An electrolyte additive of bromoxoindole enables uniform Li-ion flux and tunable Li2S deposition for high-performance lithium–sulfur batteries

Abstract

Low discharge capacity and rapid capacity decay are the main causes that hinder the commercialization of lithium–sulfur (Li–S) batteries. A feasible strategy is to modify electrolytes which aims at accelerating sulfur conversion reactions and restraining side reactions on both electrodes. However, simultaneously increasing the reactivity of sulfur and reducing the reactivity of Li poses great challenges. Herein, an electrolyte additive of 6-bromoxoindole (BOD) was introduced into Li–S batteries. Through a systematic study, it is found that the BOD can dissolve in electrolytes, undergo partial deprotonation, and react with LiTFSI/LiPSs to form BOD-Li-TFSI/BOD-Li-PS solvation structures. The Li⁺ transfer kinetics and sulfur conversion reactions were significantly enhanced with 0.5 wt% BOD addition, and meanwhile, the Li anode maintained stable plating/stripping behavior. As a result, the cell delivered a high discharge capacity of 811 mA h g⁻¹ at a sulfur loading of 5 mg cm⁻² and E/S ratio of 8 μL mg⁻¹ at ambient temperature and could also work stably at a low temperature of −20 °C.