High surface area of crystalline/amorphous ultrathin MnO2 nanosheets electrode for high-performance flexible micro-supercapacitors

Herein, ultrathin MnO2 nanosheets consisting of crystalline/amorphous phases are prepared by a simple interfacial reaction of surfactant assistance in water-bath with high specific surface area of 317.56 m2 g−1, which can accelerate ionic transport and provide more active sites during electrochemical reaction. The as-synthesized MnO2 nanosheets electrode exhibits remarkable specific capacitance of 163.3 F g−1 at 1 A g−1 in a three-electrode system using 1 M NaSO4 as electrolyte, and long-term stability with a capacity retention of 89.3% at 3 A g−1 after 10,000 cycles, which can be ascribed to a structural transition from crystalline to amorphous phase during cycling. Meanwhile, the flexible all-solid-state MnO2 symmetry micro-supercapacitors (MSCs) using sodium alginate bio-hydrogel as electrolyte, the voltage window of the MSCs can be extended to 1.4 V. The MSCs reveals a high areal capacitance of 129.29 mF cm−2 at a current density of 0.2 mA cm−2. The capacity retention rate up to 94.5% after 4000 cycles, and higher energy density of 35.19 μWh cm−2 at 0.07 mW cm−2, as well as there is no significant capacitance degradation under different bending states, exhibiting a remarkable electrochemical performance and mechanical flexibility. The crystalline/amorphous ultrathin MnO2 sheets with high specific surface area were prepared through simple preparation process and found the mechanism of the transformation of MnO2 from crystalline phase to amorphous phase during the cycle providing a new perspective for the design of high-performance MnO2 electrodes.