Vacuum annealed MnO2 ultra-thin nanosheets with oxygen defects for high performance supercapacitors

Defect engineering is one of the main ways to adjust the internal structure and electrochemical performance of materials. Therefore, we prepared MnO2 ultra-thin nanosheets loading on Ni foam as a supercapacitor electrode through hydrothermal process and vacuum annealing, which not only obtained oxygen defects on the surface, but also led to a shift in the valence of Mn4+ to Mn3+. The results indicate that the optimal electrode can achieve high specific capacitance of 522.5 F/g at a current density of 1 A/g and excellent cycling performance of 95.24% at 5000 cycles. Then, we assembled an asymmetric supercapacitor using the sample annealed for 45 min as the positive electrode and activated carbon as the negative electrode in the 1 M Na2SO4 electrolyte to investigate the practical application. The asymmetric supercapacitor with a large voltage window of 0-2 V and an energy density of 18.06 Wh/kg at a power density of 1 kW/kg with a promising electrochemical stability was assembled. This article provides a simple method to improve electrochemical performance of transition metal oxide electrodes in supercapacitors by introducing defects.