High mass loading NiCo2O4 with shell-nanosheet/core-nanocage hierarchical structure for high-rate solid-state hybrid supercapacitors

Abstract Rational design of advanced structure for transition metal oxides (TMOs) is attractive for achieving high-performance supercapacitors. However, it is hampered by sluggish reaction kinetics, low mass loading, and volume change upon cycling. Herein, hierarchical NiCo2O4 architectures with 2D-nanosheets-shell and 3D-nanocages-core (2D/3D h-NCO) are directly assembled on nickel foam via a facile one-step way. The 2D nanosheets are in-situ generated from the self-evolution of initial NCO nanospheres. This 2D/3D hierarchical structures ensure fast ion/electron transport and maintain the structural integrity to buffer the volume expansion. The 2D/3D h-NCO electrode with an ultrahigh mass loading (30 mg cm-2) achieves a high areal capacity of 4.65 C cm-2 (equivalent to1.29 mAh cm-2) at a current density of 4 mA cm-2, and retains 3.7 C cm-2 even at 50 mA cm-2. Furthermore, the assembled solid-state hybrid supercapacitor yields a high volumetric energy density of 4.25 mWh cm-3 at a power density of 39.3 mW cm-3, with a high capacity retention of 92.4% after 5000 cycles. Therefore, this work provides a new insight to constuct hierarchical electrodes for energy storage application.