We present a comparative study on the static and dynamical properties of bare Ti3C2 and T-terminated Ti3C2T2 (T = O, F, OH) monosheets using density functional theory calculations. First, the crystal structures are optimized to be of trigonal configurations (P3[combining macron]m1), which are thermodynamically and dynamically stable. It is demonstrated that the terminations modulate the crystal structures through valence electron density redistribution of the atoms, particularly surface Ti (Ti2) in the monosheets. Second, lattice dynamical properties including phonon dispersion and partial density of states (PDOS) are investigated. Phonon PDOS analysis shows a clear collaborative feature in the vibrations, reflecting the covalent nature of corresponding bonds in the monosheets. In the bare Ti3C2 monosheet, there is a phonon band gap between 400 and 500 cm(-1), while it disappears in Ti3C2O2 and Ti3C2(OH)2 as the vibrations associated with the terminal atoms (O and OH) bridge the gap. Third, both Raman (Eg and A1g) and infrared-active (Eu and A2u) vibrational modes are predicted and conclusively assigned. A comparative study indicates that the terminal atoms remarkably influence the vibrational frequencies. Generally, the terminal atoms weaken the vibrations in which surface Ti atoms are involved while strengthening the out-of-plane vibration of C atoms. Temperature-dependent micro Raman measurements agree with the theoretical prediction if the complexity in the experimentally obtained lamellae for the Raman study is taken into account.

Sodium and potassium ion batteries hold promise for next-generation energy storage systems due to their rich abundance and low cost, but are facing great challenges in optimum electrode materials for actual applications. Here, ultrathin nanoribbons of sodium titanate (M-NTO, NaTiO) and potassium titanate (M-KTO, KTiO) were successfully synthesized by a simultaneous oxidation and alkalization process of TiC MXene. Benefiting from the suitable interlayer spacing (0.90 nm for M-NTO, 0.93 nm for M-KTO), ultrathin thickness (<11 nm), narrow widths of nanoribbons (<60 nm), and open macroporous structures for enhanced ion insertion/extraction kinetics, the resulting M-NTO exhibited a large reversible capacity of 191 mAh g at 200 mA g for sodium storage, higher than those of pristine TiC (178 mAh g) and commercial TiC derivatives (86 mAh g). Notably, M-KTO displayed a superior reversible capacity of 151 mAh g at 50 mA g and 88 mAh g at a high rate of 300 mA g and long-term stable cyclability over 900 times, which outperforms other Ti-based layered materials reported to date. Moreover, this strategy is facile and highly flexible and can be extended for preparing a large number of MXene-derived materials, from the 60+ group of MAX phases, for various applications such as supercapacitors, batteries, and electrocatalysts.

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MXenes represent an emerging family of conductive two-dimensional materials. Their representative, TiCT, has been recognized as an outstanding member in the field of electrochemical energy storage. However, an in-depth understanding of fundamental processes responsible for the superior capacitance of TiCT MXene in acidic electrolytes is lacking. Here, to understand the mechanism of capacitance in TiCT MXene, we studied electrochemically the charge/discharge processes of TiCT electrodes in sulfate ion-containing aqueous electrolytes with three different cations, coupled with in situ Raman spectroscopy. It is demonstrated that hydronium in the HSO electrolyte bonds with the terminal O in the negative electrode upon discharging while debonding occurs upon charging. Correspondingly, the reversible bonding/debonding changes the valence state of Ti element in the MXene, giving rise to the pseudocapacitance in the acidic electrolyte. In stark contrast, only electric double layer capacitance is recognized in the other electrolytes of (NH)SO or MgSO. The charge storage ways also differ: ion exchange dominates in HSO, while counterion adsorption in the rest. Hydronium that is characterized by smaller hydration radius and less charge is the most mobile among the three cations, facilitating it more kinetically accommodated on the deep adsorption sites between the MXene layers. The two key factors, i.e., surface functional group-involved bonding/debonding-induced pseudocapacitance, and ion exchange-featured charge storage, simultaneously contribute to the superior capacitance of TiCT MXene in acidic electrolytes.

This review provides a comprehensive understanding of the emerging 2D MXene electrode materials for supercapacitor application.