Ultrasmall NiS2 Nanocrystals Embedded in Ordered Macroporous Graphenic Carbon Matrix for Efficiently Pseudocapacitive Sodium Storage

Abstract

Sodium-ion hybrid capacitor (SIHC) is one of the most promising alternatives for large-scale energy storage due to its high energy and power densities, natural abundance, and low cost. However, overcoming the imbalance between slow Na+ reaction kinetics of battery-type anodes and rapid ion adsorption/desorption of capacitive cathodes is a significant challenge. Here, we propose the high-rate-performance NiS2@OMGC anode material composed of monodispersed NiS2 nanocrystals (8.8 ± 1.7 nm in size) and N, S-co-doped graphenic carbon (GC). The NiS2@OMGC material has a three-dimensionally ordered macroporous (3DOM) morphology, and numerous NiS2 nanocrystals are uniformly embedded in GC, forming a core–shell structure in the local area. Ultrafine NiS2 nanocrystals and their nano–microstructure demonstrate high pseudocapacitive Na-storage capability and thus excellent rate performance (355.7 mAh/g at 20.0 A/g). A SIHC device fabricated using NiS2@OMGC and commercial activated carbon (AC) cathode exhibits ultrahigh energy densities (197.4 Wh/kg at 398.8 W/kg) and power densities (43.9 kW/kg at 41.3 Wh/kg), together with a long life span. This outcome exemplifies the rational architecture and composition design of this type of anode material. This strategy can be extended to the design and synthesis of a wide range of high-performance electrode materials for energy storage applications.