First-Principles Study of Transition Metal Ti-Based MXenes (Ti₂MC₂T_x and M₂TiC₂T_x) as Anode Materials for Sodium-Ion Batteries

To promote the application of two-dimensional transition metal MXenes in Na-ion batteries, it is essential to expand the MXene family and figure out the mechanism. Density functional theory is employed to predict the performance of different subgroups of transition metal Ti-based MXenes Ti2MC2O2 and M2TiC2O2 (M represents early transition metals from the IIIB–VIB group including V, Cr, Y, Zr, and Nb) as anode materials of Na-ion batteries. Among them, we focused on eight stable MXenes, including Ti2VC2O2, Ti2YC2O2, Ti2ZrC2O2, Ti2NbC2O2, V2TiC2O2, Cr2TiC2O2, Zr2TiC2O2, and Nb2TiC2O2 compared with common Ti3C2 MXene. Ti2ZrC2O2, Zr2TiC2O2, and Nb2TiC2O2 bimetallic MXenes possess a stronger ability to adsorb Na and lower Na diffusion barriers than Ti3C2 MXene. Moreover, the OCV values of Ti2ZrC2O2 and Zr2TiC2O2 MXenes are unique: the value decreases as the number of Na decreases at low Na coverage during discharge, implying that the assembled batteries have a higher energy density than those from Ti3C2 MXene. Furthermore, Zr in Zr2TiC2O2 has a stronger adsorption capacity for Na, and the interaction of Zr atoms with O is also beneficial. Zr2TiC2Tx MXene is expected to be prepared through experiments accompanying a larger capacity (Zr2TiC2O2Na4), better rate performance (diffusion barrier with 0.101 eV), and lower OCV (about 1 V). This finding can provide a reference for future theoretical studies and material selection in experiments on bimetallic MXenes used for Na-ion batteries.