First principles theoretical of designing sandwich-like metallic BP₄monolayer as anode for alkali metal-ion batteries

Phosphorene has been widely used as anode material for batteries. However, the huge volume change during charging and discharging process, the semiconductor properties, and the high open circuit voltage limit its application. Based on this, by introducing the electron-deficient boron atoms into blue phosphorene, we proposed a P-rich sandwich-like BP₄monolayer by density functional theory calculation and particle swarm optimization. The BP₄monolayer shows good thermodynamic and dynamic stability, as well as chemical stability in O₂atmosphere, mainly due to the strengthened P-P bond of the outer layer by the middle boron atoms adoptingsphybridization. According to the band structure, the BP₄monolayer shows metallic property, which is beneficial to electron conductivity. Furthermore, compared with blue phosphorene and black phosphorene, the P-rich BP₄monolayer shows higher theoretical capacity for Li, Na, and K of 1193.90, 1119.28, and 397.97 mA h g^-1, respectively. The lattice constant of BP₄monolayer increases only 3.73% (Li), 2.52% (Na) after Li/Na fully adsorbed on the anode. More importantly, the wettability of BP₄monolayer in the electrolyte is comparable to that of graphene. These findings show that the stable sandwich-like BP₄monolayer has potential as a lightweight anode material.