Synthesis of CoxNi1-xS2 electrode material with a greatly enhanced electrochemical performance for supercapacitors by in-situ solid-state transformation

Transition metal sulfides such as Ni-Co sulfides are promising electrode materials for supercapacitors owing to their advantageous electrical conductivity. Current approaches for the synthesis of Ni-Co sulfides normally involve wet chemical sulfurization. In this work, we demonstrate a facile method for synthesizing CoxNi1-xS2 (0 < x < 1) by solid-state sulfurization of Ni-Co layered double hydroxides (Ni-Co LDHs) at a lower temperature (300 degrees C). The Co(x)Ni(1-x)S(2)is found to retain the original porous morphological structure of Ni-Co LDHs which consists of hierarchical flower-like nanosheets. This material demonstrates much enhanced electrochemical properties. The specific capacitance of the as-prepared CoxNi1-xS2 nanosheets is 1622.4 F g(-1) (811.2 C g(-1)) at a current density of 1 A g(-1), which is 50% higher than the capacitance of the pristine Ni-Co LDHs (specific capacitance: 1040.4 Fg(-1)) at the same discharge current density. Even at a large discharge current density of 20 A g(-1), the CoxNi1-xS2 still shows specific capacitance of 1220 F g(-1), which is nearly 3 times higher compared to the Ni-Co LDHs. Electrochemical stability test shows that the CoxNi1-xS2 maintains 69% capacitance retention after 2000 charge/discharge cycles. This work demonstrates that Ni-Co sulfides possess superior intrinsic electrochemical properties to Ni-Co LDHs. Importantly, the in-situ solid-state sulfurization annealing process is a facile and viable approach for large-scale synthesis of high-performance Ni-Co-S materials. (C) 2019 Elsevier B.V. All rights reserved.