Ti₄C₃ and Ti₄N₃ MXenes: ultrafast Na-ion diffusion and figure-of-merit benchmarking for sodium-ion battery anodes

Here, density functional theory calculations are used to evaluate Ti₄C₃ and Ti₄N₃ MXenes as sodium ion-battery (SIB) anodes. Both MXenes offer multiple energetically favorable Na adsorption sites with strong binding, supporting uniform Na accommodation and mitigating Na clustering. The Na adsorption increases the electronic density near the Fermi level that enhances metallic conductivity for efficient charge transport. Importantly, Na diffusion is highly facile with migration barriers of 0.049 eV and 0.028 eV and high theoretical capacities of 314.54 mAh/g and 306.47 mAh/g for Ti₄C₃ and Ti₄N₃ respectively. The predicted open-circuit voltages of 0.259 V (Ti₄C₃) and 0.345 V (Ti₄N₃) fall within the desirable window for stable SIB operation. Surface termination effects are also considered with explicit calculations for –F terminated structures show minimal changes in the predicted anode descriptors, while screening tests indicate that Na adsorption remains favorable on –O and –OH terminations. Finally, we introduce a generalized figure of merit that integrates capacity, open-circuit voltage, and diffusion barrier into a single benchmarking metric, highlighting Ti₄C₃ and Ti₄N₃ as competitive candidates among reported MXene and other two-dimensional anodes.