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Cu-doped δ-MnO 2 microspheres are rationally synthesized via a simple and facile hydrothermal process without using any template and surfactant. The samples are characterized by XRD, EDS, SEM, TEM and BET for their structure, morphology, elemental composition and specific surface area. The prepared Cu-doped δ-MnO 2 samples are further investigated as electrodes for solid state flexible supercapacitor, which is expected to deliver a high specific capacitance (300 F g −1 at scan rate of 5 mV s −1 ), a maximum energy density of 47.4 Wh kg −1 at a power density of 259 W kg −1 and long-term cycling stability (89.7% capacitance retention after 5000 times repetition at current density of 5 A g −1 ). Electrochemical impedance spectroscopy (EIS) demonstrates the low resistance nature of the prepared sample. All of the results suggest that the solid state MnO 2 device with excellent electrochemical performance and low cost has a vast potential for significant and clean power source in the future. [ABSTRACT FROM AUTHOR]