Covalent bonding of sulfur nanoparticles to unzipped multiwalled carbon nanotubes for high-performance lithium-sulfur batteries.

The use of sulfur as a cathode material for lithium-sulfur (Li-S) batteries has attracted significant attention due to its high theoretical specific capacity (1675 mA h g ^-1 ); however, practicality is hindered by a number of obstacles, including the shuttling effect of polysulfides and the low electrical conductivity of sulfur. Herein, ball milling sulfur with unzipped multiwalled carbon nanotubes (UMWNTs) was found to covalently immobilize sulfur nanoparticles to the UMWNTs and resulted in composites (designated as S@UMWNTs) with high electrical conductivity. The unzipping degree of MWNTs was first controlled to optimize the immobilization of sulfur nanoparticles to UMWNTs and the electrochemical performance of the resulting Li-S batteries. The presence of C-S covalent bonds between the UMWNTs and sulfur nanoparticles was verified using x-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy, and the formation of C-S bonds was ascribed to the reactions between the mechanically-induced sulfur radicals and the functional groups of UMWNTs. As a result, when used as a cathode material for Li-S batteries, the S@UMWNTs exhibited excellent electrochemical performance, including a good long-term cycling stability and low capacity decay (e.g., ca. 0.09% per cycle over 500 charge/discharge cycles at 1 C) due to the suppression of the shuttling effect by the C-S covalent bonds.