Janus MgAlB2 MBene: a dipole-engineered anode for ultrafast Li-ion transport and exceptional lithium storage.

In this work, we propose a group II/IIIA-based Janus MBene, MgAlB2, and investigate its electrochemical properties using first-principles calculations. The substitution of one Mg layer in Mg2B2 MBene by an Al layer breaks the structural symmetry and generates a permanent out-of-plane polarization, giving rise to a distinct electronic environment compared with the parent Mg2B2 and Al2B2 monolayers. Electronic-structure analysis reveals enhanced orbital hybridization among B, Mg, and Al states near the Fermi level, resulting in improved electronic delocalization across the monolayer. The Janus MgAlB2 monolayer is found to possess excellent dynamical, mechanical, and thermal stability. Owing to its polarization-modified energy landscape, Li ions migrate with an exceptionally low diffusion barrier of 17.1 meV, corresponding to a room-temperature diffusion coefficient of 1.28 × 10-3 cm2 s-1. Unlike the pristine Mg2B2 and Al2B2 monolayers, which support only a single stable adsorption layer, MgAlB2 accommodates two complete Li layers. Detailed analysis shows that the residual polarization retained after first-layer lithiation continues to promote Li adsorption, whereas increasing Li-Li electrostatic interactions eventually limit further storage. As a result, the Janus monolayer delivers a high theoretical specific capacity of 1470.24 mAh g-1 together with a small volume expansion of only 3.7% during maximum lithiation. The calculated open-circuit voltage exhibits an average value of approximately 1.20 V, indicating a favorable operating voltage for an anode material. These results demonstrate that Janus-induced dipole-engineering can simultaneously regulate the thermodynamics and kinetics of Li storage, providing an atomistic strategy for designing high-capacity, fast-charging, and structurally favorable two-dimensional anode materials for lithium-ion batteries.