  Combining first-principles density functional method and crystal structure
prediction techniques, we report a series of hexagonal two-dimensional (2D)
transition metal borides (TMBs) including Sc2B2, Ti2B2, V2B2, Cr2B2, Y2B2,
Zr2B2, and Mo2B2. Their dynamic and thermal stabilities are testified by phonon
and molecular dynamics simulations. We investigate the potential of 2D Ti2B2
monolayer as the anode material for Li-ion batteries (LIBs) and Na-ion
batteries (NIBs). The Ti2B2 monolayer possesses high theoretical specific
capacities of 456 and 1027 mAhg-1 for Li and Na, respectively. The very high
Li/Na diffusivity with ultralow energy barrier of 0.017/0.008 eV indicates an
excellent charge-discharge capability. In addition, the good electronic
conductivity during the whole lithiation process is found by electronic
structure calculations. The very small change in volume after the adsorption of
one, two, and three layers of Li and Na ions indicates that the Ti2B2 monolayer
is robust. These results highlight the suitability of Ti2B2 monolayer as well
as the other 2D TMBs as excellent anode materials for both LIBs and NIBs.
