An investigation of halogen induced improvement of β12 borophene for Na/Li storage by density functional theory

Pristine and halogen doped β12 borophene, as anode of lithium-ion batteries (LIBs) and sodium-ion batteries (SIBs), was considered by first-principles study based on density functional theory. Li and Na were adsorbed on β12 borophene with adsorption energies of -3.18 eV and -2.33 eV, respectively. The effect of halogen addition, X = F, Cl, Br, and I, to borophene sheet on adsorption and also diffusion pathways of Li and Na was studied. The adsorption energy calculations show that the halogen atoms improve Li/Na adsorption on borophene sheet. Also, the results indicate that Li/Na adsorption energies on Brominated borophene sheet are higher compared to other halogen types. Diffusion calculations show that Br addition induces an electron deficiency on BoBr surface which lowers the energy barrier of migration of Li and Na ions compared to the pristine borophene. According to density of states analysis, electron charge is transferred from Li and Na atoms toward halogenated borophene sheet. Also, it can be concluded that electron transfer from Li/Na to borophene host in BoX is higher compared to pristine borophene which is in agreement with adsorption energies. The fully lithiated/sodiated complexes of BoBr are Li_0.71BoBr and Na_0.50BoBr which is equivalent to theoretical specific capacities of 1401 and 981 mAh/g which are about 3.5 and 2.6 times higher than graphite for Li and Na adsorption, respectively. Higher specific capacity of Li compared to Na is mainly attributed to steric hindrance of Na regarding its greater size. Open circuit voltage values of 1.6 V and 1.4 V were obtained for Li and Na intercalation processes, respectively, into halogen added β₁₂ borophene indicating that this structure can be applied as anode for both LIB and SIB systems.