Synergic effect of covalent and chemical sulfur fixation enhancing the immobilization-conversion of polysulfides in lithium-sulfur batteries

Lithium-sulfur batteries (LSBs) are promising as the next generation energy storage options. However, their wide applications have been technically challenged by the diffusion losses of polysulfides and polysulfide shuttle effect. In this work, the small organic molecules of 2,5-dichloropyrazine (2,5-DCP) were combined with Co-doped carbon (CoNC) flakes to achieve the synergic effect of the covalent and chemical sulfur fixation, so as that the immobilization-conversion of polysulfides in LSBs was greatly enhanced. More specifically, the nucleophilic substitution of the 2,5-DCP additive in the electrolyte with polysulfides formed the CS bonds. Through the further covalent N-Li bonds between the N atoms in 2,5-DCP and polysulfides, sulfur fixation was achieved in the form of solid organosulfur. Meanwhile, the CoNC flakes served as the sulfur cathode to chemically anchor the polysulfides. The interaction mechanism between CoNC/2,5-DCP and polysulfides was explored by the density functional theory (DFT) calculations and in-situ infrared spectroscopy. The results showed that the optimal “with 2,5-DCP” sample-assembled LSB exhibited an initial discharge specific capacity of 1244 mA h g−1 at 0.2C, and a capacity decay rate of 0.053% per cycle was displayed after 800 cycles at 1C. The good cycling stability with a high sulfur-loaded electrode sample suggested that the synergic effect of covalent/chemical sulfur fixation enabled the enhancement of polysulfides immobilization-conversion in LSBs.