First-Principles Calculations of Ti₂N and Ti₂NT₂ (T = O, F, OH) Monolayers as Potential Anode Materials for Lithium-Ion Batteries and Beyond

The potential of a Ti2N monolayer and its Ti2NT2 derivatives (T = O, F, and OH) as anode materials for lithium-ion and beyond-lithium-ion batteries has been investigated by the first-principles calculations. The bare and terminated monolayers are metallic compounds with high electronic conductivity. The diffusion barriers on bare Ti2N monolayer are predicted to be 21.5 meV for Li+, 14.0 meV for Na+, 7.0 meV for K+, 75.9 meV for Mg2+, and 38.0 meV for Ca2+, which are the lowest values reported for state-of-the-art two-dimensional energy storage materials. The functional groups on Ti2NT2 increase the diffusion barriers by about 1 order of magnitude. The calculated capacities for the monovalent cations on Ti2N and Ti2NT2 are close to that of the conventional graphite anode in lithium-ion batteries. In comparison, the capacities for Mg2+ on Ti2N and Ti2NT2 are more than 2000 mAh g–1 due to the two-electron reaction and multilayer adsorption of Mg2+. Comparison of the electrochemical performances of Ti2N and Ti2C suggests that Ti2N is a more promising anode material than Ti2C due to its lower diffusion barriers for various cations.