Engineering 3D electron and ion transport channels by constructing sandwiched holey quaternary metal oxide nanosheets for high-performance flexible energy storage

Due to the enhanced electrochemical activities, mixed metal oxides offer new and fascinating opportunities for high-performance supercapacitor electrodes. However, sluggish ionic and electronic kinetics within the electrode fundamentally limit further improvement of their electrochemical performance. To compensate for the deficiency, a flexible electrode (CNTF/Ni-Co-Mn-Mo NS/CNTN) composed of vertically-aligned areolate quaternary metal oxide nanosheets sandwiched between carbon nanotubes (CNTs) is constructed in this study, which demonstrates a unique hierarchical porous structure that can provide three-dimensional transport channels for both ions and electrons. The vertically aligned areolate quaternary metal oxide nanosheets enable increased exposed surface area and paths for ion transport, diffusion and redox reactions, resulting in an evident enhancement in electrochemical activities. Besides, the CNT networks provide improved conductivity, which can accelerate the electron transport. As a result, the flexible supercapacitor based on the CNTF/Ni-Co-Mn-Mo NS/CNTN electrode demonstrates a specific areal capacitance of 3738 mF cm−2, corresponding to a high energy density of 1.17 mW h cm−2, which outperforms most of the flexible devices reported recently. Additionally, excellent flexibility of up to 180° bend and superior performance stability of 87.87% capacitance retention after 10,000 charge-discharge cycles can be obtained. This unique design opens up a new way in the development of flexible energy storage devices with high performance.