Fluorination of Mxene by Elemental F2 as Electrode Material for Lithium-Ion Batteries.

Herein we show, for the first time, the transformation of MXene sheets into TiOF2 2D sheets with superior electrochemical performance. MXene synthesized from Ti3AlC2 has been fluorinated for 3-, 6-, and 24 hours, respectively, by means of a direct fluorination process. Exposure of MXene powder to elemental fluorine for 3 hours induces the formation of CF2 groups and TiF3 on the surface, which have beneficial effects on the electrochemical performance. X-ray photoelectron spectroscopy suggests that after fluorinating the MXene sample for 6 hours, that 2+ and 3+ Ti oxidation states are not present on the surface, with only Ti4+ present indicating TiOF2. X-ray diffraction indicates that there is TiOF2 present after fluorinating for 3 hours and after 24 hours the MXene has transformed to TiOF2 with minor impurities remaining, while maintaining the 2D layer morphology. The 24 hours fluorinated sample with its TiOF2 phase has superior capacity that increases with cycle number. It also has a better rate capability as compared to a non 2D layered TiOF2, indicating the advantage of the 2D layered morphology derived from the parent MXene phase.