Spectroscopic Monitoring of the Electrode Process of MnO 2 @rGO Nanospheres and Its Application in High-Performance Flexible Micro-Supercapacitors.

Structural instability is a major obstacle to realizing the high performance of a MnO 2 -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 MnO 2 @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 MnO 2 @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-MnO 2 transforms into the spinel-Mn 3 O 4 phase with K + ion intercalation. From the second charging, the spinel-Mn 3 O 4 phase is gradually adjusted to a more stable λ-MnO 2 with a three-dimensional tunnel structure, finally realizing the reversible intercalation/deintercalation of K + ions in the λ-MnO 2 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 MnO 6 octahedral unit with K + ion intercalation/deintercalation. Meanwhile, the MnO 2 @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.