AB-stacked bilayer β12-borophene as a promising anode material for alkali metal-ion batteries

As the lightest 2D material, monolayer borophene exhibits a specific charge capacity of 1860 mA h g(-1) for Li-ion batteries, which is four times higher than that of graphite and is one of the highest specific charge capacities ever reported for 2D anode materials. Additionally, it showed high mechanical strength and a low diffusion barrier. However, monolayer borophene suffers from stability issues in its free-standing form, which restricts its real-life applications. Inspired by the recent experimental investigations, which proved the higher stability of bilayer borophene polymorphs (BBPs) over their monolayer counterparts, in this work, we investigated the dynamical and thermodynamical stabilities of both AA- and AB-stacked BBPs in their beta(12) phase using first-principles calculations. Between the two stacking patterns, we found that only the AB-stacked beta(12)-BBP is both energetically and dynamically stable, and we further investigated its potential as a high-performance anode material for alkali metal-ion batteries. Our investigations show that AB-stacked beta(12)-BBP exhibits good electrical conductivity before and after metal atom (Li/Na/K) adsorption onto it. Further, AB-stacked beta(12)-BBP adsorbs the metal atoms strongly with adsorption energies ranging between -0.89 to -1.44 eV, indicating that there is a lesser possibility of forming dendrites on this anode. Similarly, it has a low diffusion energy barrier (similar to 0.13-0.49 eV) for metal atoms, meeting the fast charge/discharge rate requirements. Moreover, it exhibits a reasonably low average metal-insertion voltage (0.43 to 0.65 V) and a specific charge capacity of 330-413 mA h g(-1) that is comparable to graphite. All the above findings suggest that the AB-stacked bilayer beta(12)- borophene can be a potentially favorable anode material.