In situ tailoring bimetallic–organic framework-derived yolk–shell NiS2/CuS hollow microspheres: an extraordinary kinetically pseudocapacitive nanoreactor for an effective sodium-ion storage anode

Abstract

Pseudocapacitive electrochemical Na⁺-storage has been highlighted as one of the exploitable strategies for overcoming the sluggish diffusion-limited redox kinetics due to the effective structural preservation and fast ion-adsorption/desorption at the surface or quasi-surface of electrode materials. However, exploiting pseudocapacitive hosts with a micro–nano hierarchitecture and further achieving competitive pseudocapacitive contributions are still in their infancy so far. Herein, a yolk–shell NiS₂/CuS hollow microspherical architecture with superb kinetically pseudocapacitive features was successfully constructed through an in situ hydrothermal sulfidation and subsequent ion-exchange route using Ni-based bimetallic (NiZn) organic frameworks (NiZn-MOFs) as a template precursor. As expected, the strongly synergistic coupling effect and hollow structural characteristic of the NiS₂/CuS heterostructure enabled fast charge transfer and Na⁺ immigration, as well as the release of the mechanical stress/strain induced by the conversion reaction, and not unexpectedly, the NiS₂/CuS electrode afforded extraordinary Na⁺-storage capability, including a remarkable specific capacity of 410.9 mA h g⁻¹ after 750 cycles at 2.0 A g⁻¹, excellent rate capability, and prolonged cyclability in terms of a remarkable 283.4 mA h g⁻¹ even after 4200 cycles at 20.0 A g⁻¹. More significantly, the kinetic analysis demonstrated that the electrochemical charge storage of the NiS₂/CuS electrode manifested considerable pseudocapacitive contributions at all rates (90.0% to 96.9%), distinctly outperforming the previously reported NiS₂-/CuS-based anodes. Furthermore, the density functional theoretical calculations suggested a fast Na⁺-transport kinetics and enhanced antibonding state energy level and Na₂S adsorption energy due to the electronic redistribution and lattice distortion in the NiS₂/CuS heterointerfaces.