Comparative Investigation on Two-Dimensional Ti2CY2 (Y = O, S) MXene/Graphene Van der Waals Heterostructure as Potential Anode Material for Lithium-Ion Batteries: A First-Principles Calculation

Abstract

With the increasing demand for high-performance energy storage devices, the demand for alternative anode materials with high energy density and operational voltage is becoming urgent. Two-dimensional van der Waals (vdW) heterostructures gained popularity due to their large surface area and adjustable interlayer spacing. In this work, we have employed first-principles calculations to compare the structural, electronic, adsorption, and electrochemical properties of O and S functionalized Ti₂CY₂/graphene (Y = O, S) vdW heterostructures. The optimized heterostructure formed by O and S functionalized MXene and graphene layers are separated by 3.04 and 3.40 Å, respectively, giving the binding energy per atom as −0.019 and −0.018 eV. It is found that the intercalation of lithium (Li) atoms in between the Ti₂CY₂/Graphene layers is thermodynamically more favorable in comparison with intercalation on the top or below the heterostructures. The Bader charge transfer analysis confirms that O atoms gain less charge −0.13 e during Li intercalation compared to S atoms with charge transfer of −0.47 e due to the larger size of the 3p orbital of S atoms. Each Li atom contributes ~0.88–0.89 e during the intercalation process. The diffusion energy barrier for lithium atom intercalation is lower for Ti₂CS₂/graphene (0.27, 0.22, 0.12, and 0.18 eV) than for Ti₂CO₂/graphene (0.45, 0.40, 0.34, and 0.28 eV) when + nLi, n = 1, 2, 3, and 17, respectively. The CI-NEB study also confirms that the activation energy barrier decreases with the increase of intercalated Li atoms for both the heterostructures, indicating that Li atoms exhibit weak repulsive interaction. The positive open-circuit voltage (OCV) of less than 2.20 V indicates that both the heterostructures are useful as anode materials. The theoretical specific capacity is 302.36 mAh/g for Ti₂CO₂/graphene and 255.97 mAh/g for Ti₂CS₂/graphene. Ab initio MD simulations reveal that the Li diffusion rate is 8.4 × 10⁻⁷ and 8.5 × 10⁻⁷ cm²/s for Ti₂CO₂/graphene and Ti₂CS₂/graphene. Therefore, both the Ti₂CO₂/graphene and Ti₂CS₂/graphene heterostructures can be considered promising anode materials for Li-ion batteries due to their structural stability, lower diffusion energy barrier, high Li diffusion rate, and positive calculated average voltage of less than 2.2 V.