Structural engineering of MXenes for enhanced magnesium ion diffusion: a computational study

The unique layered structure and tunable surface terminations of MXenes play a critical role in Mg²⁺ storage and diffusion dynamics. This study systematically investigates the behavior of Mg²⁺ in Ti₃C₂O₂ and its nitrogen-doped derivatives through theoretical calculations. In Ti₃C₂O₂ monolayers, Mg²⁺ exhibits a high diffusion barrier of 0.81 eV due to strong electrostatic interactions. However, AA-stacking reduces this barrier to 0.32 eV by introducing staggered active sites. The instability caused by interlayer O-O repulsion is mitigated by modulating the N/O ratio (Ti₃C₂O_1.78N_0.22), resulting in a diffusion barrier of 0.27 eV. Transition metal substitution further optimizes performance, as exemplified by Nb₃C₂N₂, which achieves an ultralow barrier of 0.23 eV through weakened N-N covalency and enhanced metal-N interactions. Voltage analysis reveals that Nb₃C₂N₂ possesses dual functionality as both cathode (4.00 V) and anode (0.64 V), contrasting with the anode-specific behavior observed in Ti-based MXenes.