Hierarchically constructed NiCo₂S₄@Ni_(1-x)Co_x(OH)₂ core/shell nanoarrays and their application in energy storage

We report a new type of core-shell heterostructure consisting of a rod-like NiCo₂S₄ (NCS) core and an urchin-like Ni_(1-x)Co_x(OH)₂ (NCOH) shell via a simple hydrothermal route coupled with a facile electrodeposition. NCS nanorod arrays (NRAs) can not only act as excellent electrochemically active materials by themselves, but they can also serve as hierarchical porous scaffolds capable of fast electron conduction and ion diffusion for loading a large amount of additional active materials. Moreover, it is observed that the urchin-like NCOH nanosheets coating could bind the inner NCS nanorods together and thereby reinforce the whole structure mechanically. Meanwhile, more effective pathways for electrons are available in the NCS@NCOH hybrids than an individual NCS nanorod. Benefiting from both structural and compositional features, the NCS@NCOH electrode exhibits greatly improved electrochemical performance with high capacity (3.54 C cm^-2 at 1 mA cm^-2) and excellent cycling stability (78% capacity retention after 4000 cycles). Moreover, a battery-type device is also fabricated by using NCS@NCOH as a positive electrode and activated carbon (AC) as a negative electrode, displaying high capacity (2.51 C cm^-2 at 2 mA cm^-2) and good durability (88.8% capacity retention after 4000 cycles) as well.