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

The transformation of MXene sheets into TiOF₂ 2D sheets with superior electrochemical performance was developed. MXene synthesized from Ti₃ AlC₂ was fluorinated for 3, 6, and 24 h, respectively, by means of a direct fluorination process. Exposure of MXene powder to elemental fluorine for 3 h induced the formation of CF₂ groups and TiF₃ on the surface, which have beneficial effects on the electrochemical performance. X-ray photoelectron spectroscopy suggested that after fluorinating the MXene sample for 6 h Ti²⁺ and Ti³⁺ were not present on the surface but only Ti⁴⁺ , indicating the formation of TiOF₂ . XRD indicated that TiOF₂ was present after fluorinating for 3 h, and after 24 h the MXene had transformed to TiOF₂ with minor impurities remaining, maintaining its 2D layer morphology. The 24 h fluorinated sample with its TiOF₂ phase showed superior capacity that increased with cycle number. It also had a better rate capability than non-2D-layered TiOF₂ , indicating the advantage of the 2D-layered morphology derived from the parent MXene phase.