ni1.5Co1.5S4/rGO electrode with sandwich-like structure exhibits superior electrochemical performances, such as high specific capacity (347.5 mAh g−1 based on metal sulfide), high areal specific capacity (4.3 mAh cm−2), high cyclic stability (85.0% retention after 2000 cycles). An asymmetric device, with the Ni1.5Co1.5S4/rGO as positive material and a home-made activated carbon as negative material, delivers high energy density of 61.6 Wh kg−1 at the power density of 563.1 W kg−1 and still remains 49.3 Wh kg−1 even at high power density of 11.0 kW kg−1.