Two-dimensional functionalized MBene MgBT (T = O, H, and F) monolayers as anode materials for high-performance K-ion batteries.

Two-dimensional metal borides have received attention as high performance battery anode materials. During the practical application, the 2D surface terminalization is an inevitable problem. This study employs first-principles calculations to investigate the termination of the MgB monolayer with O, H, F, and Cl groups. These structures' stabilities are examined through energetic, mechanical, kinetic and thermodynamic stability studies. Electronic property analysis shows that MgBT (T = O, H, F, and Cl) monolayers are all metallic. Calculated results reveal that the MgBO, MgBH, and MgBF monolayers exhibit high K ion storage capacities (up to 826 mA h g, 980 mA h g, and 804 mA h g, respectively), with diffusion barriers of 0.338 eV, 0.490 eV, and 0.507 eV, respectively. More importantly, the calculated in-plane lattice constants of the substrate materials exhibit a minimal variation and the observed volume expansion is almost negligible (less than 0.08%) during the entire potassization process, which is much lower than that of the pristine MgB monolayer. This structural stability is attributed to the presence of surface functional groups. These results provide helpful insights into designing and discovering other high-capacity anode materials for batteries.