Janus-Structured 2D Transition-Metal Borides as Anode Materials for Metal-Ion Batteries: A Density Functional Theory Study

Two-dimensional (2D) transition-metal borides, the MBenes, are nanomaterials that have attracted widespread attention for their applications as anode materials in metal-ion batteries. 2D Mo2B2 is one such MBene with remarkable electrochemical properties and excellent conductivity. A Janus MBene nanostructure is obtained when the reflection symmetry between the identical transition-metal layers is removed by replacing one of these with a different metal, leading to an asymmetric structure. In this work, we employ plane-wave density functional theory simulations to design and investigate a series of Janus MBenes derived from Mo2B2. The structural and thermal stability of Janus MoMB2 (M = Sc–Cr, Y–Nb, and W) is investigated and confirmed through ab initio molecular dynamics and phonon calculations. The stable Janus MBenes investigated display favorable adsorption for Li, Na, and Mg atoms. Notably, MoVB2 and MoWB2 show appreciable theoretical specific capacities of 1908 and 1067 mAh g–1, respectively, for a magnesium-ion battery and 954 and 533 mAh g–1 for a lithium-ion battery, along with suitable open-circuit voltages. Low diffusion barriers in the range 0.08–0.34 eV indicate fast ion kinetics, while the metallic state is preserved upon adsorption, ensuring efficient charge transport. Functionalization of the 2D material with oxygen leads to enhanced metal adsorption and therefore better storage. Our modeling results imply Janus MBenes as suitable nanomaterials for electrode applications in metal-ion batteries.