Understanding diffusion behavior of multiple Li and Na-ions on a β12-borophene electrode: A first-principles study

Abstract

An increasing emphasis on 2D materials such as ${\beta }_{12}$-borophene has emerged in the pursuit of enhancing lithium and sodium-ion batteries, owing to their exceptional structural and electronic properties including low diffusion energy barriers. Although most of the results on diffusion mechanisms have been reported, they are largely limited to infinitely dilute concentrations. In this study, we used density functional theory to investigate the electronic properties of ${\beta }_{12}$-borophene adsorbed with high concentration of ions and determined its use as an electrode in lithium and sodium-ion batteries. Our systematic exploration involved the investigation of adsorption energies and diffusion mechanisms for single ions, vacancy, and knock-off at multiple ion levels. The findings indicate that ${\beta }_{12}$-borophene exhibits spontaneous adsorption energies towards Li and Na ions of -2.56 and -2.70 eV, respectively. Additionally, low open circuit voltages of 0.21 V for Li and 0.91 V for Na were obtained, suggesting that the formation of dendrites can be suppressed. At a high concentration of 24 ions, the storage capacity was calculated to be 1487.68 mAh/g for both Li and Na, surpassing that of commercial graphite electrodes and other 2D materials. We also observed charge transfer from the adsorbates to the substrate, with charge distributions primarily located between the first layers of ions and the substrate, indicating a significant concentration of electrons being transferred towards the substrate. We also investigated the energy barriers associated with diffusing vacancies (Li = 0.55 eV, Na = 0.22 eV) and knock-off mechanisms (Li = 0.56 eV, Na = 0.7 eV) at a high concentration of adsorbed ions and on a supercell that was twice the size of some previous studies. These results revealed varying energy barriers due to the presence of multiple ions, with the knock-off mechanism exhibiting the highest energy barrier. The increased energy barriers due to high concentration of ions is attributed to the repulsive forces between the ions. Furthermore, despite the adsorption of multiple Li and Na ions, ${\beta }_{12}$-borophene maintained its metallic properties, signifying its potential for use in battery operation cycles. Lastly, high structural stability at 300 K confirmed the viability of ${\beta }_{12}$-borophene for normal battery operations. Altogether, these properties underscore the potential of ${\beta }_{12}$-borophene as an effective electrode material for lithium and sodium-ion batteries.