Strongly trapping soluble lithium polysulfides using polar cysteamine groups for highly stable lithium sulfur batteries

Abstract

Sulfur has become one of the most promising positive electrode materials for lithium sulfur batteries due to its high theoretical capacity and high energy density (2500 Wh kg−1). The use of common nonpolar carbon/sulfur composites has proved to be a good way to improve the performance, but they still cannot efficiently trap highly polar lithium polysulfides due to the weak interactions between nonpolar carbon and polar polysulfides. Herein, we report a new strategy of using polar cysteamine groups to trap polar polysulfides, leading to greatly enhanced capacity of ∼920 mAh g−1 at 1 C with a high Coulombic efficiency of ∼99.1%, and a long cycle life of over 600 cycles with a capacity retention higher than 80%. Importantly, in situ UV/Vis spectroscopy was employed to identify intermediates during cycling, which demonstrates the constructed unique polar cysteamine functionalized carbon nanotubes (CNTs) can greatly reduce the production of polysulfides and suppress the shuttle effect. The broken-bond model of linear polysulfane during cycling was further demonstrated by density functional theory calculations. The present strategy of using polar cysteamine-functionalized CNTs to trap soluble intermediates is promising and has significant potential for the development of highly efficient lithium sulfur batteries.