In Situ Synthesis of Bipyramidal Sulfur with 3D Carbon Nanotube Framework for Lithium-Sulfur Batteries

A one-pot synthesis of three-dimensional carbon nanotube frameworks with bipyramidal sulfur particles and the application of these materials for a cathode in lithium-sulfur (Li-S) battery are reported. By simple mixing of multi-walled carbon nanotubes (MWCNTs), sulfur powder, and capping agents in water/tetrahydrofuran, micrometer bipyramidal sulfur particles enclosed with MWCNTs are synthesized. The MWCNTs spontaneously form a 3D conducting network inside and outside the sulfur particle. Along the edge of MWCNT framework, a sulfur particle-free region is present, which comprises approximate to 35 vol% based on the total volume. These sulfur-MWCNT bipyramidal particles are mixed with conductive carbon additive to prepare binder-free cathode for Li-S cells. The Li-S cells deliver a specific discharge capacity of approximate to 1600 mAh g(-1) at 0.05 C on the first cycle. In particular, these Li-S cells show high rate stability and Coulombic efficiency with deep discharge and charge (1.0-3.0 V vs Li/Li+). This resultant performance can arise from 1) homogeneous distribution of the conducting MWCNT framework and the carbon additive coating layer on the sulfur particle, which allow rapid Li+ ion/electrolyte diffusion and mitigation of polysulfide shuttle, respectively, and 2) the sulfur-free buffer space accommodating volume expansion. It is expected that this new cathode design with the simple synthetic process can reduce the number of preparation steps, thus allowing the construction of a low-cost Li-S battery.