Ultrahigh Storage Capacity of Alkali Metal Ions in Hexagonal Metal Borides with Orderly Multilayered Growth Mechanism

The global energy shortage and the gradual depletion of lithium resources have become increasingly prominent. Improving the energy density of lithium-based secondary batteries and developing other high-performance alkali-metal secondary batteries have become the research focus. In this study, two-dimensional (2D) hexagonal metal borides (<i>h</i>-MBenes) are investigated as ordered alkali metal adsorption substrates for alkali-metal-based battery anode materials using density functional theory (DFT). Twelve thermodynamically stable <i>h</i>-MBenes are screened out from thirty-three structures, and their excellent stability and metallic electronic characteristics are confirmed. The ordered multilayered growth in alkali metal adsorption is found to depend on two factors: low lattice mismatching and dynamic matching of the work function. In particular, Mg/Al/V-based <i>h</i>-MBenes exhibit excellent lithium lattice matching (<3.35% mismatch), enabling layer-by-layer hexagonal (001) Li growth for ≥5 layers. They have ultrahigh lithium capacities (2170-3818 mAh·g^-1), low migration barriers (0.01-0.05 eV), and low voltages (0.003-0.714 V). Mg/Y-based <i>h</i>-MBenes enable three Na layers' adsorption with a capacity of 1717/605 mAh·g^-1, and Al₂B₂ achieves a 472 mAh·g^-1 potassium storage capacity, respectively. Due to the orderly multilayered growth mechanism, Mg/Al/V-based <i>h</i>-MBenes show great potential as high-safety and ultrahigh-capacity alkali-metal battery anode materials.