Bifunctional 6-bromohexanenitrile additive enhancing lithium anode protection and sulfur redox reactions for high-stability lithium-sulfur batteries

Abstract

Lithium-sulfur (Li-S) batteries are considered promising candidates for next-generation energy storage technologies due to their high theoretical energy density. However, the instability of the lithium anode and the sluggish redox kinetics of sulfur significantly hinder their practical application. To overcome these challenges, the development of electrolyte additives with dual functions, protecting the lithium anode and promoting sulfur redox reactions has emerged as a key strategy for breaking through current performance bottlenecks. In this study, the effect of 6-bromohexanenitrile (BHN) as a bifunctional electrolyte additive on Li-S battery performance is investigated. The findings reveal that the bromo terminal group in BHN participates in the formation of the SEI layer on the lithium anode, generating inorganic compounds such as LiBr, thereby enhancing interface stability, suppressing lithium dendrite growth, and protecting the lithium anode from side reactions with polysulfides. Furthermore, density functional theory (DFT) calculations show that the BHN additive can effectively modulate the molecular orbital energy levels of lithium polysulfides, resulting in reduced molecular orbital energy gaps and enhanced redox activity. This accelerates polysulfide interconversion and effectively improves interfacial reaction processes. Li-S cells incorporating BHN exhibit an initial discharge specific capacity of 1360 mAh g− 1 at a 0.1 C rate. Moreover, at a current density of 0.2 C, the cells demonstrate excellent cycling stability, retaining 80.4% of their capacity after 120 charge-discharge cycles. This work presents a new strategy for the design of high-performance lithium-sulfur battery electrolytes and provides valuable theoretical insights into the development of bifunctional electrolyte additives.