Ultrahigh Capacity 2D Anode Materials for Lithium/Sodium-Ion Batteries: Entirely Planar B7P2 monolayer with Proper Pore Size and Distribution

Lithium-ion batteries (LIBs) are widely used energy storage devices, and sodium-ion batteries (SIBs) are promising alternative to LIBs because sodium is of high abundance and low toxicity. However, a dominant obstacle for the advancement of LIBs and SIBs is the lack of high capacity anode materials, especially for SIBs. Here, we propose that three characteristics, namely appropriate pore size, proper porous distribution, and an entirely planar topology, can help achieve ultrahigh capacity 2D anode materials. Under such guidelines, we constructed the B7P2 monolayer, and investigated its potential as LIB/SIB anode materials by means of density functional theory (DFT) computations. Encouragingly, the B7P2 monolayer possesses all the essential properties for a high-capacity LIB/SIB anode: its high stability ensures the experimental feasibility for the synthesis, its metallicity does not change upon Li/Na adsorption and desorption, the Li/Na can well diffuse on the surface, and the open-circuit voltage is in the good range. Most importantly, B7P2 monolayer has a high storage capacity of 3117 mA h g-1 for both LIB and SIBs, this capacity value ranks among the highest for 2D SIB anode materials. This study not only identifies a promising LIB/SIB anode material with a ultrahigh capacity, but also provides good clues to design/discover other anode materials.