  Two-dimensional (2D) materials as electrodes is believed to be the trend for
future Li-ion and Na-ion batteries technologies. Here, by using
first-principles methods, we predict that the recently reported borophene (2D
born sheet) can serve as an ideal electrode material with high electrochemical
performance for both Li-ion and Na-ion batteries. The calculations are
performed on the two experimentally stable borophene structures, namely
\b{eta}12 and \c{hi}3 structures. The optimized Li and Na adsorption sites are
identified, and the host materials are found to maintain good electric
conductivity before and after adsorption. Besides advantages including small
diffusion barriers and low average open-circuit voltages, most remarkably, the
storage capacity can be as high as 1984 mA h g-1 in \b{eta}12 borophene and
1240 mA h g-1 in \c{hi}3 borophene for both Li and Na, which is several times
higher than the commercial graphite electrode and is the highest among all the
2D materials discovered to date. Our results highly support that borophenes can
be appealing anode materials for both Li-ion and Na-ion batteries with
extremely high power density.
