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 Ni 0.75 Mn 0.25 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 Ni 0.75 Mn 0.25 O/C solid solution. The resultant 3D‐Ni/Ni 0.75 Mn 0.25 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/Ni 0.75 Mn 0.25 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/Ni 0.75 Mn 0.25 O/C electrode for efficient energy storage applications.