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.

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