Theoretical Insights into the Favorable Functionalized Ti₂C-Based MXenes for Lithium-Sulfur Batteries

Because of the high specific surface area, excellent electronic conductivity, facile Li diffusion, and rich functional groups, Ti₂C-based MXenes have been widely used to improve the electrochemical property of lithium-sulfur batteries. The complex surface functionalization (such as -OH, -S, -F, and -O) of MXenes boosts the performance but also causes controversies about the favorable functionalized surface in the electrochemical reaction during the charge and discharge process. In the present work, a theoretical study based on density functional theory has been carried out to clarify the favorable functionalized surface by comparing pristine Ti₂C and -OH-, -S-, -F-, and -O-functionalized Ti₂C surfaces from the aspects of adsorption ability, electronic conductivity, and kinetic conversion ability. It is found that compared with severe polysulfide deformation on pristine Ti₂C and Ti₂C(OH)₂ surfaces, Ti₂CO₂, Ti₂CS₂, and Ti₂CF₂ have effective polysulfide adsorption. Ti₂CO₂ has the largest surface adsorption energy, followed by Ti₂CS₂, and Ti₂CF₂ is the weakest. Meanwhile, the narrow-band gap semiconductor property of Ti₂CO₂ during adsorption indicates worse electronic conductivity than metallic Ti₂CS₂ and Ti₂CF₂. In addition, for the kinetic conversion ability, the Ti₂CS₂ surface has the fastest polysulfide conversion and Li diffusion, followed by Ti₂CF₂, and Ti₂CO₂ represents the slowest conversion and diffusion. Accordingly, because of the medium binding energy, good electronic conductivity, and fast polysulfide conversion and Li diffusion, Ti₂CS₂ is revealed to be the favorable functionalized surface. More importantly, the origin for the Ti₂CS₂ surface with medium adsorption ability represents the fastest polysulfide conversion, and Li diffusion is further clarified. The great affinity of the Ti₂CS₂ surface to the product Li₂S leads to facile polysulfide conversion. The uniform charge distribution on the Ti₂CS₂ surface contributes to the fast Li diffusion.