Adsorption and diffusion of lithium polysulfides over blue phosphorene for Li-S batteries

Lithium-sulphur (Li-S) batteries suffer from capacity loss due to the dissolution of lithium polysulfides (LiPSs). Although finding cathodes that can trap LiPSs strongly is a possible solution to suppress the "shuttle" effect, fast diffusion of lithium and LiPSs is also pivotal to prevent agglomeration. We report that monolayer blue phosphorene (BP), a recently synthesized two-dimensional material, possesses these characteristics as a cathode in Li-S batteries. Density functional theory calculations showed that while the adsorption energies (E_b) of various LiPSs over pristine BP are reasonably strong (from -0.86 eV to -2.45 eV), defect engineering of the lattice by introducing a single vacancy (SV) increased the binding strength significantly, with E_b in the range of -1.41 eV to -4.34 eV. Ab-initio molecular dynamics simulations carried out at 300 K showed that the single vacancies trap the Li atoms in the LiPSs compared to pristine BP. Projected density of states revealed that the creation of an SV induces metallicity in the cathode. Furthermore, an increase in the adsorption strength did not cause significant structural deformation, implying that the soluble large LiPSs did not decompose, which is essential to suppress capacity fading. The energy barriers for LiPSs' migration over pristine BP are minimal to ensure ultrafast diffusion, with the lowest diffusion energy barriers being 0.23 eV, 0.13 eV and 0.18 eV for Li₂S₄, Li₂S₆ and Li₂S₈, respectively. Furthermore, the energy barrier associated with the catalytic oxidation of Li₂S over pristine and defective BP was found to be greater than three times smaller compared to graphene, which suggests that charging processes could be faster by orders of magnitude. Therefore, BP with a suitable combination of defects would be an excellent cathode material in Li-S batteries.