Fabrication of nickel-cobalt sulfide heterostructures with stable thermodynamic interfaces and enhanced electrochemical performance

Transition metal sulfides (TMS) with multicomponent heterostructure have been widely used as electrodes in electrochemical energy storage devices due to their abundant electroactive sites and fast charge transfer across the interfaces. Different kinds of heterostructures with specific TMS are fabricated by various routes to date, but it is still poorly understood how specific thermodynamic interfaces can be designed and easily coupled. Herein, the spinel and cobalt-pentlandite crystal phases of nickel–cobalt sulfides were chosen as models to demonstrate the formation of epitaxial heterointerface. The density functional theory calculations revealed the easy combination and high compatibility of these two crystal phases at the (111) plane. Along this guideline, a novel NiCo2S4/[Ni, Co]9S8 heterostructure was synthesized by a facile one-step hydrothermal strategy, with nanosheet-like shapes and well-defined heterointerfaces along the [111] crystal direction. The formation mechanism of the epitaxial heterointerface was proposed through the combination of solid and liquid phase reaction fields. As a proof-of-concept application, the obtained NiCo2S4/[Ni, Co]9S8 heterostructure as redox electrode for supercapacitor exhibits an enhanced specific capacitance of 1780F g−1 at 1 A/g and high capacitance retention of 70 % at 20 A/g. Moreover, the assembled aqueous hybrid supercapacitor of NiCo2S4/[Ni, Co]9S8//activated carbon delivers a high energy density of 62.8 Wh kg−1 at 0.4 kW kg−1 and excellent electrochemical stability after 3000 cycles. This work provides a new perspective on the epitaxial design of nickel–cobalt sulfide heterostructure with good lattice match as a promising electrode in energy storage devices.