Spectroscopic Monitoring of the Electrode Process of MnO₂@rGO Nanospheres and Its Application in High-Performance Flexible Micro-Supercapacitors

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.