Functionalized M2TiC2Tx MXenes (M=Cr and Mo; T = O, F, and OH) as High Performance Electrode Materials for Sodium Ion Batteries

First-principles calculations were performed to study the electrochemical performance of M2TiC2 (M= Cr or Mo) and M2TiC2Tx (T = O, F or OH) used as anode materials for sodium ion batteries (SIBs) at 0 K. The geometrical optimization results show that the Cr2TiC2O2 and Mo2TiC2O2 own more stable structure than F and OH terminated MXenes. The calculated electronic structures reveal that the Cr2TiC2O2 and Mo2TiC2O2 show metallic characteristic, and therefore they own high electronic conductivity. The main factor affecting the diffusion performance of Na in MXenes materials is the T groups. The lowest energy barriers of functionalized MXenes are about one order larger in magnitude than that of bare MXenes. Although the introduction of O-groups hinders the diffusion of Na, it greatly improves the theoretical Na storage capacities. Meanwhile, the concentration of Na affects the diffusion paths and diffusion energy barriers of Na and the interaction between two terminations has little effect on the diffusion barriers of Na. The enlarged interlayer distance of M2TiC2O2 monolayers allows them to adsorb multiple layers of sodium atoms stably, thus significantly increasing their theoretical capacities. The theoretical sodium storage capacities of M2TiC2O2 monolayers could reach to 515.70 mA h g-1 (M=Cr) and 362.46 mA h g-1 (M=Mo) and the OCVs have approached to 0.034 V (M=Cr) and 0.042 V (M=Mo). The Cr2TiC2O2 and Mo2TiC2O2 monolayers are expected to be promising anode materials for SIBs due to their excellent properties, such as good electronic conductivity, low sodium diffusion barrier, and high theoretical sodium storage capacity.