Partially delocalized charge in Fe-doped NiCo2S4 nanosheet–mesoporous carbon-composites for high-voltage supercapacitors

Abstract

Unraveling the effect of transition-metal doping on the energy storage properties of bimetallic sulfides remains a grand challenge. Herein, we construct bimetallic sulfide nanosheets and hence deliberately introduce transition-metal doping domains on their surface. The resulting materials show not only an enhanced density of states near the Fermi level but also partially delocalized charge as shown by density functional theory (DFT) calculations. Fe-doped NiCo₂S₄ nanosheets wrapped on N,S-doped ordered mesoporous carbon (Fe-NiCo₂S₄@N,S-CMK-3) are prepared, which show an enhanced specific capacitance of 197.8 F g⁻¹ in ionic liquid-based supercapacitors at a scan rate of 2 mV s⁻¹. This is significantly higher as compared to the capacitance of 155.2 and 135.9 F g⁻¹ of non-iron-doped NiCo₂S₄@N,S-CMK and Fe-NiCo₂S₄@CMK-3 electrodes, respectively. This result arises from the enhanced ionic liquid polarization effect and transportation ability from the Fe-NiCo₂S₄ surface and N,S-CMK-3 structure. Furthermore, the importance of matching multi-dimensional structures and ionic liquid ion sizes in the fabrication of asymmetric supercapacitors (ASCs) is demonstrated. As a result, the ASC device exhibits a high energy density of 107.5 W h kg⁻¹ at a power density of 100 W kg⁻¹ in a working-voltage window of 4 V when using Fe-NiCo₂S₄@N,S-CMK-3 and N,S-CMK-3 as positive and negative electrodes, respectively. This work puts forward a new direction to design supercapacitor composite electrodes for efficient ionic liquid coupling.