Snowflake-like core-shell α-MnO2@δ-MnO2 for high performance asymmetric supercapacitor

Novel three-dimensional (3D) snowflake-like α-MnO2@δ-MnO2 (3D-MnO2), with hierarchical core-shell nanostructures, was synthesized by a facile hydrothermal route and chemical bath deposition method. The 3D-MnO2 can improve the defects of single-phase MnO2 as electrode materials. The microstructure and chemical composition of as-prepared samples were characterized by a series of means, such as XRD, SEM, TEM, HRTEM, XPS and BET. The morphology of α-MnO2, δ-MnO2 and 3D-MnO2 were 3D star-like nanowires, tremella-like nanosheets and 3D snowflake-like hierarchical nanostructures, respectively. From its unique interconnected hierarchical structure, the 3D-MnO2 possessed the highest specific capacitance (Cm) (260.5 F g−1 at a current density of 0.3 A g−1, about 2.7 times of α-MnO2 and 1.5 times of δ-MnO2), good rate performance and excellent cycling stability (94.7% capacitance retention after 3000 cycles). Furthermore, an asymmetric supercapacitor (3D-MnO2//AC) was successfully assembled, 3D-MnO2 as positive electrode and activated carbon (AC) as negative electrode. Meanwhile, the optimized 3D-MnO2//AC device with a voltage window of 2.0 V delivered a maximum energy density of 36.6 Wh kg−1 at a power density of 0.3 kW kg−1.