MnO₂ nanomaterials for flexible supercapacitors: performance enhancement via intrinsic and extrinsic modification

Increasing power and energy demands for next-generation portable and flexible electronics have raised critical requirements (flexibility, stretch-ability, environmental friendliness, lightweight, etc.) for the energy storage devices. Flexible supercapacitors (SCs), as one of the most promising next-generation energy storage devices, have stimulated intensive interest owing to their outstanding features including small size, low weight, ease of handling, excellent reliability, and high power density. Manganese oxide (MnO₂), has attracted much interest in the development of flexible SCs with high electrochemical performance. Yet, the poor electronic and ionic transport in MnO₂ electrodes still limits its promotion in practical applications. This review aims to describe the recent progress in the application of MnO₂ materials in the development of flexible SCs and summarizes the intrinsic modification of MnO₂via crystallinity, crystal structure, and oxygen vacancy introduction and the extrinsic modification of MnO₂via non-three-dimensional (3D) and 3D flexible conductive scaffolds for high performance flexible SCs. Moreover, we also discuss briefly on the current challenges, future directions, and opportunities for the development of high-performance MnO₂ based flexible SCs.