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.

Electrochemical and in-situ X-ray diffraction studies of Ti 3 C 2 T x MXene in ionic liquid electrolyte

“…However, surface functional groups (O or F) and Ti are not supposed to be electrochemically active in aprotic ionic liquids. Intercalation of cations between MXene layers from aqueous electrolytes was observed by insitu XRD [6], and similar intercalation behavior was reported for MXene battery electrodes [18,19]. It is reasonable to assume that intercalation may occur in ionic liquid electrolytes as well.…”

Emerging nanostructured electrode materials for water electrolysis and rechargeable beyond Li-ion batteries

“…The mechanism of sodium intercalation into Ti 3 C 2 T x electrode was investigated using aberration-corrected scanning transmission electron microscopy [153] and solid-state 23 Na magic angle spinning NMR spectroscopy [154]. The atomic scale study by STEM showed that double layer of Na + ions can accommodate within the Ti 3 C 2 T x interlayer space upon deep discharge down to 0.0 V vs. Na/Na + ( Figure 14(e) and (f)).…”

“…Ti 3 C 2 T x electrode exhibited a reversible capacity of ≈200 mAh g −1 in Na-ion cells for 200 cycles. This high performance was attributed to the increased number of intercalation sites and large interlayer spacing of 9.8 Å which allow for the double-layer intercalation mechanism [153]. Solid-state NMR and XRD analyses demonstrated that the interlayer distance expands in the first sodiation process by 2.8 Å [154].…”

Transmission Electron Microscopy of 2D Materials: Structure and Surface Properties

“…The unit cell of MXene Ti 3 C 2 has lattice constants a and b = 3.05Å, while the lattice constant c often describes the height of two adjacent sheets, see Figure 3.1d-f. The lattice constant c is 19.86Å for ideal Ti 3 C 2 MXene [122], while surface groups and intercalants expand the separation between sheets and increase the lattice constant between 0.18-9.8Å [114,127]. Apart from changing the separation between sheets, these surface groups and intercalants also influence the properties of MXene.…”