Hierarchical bilayered hybrid nanostructural arrays of NiCo2O4 micro-urchins and nanowires as a free-standing electrode with high loading for high-performance lithium-ion batteries.

Fabrication of free -standing binary transition metal oxides, especially NiCo2O4, has attracted significant research interests since these metal oxides are promising candidates for free-standing anodes of lithium-ion batteries (LIBs). However, there remain some problems, especially low loading, for the existing NiCo2O4 anodes. To address the abovementioned issue, it will be a quite feasible solution to combine the advantages of both hierarchical micro/nano-structures and free-standing electrodes to fabricate a free-standing hierarchical micro/nano-structural NiCo2O4 electrode. Herein, we proposed an effective method to controllably synthesize hierarchical bilayered hybrid nanostructural arrays of NiCo2O4(HNAs) micro-urchins and nanowires, denoted as NiCo2O4 HNAs/NF, based on Ni foam (NF) with a high loading via a simple surfactant-assisted hydrothermal and subsequent annealing treatment. In this synthesis, NF was applied as a Ni source for NiCo2O4 without the addition of other Ni-containing reagents, and the pH value played an important role in the synthesis of NiCo2O4 HNAs/NF. Furthermore, the reasonable reaction mechanism of NiCo2O4 HNAs/NF has been discussed in detail and proposed. The as-synthesized NiCo2O4 HNAs/NF possess unique structural advantages such as a large surface area, hierarchical porous structures, and robust connection of NFs and NiCo2O4 active materials. Thus, these unique NiCo2O4 HNAs/NF display excellent electrochemical performance such as a large reversible capacity of 1094 mA h g-1 at a current density of 500 mA g-1 and a good rate capability of 875 mA h g-1 at a large 1000 mA g-1. Especially, a high loading (7 mg cm-2) of NiCo2O4 HNAs/NF, which is much higher than those of other NiCo2O4 electrodes, is beneficial towards the achievement of lightweight and miniaturized LIBs.