Structural instability is a major obstacle to realizing the high performance of a MnO2-based pseudocapacitor material. Understanding its structure transformation in the process of electrochemical reaction, therefore, plays an important role in the efficient enhancement of rate capacity and stability. Herein, a stable MnO2@rGO core-shell nanosphere is first synthesized by a liquid-liquid interface deposition further combined with the electrostatic self-assembly method. The structural transformation process of the MnO2@rGO electrode is monitored by ex situ Raman and X-ray diffraction spectroscopy during the charging-discharging process. It is found in the first discharging process that layered-MnO2 transforms into the spinel-Mn3O4 phase with K+ ion intercalation. From the second charging, the spinel-Mn3O4 phase is gradually adjusted to a more stable λ-MnO2 with a three-dimensional tunnel structure, finally realizing the reversible intercalation/deintercalation of K+ ions in the λ-MnO2 tunnel structure during subsequent cycling, which can be attributed to the presence of oxygen vacancies formed by the lengthening of the Mn-O bond and losing oxygen in the MnO6 octahedral unit with K+ ion intercalation/deintercalation. Meanwhile, the MnO2@rGO electrode demonstrates a high specific capacitance of 378 F g-1 at 1 A g-1 and excellent cycling stability with a capacitance retention of up to 89.5% after 10 000 cycles at 10 A g-1. Furthermore, the assembled symmetric micro-supercapacitor delivers a high areal energy density of 1.01 μWh cm-2, superior cycling stability with no significant capacity decay after 8700 cycles, and a capacity retention rate of almost 100% after 2000 bending cycles, showing great mechanical flexibility and practicability.
Keywords: MnO2@rGO nanosphere; energy storage mechanism; liquid−liquid interface; micro-supercapacitors; structural transformation.