Borophene as a promising anode material for sodium-ion batteries with high capacity and high rate capability using DFT.

Two-dimensional boron synthesized by the chemical vapor deposition method is an atomically thin layer of boron with both light weight and metallicity. To investigate the potential of borophene as an anode material in sodium-ion batteries, first-principles calculations and molecular dynamics simulations were carried out. The calculated results reveal that after introducing vacancy defects, the special puckered structure becomes relatively flat and the metallic nature of the defective borophene is enhanced, while the defects in borophene can weaken sodium adsorption. A single sodium atom is preferentially absorbed on the B site. The adsorption energies gradually reduce with an increase in sodium concentration due to the increased Na-Na repulsion. The fully sodium storage phase of borophene corresponds to NaB with a theoretical specific capacity of 1240 mA h g, which is much larger than that of other two-dimensional materials. Most interestingly, sodium ion flows in the furrows of puckered borophene are extremely fast with a low energy barrier of 30 meV. Meanwhile, sodium diffusion on borophene was found to be highly anisotropic, as further verified by the results of the molecular dynamics simulations. The sodiated-borophene nanostructure shows enhanced electronic conductivity during the whole sodiation process, which is superior to other anode materials. Borophene is expected to be a promising candidate with high capacity and high rate capability for anode materials in sodium-ion batteries.