Synthesis of Co_xNi_1-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 Co_xNi_1-xS₂ (0 < x < 1) by solid-state sulfurization of Ni–Co layered double hydroxides (Ni–Co LDHs) at a lower temperature (300 °C). The Co_xNi_1-xS₂ 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 Co_xNi_1-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 F g⁻¹) at the same discharge current density. Even at a large discharge current density of 20 A g⁻¹, the Co_xNi_1-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 Co_xNi_1-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.