Synergetic compositional and morphological effects for improved Na⁺ storage properties of Ni₃Co₆S₈-reduced graphene oxide composite powders.

The electrochemical properties of binary transition metal sulfide-reduced graphene oxide (RGO) composite powders, relevant for their performance as anode materials in sodium ion batteries, were firstly studied. (Ni,Co)O-RGO composite powders prepared by spray pyrolysis are transformed into Ni3Co6S8-RGO composite powders by a simple sulfidation process. Plate-shape nanocrystals of nickel-cobalt sulfide (Ni3Co6S8) are uniformly distributed over the crumpled RGO structure. The discharge capacities of the Ni3Co6S8-RGO composite powders for 2(nd) and 100(th) cycles at a current density of 0.5 A g(-1) are 504 and 498 mA h g(-1), respectively. However, the discharge capacities of the bare Ni3Co6S8 powders for 2(nd) and 100(th) cycles are 522 and 125 mA h g(-1), respectively. The NiO-Co3O4 and (Ni,Co)O-RGO composite powders prepared by spray pyrolysis also show low discharge capacities of 122 and 119 mA h g(-1), respectively, after 100 cycles. The high structural stability of the Ni3Co6S8-RGO composite powders during repeated sodium ion intercalation/deintercalation processes results in excellent cycling and rate performances for Na(+) storage.