Enhancing Capacitance of Nickel Cobalt Chalcogenide via Interface Structural Design

Spinel NiCo ₂ X ₄ (X = O or S), comprising two geometrical cobalt ions, Co ²⁺ in the tetrahedral site (Co ²⁺ _Td ) and Co ³⁺ in the octahedral site (Co ³⁺ _Oh ), has been widely evaluated as a promising pseudocapacitor electrode material. Previous literature mainly demonstrated that much higher specific capacitance of NiCo ₂ S ₄ than that of NiCo ₂ O ₄ 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 NiCo ₂ S ₄ (1862 F g ^-1 at 4 A g ^-1 ) is 50% higher than that of NiCo ₂ O ₄ (1230 F g ^-1 at 4 A g ^-1 ), which is derived from the designed increase of Co ²⁺ _Td ions (cobalt ions in the tetrahedral site) in NiCo ₂ S ₄ . These results have significant implications for the design and optimization of the electrochemical properties of transition-metal-based pseudocapacitors.