Bifunctional functionalized two-dimensional transition metal borides for fast reaction redox kinetics in lithium–sulfur batteries

Abstract

Lithium-sulfur (Li-S) batteries are regarded as one of the most promising next-generation energy storage systems due to their high theoretical specific energy density and low cost. However, serious shuttle effect and sluggish lithium polysulfides (LiPSs) redox kinetics severely impede the practical application of Li-S batteries. Employing polar sulfur hosts is an effective strategy to alleviate the above problems. Herein, the potential of two-dimensional (2D) Ti2B based sulfur hosts for Li-S batteries were comprehensively explored by using first-principles calculations. The results show that functional groups of Ti2B can significantly modulate its structural properties, thus affecting its interaction with sulfur-containing species. Among S, Se, F, Cl, and Br elements, Ti2B terminated with S and Se atoms possess stronger adsorption capability towards soluble Li2S8, Li2S6, and Li2S4, obviously stronger than organic electrolytes, which indicates that they can completely suppress the shuttle effect. Besides, Ti2BS2 and Ti2BSe2 can powerfully expedite the electrochemical conversion of LiPSs. Moreover, the decomposition energy barrier of Li2S and diffusion energy barrier of single Li ion on them are also fairly low, manifesting their excellent catalytic performance towards the oxidation of Li2S. Finally, Ti2BS2 and Ti2BSe2 always keep metallic conductivity during the whole charge/discharge process. Taking all this into account, Ti2BS2 and Ti2BSe2 are proposed as promising bifunctional sulfur hosts for Li-S batteries. Our results suggest that increasing the proportion of S and Se groups during the synthesis of Ti2B monolayers is greatly helpful for obtaining high-performance Li-S batteries. Besides, our work not only reveals the huge potential of 2D transition metal borides in Li-S batteries but also provides insightful guidance for the design and screening of new efficient sulfur cathodes.