Covalently Grafting Sulfur-Containing Polymers to Carbon Nanotubes Enhances the Electrochemical Performance of Sulfur Cathodes

Abstract

Lithium-sulfur (Li-S) batteries have attracted extensive attention due to the high theoretical specific capacity of sulfur (1675 mA h g(-1)). Despite the advantage, the utility of Li-S batteries is limited by the low electrical conductivities of sulfur and its discharge products (i.e., Li2S2 and Li2S) as well as the "shuttle effect" of polysulfides. Herein, a strategy of covalently grafting sulfur-containing polymers to carbon nanotubes (CNTs) is proposed as a solution to the problems associated with sulfur cathodes. The sulfur-containing polymers are grafted to the CNTs using an "inverse vulcanization" method, wherein elemental sulfur is reacted with CNTs that are previously functionalized with isopropenyl groups. A morphological study of the resulting composite indicates that the material maintains the one-dimensional characteristic of CNTs and effectively establishes a porous network via overlap. Such structural features are found to facilitate electron transport in the composite and also suppress the shuttle effect of polysulfides. When compared to cells that are prepared from elemental sulfur, devices that contain the composite show improved specific capacity (i.e., 1368 mA h g(-1) at 0.05 C) and a suppressed capacity fading rate (i.e., 0.074% per cycle at 1 C over 500 cycles). In a broader context, covalently attaching sulfurcontaining polymers to CNTs provides design criteria for realizing cathode materials that may be employed in high-performance Li-S batteries.