Oxygen vacancy inducing phase transition during charge storage in MnOx@rGO supercapacitor electrode

Herein, in-situ Raman and electrochemical quartz crystal microbalance techniques combined with density functional theory (DFT) calculations are adopted to prove the charge storage mechanism for high-stability MnOx@graphene microspheres electrode in alkaline electrolyte (KOH). The results reveal that the layered MnO2 phase gradually transformed into the spinel Mn3O4 phase in the process of discharging, accompanied by the insertion of potassium ions. Moreover, the Mn-O bonds length near the potassium ions in [MnO6] octahedral structure becomes longer, while the long unstable Mn-O bonds tend to lose oxygen and create oxygen vacancies, which further transforms the layered structure into stable spinel structure. Furthermore, it is also found that potassium ions can be more easily embedded in the layered MnO2 with oxygen vacancies due to the lower energy demand compared with the pure MnO2. Meanwhile, the MnOx@graphene microspheres electrode exhibits a capacity retention as high as 90% after 10,000 cycles at 10 A g−1, indicating a superior cyclic stability.