Janus MgAlB_2 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, MgAlB_2, and investigate its electrochemical properties using first-principles calculations. The substitution of one Mg layer in Mg_2B_2 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 Mg_2B_2 and Al_2B_2 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 MgAlB_2 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 3.43x10^-10 cm^2/s. Unlike the pristine Mg_2B_2 and Al_2B_2 monolayers, which support only a single stable adsorption layer, MgAlB_2 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 together with a small volume expansion of only 3.7% during maximum lithiation. The present study demonstrates that intrinsic polarization can be utilized to regulate both the thermodynamics and kinetics of Li storage, providing a design strategy for high-rate and high-capacity two-dimensional electrode materials.