Rational design of crystalline/amorphous nickel manganese phosphate octahydrate heterostructure for high-performance aqueous and all-solid-state asymmetric supercapacitors

Designing a rational architecture and combining diverse metal cations are of vital importance to obtain high-performance bimetallic compounds for energy storage purposes. Herein, a novel crystalline/amorphous nickel manganese phosphate octahydrate heterostructure grown in-situ on nickel foam was synthesized by a facile hydrothermal method followed by annealing in argon. The composite exhibits exceptional performance by adjusting the atomic ratio of redox metals (nickel and manganese) with multiple oxidation states. Specifically, the composite shows an excellent specific capacitance of 2351.6 F g−1 at a current density of 1 A g−1 and 67.3 % capacity retention after 3000 cycles at 10 A g−1. This remarkable performance can be attributed to the unique heterogeneous structure of the composite and the synergistic effect between two metals, improving the electronic conductivity. Density functional theory (DFT) calculations further confirm the reduction of band gap and conductivity enhancement. Furthermore, the crystalline portions offer not only active sites but also good mechanical support, while the amorphous components effectively promote ion diffusion and reaction. An aqueous asymmetric supercapacitor was assembled employing nickel manganese phosphate octahydrate as cathode and active carbon as anode, displaying a high energy density of 66.22 Wh kg−1 at a power density of 400 W kg−1 and outstanding cycling performance at 60 mA cm−2 (81.03 % capacitance retention after 15,000 cycles). Additionally, the fabricated all-solid-state asymmetric supercapacitor also displays a remarkable electrochemical performance.