Two-dimensional TiCl2: a high-performance anode material for Na-ion batteries with high capacity and fast diffusion.

Na-ion batteries (NIBs) have attracted a great deal of attention for large-scale electric energy storage due to their inherent safety, natural abundant resources, and low cost. The exploration of suitable anode materials is the major challenge in advancing NIB technology. On the basis of first-principles calculations, we systematically explore the potential performance of two-dimensional (2D) TiCl2 as an electrode material for NIBs. Monolayer TiCl2 can be easily exfoliated from the bulk structure with a small exfoliation energy of 0.64 J m-2. It shows good stability, as demonstrated by its high cohesive energy, positive phonon modes, and high thermal stability. Monolayer TiCl2 has high storage capacity (451.3 mA h g-1), low diffusion energy barrier (0.02-0.14 eV), moderate average open-circuit voltage (0.81 V), and small lattice change (2.37%). Moreover, bilayer TiCl2 can significantly enhance the Na adsorption strength but reduce the Na-ion diffusion ability. These results suggest that TiCl2 is a promising anode candidate for NIBs.