Oxygen-vacancy-rich NiCo2O4 nanoneedles electrode with poor crystallinity for high energy density all-solid-state symmetric supercapacitors

Cubic spinel phase NiCo2O4 nanoneedles with poor crystallinity are grown successfully on Ni foam (NF) substrate through annealing the precursor obtained via a facile hydrothermal approach. The poor crystallinity can promote the generation of oxygen vacancy defects, which lead to improve the electrical conductivity of NiCo2O4. As electrode of supercapacitors, NiCo2O4/NF exhibits an excellent specific capacitance of 1076 F g(-1) at a current density of 0.5 A g(-1) and better cyclic stability which the specific capacity is similar to 14% higher than that of the beginning after 1000 cycles at current density of 10 A g(-1). The all-solid-state symmetric supercapacitor (SSC) based on NiCo2O4//NiCo2O4 electrodes delivers a high energy density of 30.5 W h kg(-1) at a power density of 750 W k g(-1). The results reveal that the poor crystallinity and the presence of oxygen vacancies are the important factors for improving the charge storage ability of NiCo2O4 electrode materials.