A novel hierarchical core-shell structure of NiCo2O4@NiCo-LDH nanoarrays for higher-performance flexible all-solid-state supercapacitor electrode materials

In the present work a 3D hierarchical NiCo2O4@NiCo-LDH core-shell structure has been fabricated on activated carbon cloth through a facile hydrothermal strategy and an electrodeposition step. The core of the NiCo2O4 nanowires forms the nanoarray backbone on carbon cloth while the shell of NiCo-LDH nanosheets extends the electrochemical active sites and generates mesoporous hierarchical channels to facilitate electron and ion transport. Therefore, with the optimized mass ratio of 1.9 between NiCo-LDH and NiCo2O4, such a core-shell structure enhanced the areal capacitance of the as-prepared NiCo2O4@NiCo-LDH electrode up to 6092 mF/cm2 (or areal capacity of 0.846 mAh/cm2 at 2 mA/cm2 for the battery type pseudocapacitor) and obtain high specific capacitance of 1882 F/g under 1 A/cm2. The capacity retention rate can maintain 83.9 % after 5000 cycles at 30 mA/cm2. Furthermore, an all-solid-state asymmetric supercapacitor (ASC) was assembled using NiCo2O4@NiCo-LDH//activated carbon. Such an ASC exhibits a very high energy density of 49 Wh/kg at the power density of 750 W/kg and retains 30 Wh/kg at 7500 W/kg. The ASC shows its excellent cycling stability by keeping 83.3 % of initial capacity retention rate after 2000 cycles under 5 A/g. The assembled flexible all-solid-state supercapacitor with NiCo2O4@NiCo-LDH//activated carbon exhibits good mechanical stability and high specific capacity of 157 F/g under 1 A/g. The significantly improved electrochemical properties demonstrates the practical applications of the core-shell structure of NiCo2O4@NiCo-LDH on carbon cloth for novel ASC configuration design and development of next energy storage devices.