Assessing lithium storage capacities and ion diffusion dynamics in N-doped double-transition metal Mo2Ti(CxN1-x)2 and Mo2Ti(CxN1-x)2O2 MXenes: A first-principles calculations

We employ the first -principles calculations to investigate the atomic structure, thermodynamic stability, and electrochemical energy storage properties of N -doped carbonitride and nitride double -transition metal MXenes including Mo2TiC2-Mo2TiN2 and Mo2TiC2O2-Mo2TiC2O2 series. The thermodynamic feasibility for the ammonization or nitrogenation of Mo2TiC2 and Mo2TiC2O2 monolayers to obtain Mo2Ti(CxN1-x)2 and Mo2Ti(CxN1-x)2O2 MXenes are evaluated by calculating the reaction enthalpies as a function of N content. It is found that ammonization of both bare and O -terminated MXenes is exothermic with a N content below 50 at.%, and which is slightly endothermic above this dopant concentration. Meanwhile, reaction enthalpy of nitrogenation of Mo2TiC2 or Mo2TiC2O2 MXenes is highly endothermic. Theoretical capacities of bare and O -terminated Mo2Ti (CxN1-x)2 and Mo2Ti(CxN1-x)2O2 MXenes are predicted to be in the range from 160 mAh/g to 350 mAh/g for lithium ion battery (LIB). Due to the presence of a multi -layer adsorption mechanism and the surface conversion reactions, Mo2Ti(CxN1-x)2O2 MXenes exhibit significantly higher theoretical capacities for LIB than bare carbide, carbonitride and nitride MXenes. For all studied bare and O -terminated MXenes, their mean open circuit voltages are found to be less than 1.0 V. For the Li+ diffusion dynamics, the diffusion coefficients of Mo2Ti(CxN1-x)2- Mo2TiN2 and Mo2Ti(CxN1-x)2O2-Mo2TiN2O2 series are predicted to be 0.45 - 0.75 x 10-8 m2/s and 0.1 - 0.2 x 10-8 m2/s at 350 K, respectively. The diffusion energy barrier heights of Li+ on Mo2Ti(CxN1-x)2 and Mo2Ti(CxN1- x)2O2 MXenes are found to be less than 0.1 eV for energetically favorable migration pathways. Overall, we predict that O -terminated carbonitride MXenes could possess both high storage capacity and high charge/ discharge rate capability for their use as electrode materials in LIB.