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.