In situ template synthesis of hollow nanospheres assembled from NiCo₂S₄@C ultrathin nanosheets with high electrochemical activities for lithium storage and ORR catalysis

Transition-metal sulfide hollow nanostructures have received intensive attention in energy-related applications due to their unique structural features and high electrochemical activities. Here, a well-designed composite of NiCo₂S₄@C is successfully fabricated using a facile in situ template removal method. The obtained composite shows unique microstructures of hollow nanospheres (∼650 nm in diameter) assembled from ultrathin NiCo₂S₄@C nanosheets, in which numerous scattered NiCo₂S₄ nanoparticles are embedded in ultrathin carbon nanosheets, exhibiting mesoporous features with a high surface area of 247.25 m² g^-1. When used as anode materials for LIBs, NiCo₂S₄@C hollow nanospheres exhibit a high reversible capacity of 1592 mA h g^-1 at a current density of 500 mA g^-1, enhanced cycling performance maintaining a capacity of 1178 mA h g^-1 after 200 cycles, and a remarkable rate capability. Meanwhile, the hollow nanospheres display excellent catalytic activity as ORR catalysts with a four-electron pathway and superior durability to that of commercial Pt/C catalysts. Their excellent lithium storage and ORR catalysis performance can be attributed to the rational incorporation of high-activity NiCo₂S₄ and ultrathin carbon nanosheets, as well as unique hollow microstructures, which offer efficient electron/ion transport, an enhanced electroactive material/electrolyte contact area, numerous active sites, and excellent structural stability.