Defective phosphorene as an anode material for high-performance Li-, Na-, and K-ion batteries: a first-principles study.

Single-layer phosphorene is a unique material with distinctive properties resulting in its high potential to be used as an anode in alkali metal ion batteries (AIBs). In this study, the improvement of the adsorption energy, the diffusion, and the storage capacity of alkali metals (Li, Na, and K) in a pristine and defective monolayer phosphorene was systematically studied using first-principles calculations. All possible defects on phosphorene were studied by electronic structure analysis. The pristine phosphorene strongly adsorbed Li, Na, and K with adsorption energies of -2.08 eV, -1.33 eV, and -2.16 eV, respectively. Interestingly, the presence of point defects significantly enhanced the binding of the alkali metals with adsorption energies in the range of 2.04-2.88 eV for Li, 1.05-2.72 eV for Na, and 2.16-3.05 eV for K. The diffusion of these alkali metals over pristine phosphorene was anisotropic, with an energy barrier of 0.1 eV for Li, 0.03 eV for Na, and 0.02 eV for K. More importantly, alkali metals could diffuse between two adjacent grooves in defective phosphorenes with a low energy barrier, which opens a novel channel for alkali metal diffusion. The lowest barrier energy (0.022 eV) was found for K atom in a single-vacancy defective structure, indicating faster migration and fast charge/discharge capability in KIBs. The theoretical Li and Na capacities on pristine phosphorene were calculated as 865 mA h g-1, which reached 882.5 mA h g-1 upon creating defects. An acceptable capacity (435 mA h g-1) was also obtained for K. Furthermore, phosphorenes have relatively low average open-circuit voltages when used as anode materials. These interesting properties indicate that the defective phosphorene has a great potential to be used as electrode material in AIBs, especially for KIBs.