In Situ Growth of Hierarchical Ni-Mn-O Solid Solution on a Flexible and Porous Ni Electrode for High-Performance All-Solid-State Asymmetric Supercapacitors.

To endow all-solid-state asymmetric supercapacitors with high energy density, cycling stability, and flexibility, we design a binder-free supercapacitor electrode by in situ growth of well-distributed broccoli-like Ni0.75 Mn0.25 O/C solid solution arrays on a flexible and three-dimensional Ni current collector (3D-Ni). The electrode consists of a bottom layer of compressed but still porous Ni foam with excellent flexibility and high electrical conductivity, an intermediate layer of interconnected Ni nanoparticles providing a large specific surface area for loading of active substances, and a top layer of vertically aligned mesoporous nanosheets of a Ni0.75 Mn0.25 O/C solid solution. The resultant 3D-Ni/Ni0.75 Mn0.25 O/C cathode exhibits a specific capacitance of 1657.6 mF cm-2 at 1 mA cm-2 and shows no degradation of the capacitance after 10 000 cycles at 3 mA cm-2 . The assembled 3D-Ni/Ni0.75 Mn0.25 O/C//activated carbon asymmetric supercapacitor has a high specific capacitance of 797.7 mF cm-2 at 2 mA cm-2 and an excellent cycling stability with 85.3 % of capacitance retention after 10 000 cycles at a current density of 3 mA cm-2 . The energy density and power density of the asymmetric supercapacitor are up to 6.6 mW h cm-3 and 40.8 mW cm-3 , respectively, indicating a fairly promising future of the flexible 3D-Ni/Ni0.75 Mn0.25 O/C electrode for efficient energy storage applications.