Hierarchical self-assembly flower-like ammonium nickel phosphate as high-rate performance electrode material for asymmetric supercapacitors with enhanced energy density

Ammonium nickel phosphate has a large specific capacitance as an electrode material at low current density, but its capacitance decays fast at high current density, which directly affects the rate performance of supercapacitors. Herein, we demonstrate a facile route for the controllable synthesis of hierarchical self-assembly flower-like ammonium nickel phosphate as a high-rate electrode material for asymmetric supercapacitors, which is an important strategy to enhance the energy density at high power density. The flower-like structures are hierarchically assembled by a mass of rectangular sheets, which can provide fast electron transport and short ion diffusion path, thereby exhibiting excellent electrochemical performance with ultrahigh specific capacitance of 1016 F g-1 at 1.0 A g-1. More importantly, the NH4NiPO4 · H2O materials exhibit outstanding rate performance (800 F g-1 even at large current density of 30 A g-1) and superior long-term cycle life (83% of capacity retention up to 3000 cycles at 5 A g-1). Furthermore, the NH4NiPO4 · H2O//AC asymmetric supercapacitors are assembled in aqueous KOH electrolyte, and exhibit high energy density (46.2 Wh kg-1 at 160 W kg-1 and 26.7 Wh kg-1 at a large power density of 4000 W kg-1, respectively). Due to the outstanding electrochemical performance, the all-solid-state asymmetric supercapacitors are successfully constructed using these materials.