Tuning the Linkers in Polymer-Based Cathodes to Realize High Sulfur Content and High-Performance Potassium-Sulfur Batteries

The development of effective rechargeable potassium-sulfur (K-S) batteries has been retarded by a severe polysulfide shuttle effect and low sulfur content in the cathode (generally less than 40 wt %). Herein, a series of sulfur-linked polymers with different numbers of allyloxy linkers have been chosen to explore their effect on the performance of K-S batteries. By taking sulfur-linked tetra(allyloxy)1,4-benzoquinone polymer (poly(S4-TABQ)) as the cathode of K-S batteries, its maximal sulfur content reaches similar to 71 wt %, which displays a high capacity retention of 94.5% after 200 cycles (only 0.027% loss per cycle). Theoretical analyses and density functional theory calculations show that the abundant allyloxy linkers in poly(S4-TABQ) cathodes play a vital role in inhibiting the polysulfide shuttle effect and realizing high capacity, owing to the strong interaction of the allyloxy moieties with potassium polysulfides and the accelerated charge transfer during charge/discharge process. Moreover, ex situ X-ray photoelectron spectroscopy and ultraviolet-visible spectroscopy analysis are conducted to explore the electrochemical mechanism in poly(S4-TABQ) cathodes, indicating that -C-S-n(-).K+ (n = 2-6) and K2Sn (n = 2-6) coexist in K-S batteries without the formation of K2S groups. This study provides a novel insight into the application of sulfur-linked polymers as cathode material in potassium-sulfur batteries.