Heteroatom functionalization of Ti₂C MXenes for enhanced Li-ion storage performance

Surface functionalization provides an effective approach for tailoring the electrochemical properties of MXenes for lithium-ion battery applications. In this study, density functional theory (DFT) calculations were performed to investigate the effect of sulfur (S) and nitrogen (N) terminations on the lithium-storage performance of Ti₂C MXene. Pristine Ti₂C together with S-terminated (Ti₂CS₂) and N-terminated (Ti₂CN₂) monolayers were investigated by evaluating their structural stability, electronic properties, Li adsorption, diffusion behavior, open-circuit voltage and theoretical specific capacity. All three monolayers are energetically stable and retain their metallic character after lithiation, ensuring continuous electronic conductivity during battery operation. Surface functionalization increases the maximum Li atoms from x _max = 2 in Li _x Ti₂C to x _max = 4 in both Li _x Ti₂CS₂ and Li _x Ti₂CN₂ resulting in theoretical specific capacities of 497.5, 623.7, and 789.7 mAh g^-1, respectively. The calculated Li-ion diffusion barriers of 0.24, 0.28 and 0.32 eV, together with the favorable open-circuit voltages (OCV) 0.87-0.08 V, 0.81-0.04 V and 0.68-0.02 V for Ti₂C, Ti₂CS₂, and Ti₂CN₂ respectively, indicating fast Li-ion transport. Overall, sulfur and nitrogen terminations significantly improve the lithium-storage characteristics of Ti₂C MXene, with Ti₂CN₂ exhibiting the highest theoretical capacity and Ti₂CS₂ offering a favorable balance between capacity and Li-ion mobility.