Enhancing Capacitance of Nickel Cobalt Chalcogenide via Interface Structural Design

Spinel NiCo₂X₄ (X = O or S), comprising two geometrical cobalt ions, Co²⁺ in the tetrahedral site (Co²⁺_Td) and Co³⁺ in the octahedral site (Co³⁺_Oh), has been widely evaluated as a promising pseudocapacitor electrode material. Previous literature mainly demonstrated that much higher specific capacitance of NiCo₂S₄ than that of NiCo₂O₄ was ascribed to the higher electronic conductivity. However, we argue that only a small amount of capacitance can be induced by the electronic conductivity, while the significance of electrochemical active species in these system has long been ignored. Here, we propose that geometrical-site-dependent pseudocapacitive activity will generate enhanced specific capacitance through the interface structural design. It reveals that specific capacitance of NiCo₂S₄ (1862 F g^-1 at 4 A g^-1) is 50% higher than that of NiCo₂O₄ (1230 F g^-1 at 4 A g^-1), which is derived from the designed increase of Co²⁺_Td ions (cobalt ions in the tetrahedral site) in NiCo₂S₄. These results have significant implications for the design and optimization of the electrochemical properties of transition-metal-based pseudocapacitors.