Hierarchical crumpled NiMn2O4@MXene composites for high rate ion transport electrochemical supercapacitors.

MXenes have received great attention due to their excellent features such as metal-like electronic conductivity, hydrophilic surface groups, and high volumetric capacitance. However, many performances of MXenes are still unsatisfactory due to their low energy density and easy horizontal stacking. In this work, an NiMn2O4@MXene composite with a crumpled surface was fabricated by a hydrothermal method and a developed dip-coating method. The maximum specific capacitance of the electrode is about 1.52 times that of NiMn2O4. Besides, it delivers a retention rate of 93.3% after 4000 cycles due to the increased transport of ions and electrons by the crumpled surface. An asymmetrical device based on the crumpled NiMn2O4@MXene composite and AC was also assembled, which shows an ultra-high energy density. This work provides an effective strategy to solve the vertical stacking problem of MXenes, which can open new avenues for large-scale applications of MXenes in energy storage.