Atomic-Scale Recognition of Surface Structure and Intercalation Mechanism of Ti₃C₂X

Abstract

MXenes represent a large family of functionalized two-dimensional (2D) transition-metal carbides and carbonitrides. However, most of the understanding on their unique structures and applications stops at the theoretical suggestion and lack of experimental support. Herein, the surface structure and intercalation chemistry of Ti₃C₂X are clarified at the atomic scale by aberration-corrected scanning transmission electron microscope (STEM) and density functional theory (DFT) calculations. The STEM studies show that the functional groups (e.g., OH^–, F^–, O^–) and the intercalated sodium (Na) ions prefer to stay on the top sites of the centro-Ti atoms and the C atoms of the Ti₃C₂ monolayer, respectively. Double Na-atomic layers are found within the Ti₃C₂X interlayer upon extensive Na intercalation via two-phase transition and solid-solution reactions. In addition, aluminum (Al)-ion intercalation leads to horizontal sliding of the Ti₃C₂X monolayer. On the basis of these observations, the previous monolayer surface model of Ti₃C₂X is modified. DFT calculations using the new modeling help to understand more about their physical and chemical properties. These findings enrich the understanding of the MXenes and shed light on future material design and applications. Moreover, the Ti₃C₂X exhibits prominent rate performance and long-term cycling stability as an anode material for Na-ion batteries