Bifunctional functionalized two-dimensional transition metal borides for fast reaction redox kinetics in lithiumesulfur batteries

Abstract

Lithiumesulfur (LieS) 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 LieS 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 LieS batteries was comprehensively explored 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 LieS 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 LieS batteries. Besides, our work not only reveals the huge potential of 2D transition metal borides in LieS batteries, but also provides insightful guidance for the design and screening of new efficient sulfur cathodes.

1. Introduction

Since their successful commercialization by Sony Corporation in 1991, rechargeable lithium-ion (Li-ion) batteries have found a wide range of applications in portable electronics devices and electric vehicles due to high (de)intercalation reversibility and cycling stability of lithium ion [1,2]. However, the energy density of Li-ion batteries is reaching its theoretical limit, though tremendous efforts have been made on battery components and packing engineering. To meet the ever-increasing demand for high energy-density batteries in portable electronic devices and electric vehicles, many post-Li-ion batteries which are composed of lithium or silicon anodes and oxygen or sulfur cathodes have emerged and gained enormous attention as promising alternatives to Li-ion batteries [3,4]. Among these, lithiumesulfur (LieS) batteries are regarded as one of the most viable next-generation energy-storage devices owing to their ultrahigh theoretical specific energy density (2600 W·h·kg−1) assuming a complete conversion reaction of S8+16Li → 8Li2S. Moreover, the light-mass S element, the active material of the cathode, has the advantage of large natural abundance, low cost, and environmental friendliness, further boosting the competitiveness of LieS batteries [5,6].

However, the practical implementation of LieS batteries is plagued with several challenges [7,8]. First, the high electronic resistivity of the active material S8 and the final discharging product (Li2S) badly retards the sulfur reduction/oxidation reaction kinetics. Then, the ~80% volume expansion of the S cathode during the cycling often causes the pulverization of the cathode material. The most serious issue in LieS batteries is the notorious “shuttle effect”, which originates from the dissolution of high-order lithium polysulfides (LiPSs) into the common electrolyte.