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.