Butyllithium-Treated TiCT MXene with Excellent Pseudocapacitor Performance.

MXenes, a family of two-dimensional (2D) transition-metal carbide and nitride materials, are supposed to be promising pseudocapacitive materials because of their high electronic conductivity and hydrophilic surfaces. MXenes, prepared by removing the "A" elements of their corresponding MAX phases by hydrofluoric acid (HF) or LiF-HCl etching, possess abundant terminal groups like -F, -OH, and -O groups. It has been proven that the MXenes with fewer -F terminal groups and more -O groups showed a higher pseudocapacitor performance. In organic reactions, -OH and -X (X = halogen) groups could turn to ether groups in strong nucleophilic reagent. Inspired by that, herein, we report an -butyllithium-treated method to turn the -F and -OH terminal groups to -O groups on the TiCT MXenes. Two types of TiCT MXenes prepared by either HF or LiF-HCl etching were systematically investigated, and a comparison with the traditional KOH/NaOH/LiOH-treated method was also carried out. It is found that most of the -F terminal groups on the TiCT MXenes can be successfully removed by -butyllithium, and abundant -O terminal groups were formed. The -butyllithium-treated TiCT MXenes show promising applications in high-performance pseudocapacitors. A record high capacitance of 523 F g at 2 mV s was obtained for the -butyllithium-treated TiCT MXenes, and 96% capacity can remain even after 10 000 cycles.