Two-Step Hydrothermal\nSynthesis of Ni<sub>3</sub>(NO<sub>3</sub>)<sub>2</sub>(OH)<sub>4</sub>@MnO<sub>2</sub> Heterojunction\nSupported α‑MnO<sub>2</sub> Material on Foam Nickel

Enhancing the performance of electrode materials is an\neffective\nstrategy for increasing the energy density, power density, and lifespan\nof supercapacitors. In this study, a one-step hydrothermal method\nis employed to fabricate a large-sized interlaced lamellar structure\nwith Ni<sub>3</sub>(NO<sub>3</sub>)<sub>2</sub>(OH)<sub>4</sub>@MnO<sub>2</sub> (NNM-2) heterojunctions loaded onto nickel foam. Then, a\nuniform layer of α-MnO<sub>2</sub> nanosheets is deposited to\ncreate a hierarchical structure of Ni<sub>3</sub>(NO<sub>3</sub>)<sub>2</sub>(OH)<sub>4</sub>@MnO<sub>2</sub>/α-MnO<sub>2</sub> (NNMM-2)\nvia a subsequent hydrothermal process. The interaction between various\ncomponents, along with an increased number of active sites, significantly\nimproves the electrochemical performance of the electrode material.\nFurthermore, the designed core–shell structure helps alleviate\nvolume changes during charge and discharge cycles, thereby improving\nthe stability of the material. Consequently, NNMM-2 demonstrates an\nimpressive specific capacitance of 1261.3 F g<sup>–1</sup> when\nmeasured at a current density of 1 A g<sup>–1</sup>. When employed\nas the positive electrode in an asymmetric supercapacitor that features\nactivated carbon as the negative electrode, it demonstrates an energy\ndensity of 36.5 W h kg<sup>–1</sup> at a power density of 471.7\nW kg<sup>–1</sup>. After 7000 charge–discharge cycles,\nthe capacity retention rate remains at 73% with a Coulombic efficiency\nof 99%, demonstrating excellent stability and capacitance retention\ncapability. This research offers important perspectives on the creation\nof enhanced electrode materials for energy storage devices with superior\nperformance.