Protective hydrothermal treatment to improve ion pathway in Ti3C2Tx MXene for high-performance flexible supercapacitors

The strong re-stacking of Ti₃C₂T_x MXene nanosheets severely blocks ion transport pathway and sacrifices electrochemical performance. Here, a protective hydrothermal treatment is used to improve ion pathway in Ti₃C₂T_x MXene. The protective hydrothermal treatment is simply conducted in the hydrothermal kettle (filled with N₂ atmosphere) placed in vacuum oven. The optimal Ti₃C₂T_x is achieved by the hydrothermal treatment of 150 °C for 12h, and recorded as h-Ti₃C₂T_x@150 °C/12h. The h-Ti₃C₂T_x@150 °C/12h film electrode achieves significantly improved ion transport pathways and enhanced ion accessibility with more pseudocapacitive active sites due to the increased interlayer spacing and pores as well as decreased flake size. According to DFT calculations, a small number of oxides (TiO₂ nanoparticles) produced during the hydrothermal treatment also facilitate to increase the capacitive performance of h-Ti₃C₂T_x@150 °C/12h. Therefore, the h-Ti₃C₂T_x@150 °C/12h film electrode achieves significantly increased capacitance and rate performance. The h-Ti₃C₂T_x@150 °C/12h film electrode exhibits excellent capacitance (498.3 F g⁻¹ and 1911 F cm⁻³ at 1 A g⁻¹) and rate performance (63 % retention from 1 to 20 A g⁻¹) with high cycling stability (98.2 % retention after 20 000 cycles), which are among the best electrochemical performances of undoped Ti₃C₂T_x based film electrodes reported so far. Thick h-Ti₃C₂T_x@150 °C/12h film electrode of 33.1 μm thickness exhibits an ultra-high areal capacitance of 3.23 F cm⁻² at 1 A g⁻¹. Moreover, the h-Ti₃C₂T_x@150 °C/12h-based flexible symmetric supercapacitor device exhibits excellent energy storage performance (117 F g⁻¹ at 0.5 A g⁻¹ and 23.4 Wh kg⁻¹ at 299.8 W kg⁻¹) with high cycling stability (84 % retention after 3000 cycles) and bending stability, outperforming most of previously reported Ti₃C₂T_x-based flexible supercapacitors. The impressive results indicate the great application potential of the h-Ti₃C₂T_x@150 °C/12h film in flexible energy storage devices.