Bimetallic Ti ₂ NbC ₂ MXene as anode material for metal ion batteries: influence of functional groups

By DFT calculations, we investigate the effect of functional groups on the electrochemical activity of the Ti₂NbC₂ MXene as an anode in alkali and alkaline earth batteries. Our findings show that the H3 site is the most favorable adsorption site for O, Cl, F, and OH surface functionalization. The electronic properties of functionalized MXenes are investigated, finding a metallic behavior in all cases. The Li, Na, K, Ca, and Mg intercalation process was evaluated in the functionalized MXenes by systematically inserting atoms. The electrochemical properties are investigated by calculating the open-circuit voltage as a function of theoretical gravimetric capacities. Results demonstrate that Ti₂NbC₂(OH)₂ is unfavorable for energy storage applications. At the same time, Cl- and F-functionalized MXenes provide the lower theoretical gravimetric capacities of less than 100 mAh g^-1 for alkali metal ions and are unstable for alkaline earth elements. On the other hand, Ti₂NbC₂O₂ MXene shows an excellent performance. The theoretical gravimetric capacities for Li, Na, and Mg ions are 274, 219, and 438 mAh g^-1, respectively. Similar values to those reported for low atomic weight MXenes, evidencing their capacity to store metal ions. Our findings demonstrate the capacity of oxidized Ti₂NbC₂ to be implemented in energy storage devices.