Capacitive properties of reduced graphene oxide microspheres with uniformly dispersed nickel sulfide nanocrystals prepared by spray pyrolysis

Nickel sulfide-reduced graphene oxide (RGO) composite powders with spherical shapes are prepared by a one-pot spray pyrolysis process. The optimum mole ratio of nickel nitrate and thiourea to obtain nickel sulfide-RGO composite powders with high initial capacities and good cycling performance is 1:8. The bare nickel sulfide and nickel sulfide-RGO composite powders prepared directly by spray pyrolysis have mixed crystal structures of hexagonal α-NiS and cubic Ni₃S₄ phases. The bare nickel sulfide powders are prepared from the spray solution without graphene oxide sheets. The nickel sulfide-RGO composite powders have sharp mesopores approximately 3.5 nm in size. The discharge capacities of the nickel sulfide-RGO composite powders for the 1st and 200th cycles at a current density of 1000 mA g^-1 are 1046 and 614 mA h g^-1, respectively, and the corresponding capacity retention measured from the second cycle is 89%. However, the discharge capacities of the bare nickel sulfide powders for the 1st and 200th cycles at a current density of 1000 mA g^-1 are 832 and 16 mA h g^-1, respectively. The electrochemical impedance spectroscopy (EIS) measurements reveal the high structural stability of the nickel sulfide-RGO composite powders during cycling.