First-Principles Study of Transition Metal Ti-Based MXenes (Ti2MC2Tx and M2TiC2Tx) 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 Ti 2 MC 2 O 2 and M 2 TiC 2 O 2 (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 Ti 2 VC 2 O 2, Ti 2 YC 2 O 2, Ti 2 ZrC 2 O 2, Ti 2 NbC 2 O 2, V 2 TiC 2 O 2, Cr 2 TiC 2 O 2, Zr 2 TiC 2 O 2, and Nb 2 TiC 2 O 2 compared with common Ti 3 C 2 MXene. Ti 2 ZrC 2 O 2, Zr 2 TiC 2 O 2, and Nb 2 TiC 2 O 2 bimetallic MXenes possess a stronger ability to adsorb Na and lower Na diffusion barriers than Ti 3 C 2 MXene. Moreover, the OCV values of Ti 2 ZrC 2 O 2 and Zr 2 TiC 2 O 2 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 Ti 3 C 2 MXene. Furthermore, Zr in Zr 2 TiC 2 O 2 has a stronger adsorption capacity for Na, and the interaction of Zr atoms with O is also beneficial. Zr 2 TiC 2 T x MXene is expected to be prepared through experiments accompanying a larger capacity (Zr 2 TiC 2 O 2 Na 4 ), 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.