Interlayer Structure Engineering of MXene-Based Capacitor-Type Electrode for Hybrid Micro-Supercapacitor toward Battery-Level Energy Density.

Micro-supercapacitors are notorious for their low energy densities compared to micro-batteries. While MXenes have been identified as promising capacitor-type electrode materials for alternative zinc-ion hybrid micro-supercapacitors (ZHMSCs) with higher energy density, their tightly spaced layered structure renders multivalent zinc-ions with large radii intercalation inefficient. Herein, through insertion of 1D core-shell conductive BC@PPy nanofibers between MXene nanosheets, an interlayer structure engineering technique for MXene/BC@PPy capacitor-type electrodes towards ZHMSCs is presented. Owing to simultaneously achieving two objectives: (i) widening the interlayer space and (ii) providing conductive connections between the loose MXene layers, enabled by the conductive BC@PPy nanospacer, the approach effectively enhances both ion and electron transport within the layered MXene structure, significantly increasing the areal capacitance of the MXene/BC@PPy film electrode to 388 mF cm-2 , which is a 10-fold improvement from the pure MXene film electrode. Pairing with CNTs/MnO2 battery-type electrodes, the obtained ZHMSCs exhibit an areal energy density up to 145.4 μWh cm-2 with an outstanding 95.8% capacity retention after 25000 cycles, which is the highest among recently reported MXene-based MSCs and approaches the level of micro-batteries. The interlayer structure engineering demonstrated in the MXene-based capacitor-type electrode provides a rational means to achieve battery-levelenergy density in the ZHMSCs.