Polysulfide Stabilization: A Pivotal Strategy to Achieve High Energy Density Li-S Batteries with Long Cycle Life

摘要

The disproportionation of polysulfide (PS) is a long-neglected and vital issue that causes the fast capacity fading of Li-S batteries. Based on the hard and soft acids and bases (HSAB) theory, a large size N-methyl-N-ethyl pyrrolidinium (MEP+) cation is proposed to complex and stabilize the PS in electrolyte. The disproportionation of PS is successfully suppressed by this simple method, thereby avoiding the precipitation of sulfur in the electrolyte and reducing the loss of the active materials. The mutual interaction mechanism between MEP+ and S-n(2-) in electrolyte is comprehensively investigated and verified for the first time, via both density functional theory (DFT) calculation and experimental characterization. It enables the 5000 mA h Li-S batteries (soft package type) to achieve initial specific energy over 300 Wh kg(-1) and maintain over 65% after 100 charge/discharge cycles at 1/20 C, while merely 24% is remained at 59 cycles without MEP+. This interesting finding is believed to shed light on the further development of Li-S batteries.