C–S Bonds in Sulfur-Embedded Graphene, Carbon Nanotubes, and Flake Graphite Cathodes for Lithium–Sulfur Batteries

High Resolution Image Download MS PowerPoint Slide Lithium–sulfur (Li–S) batteries are excellent rechargeable battery candidates which are extraordinarily promising as they exhibit superior specific capacity and well-known energy density; they are cost-effective and environmentally benign. Nevertheless, a few technical issues pose a significant challenge on the path to industrial applications, namely, capacity fade and Coulombic efficiency decay, which are inherent in the soluble polysulfide shuttle effect during charge/discharge cycling. Carbon materials which have excellent conductive scaffold and flexible structure with a variety of morphologies can serve as a remedy to this issue. Herein, with a well-designed melt-diffusion procedure, we prepared three carbon-based sulfur-embedded cathodes with diverse structures [graphene, carbon nanotubes (CNTs), and flake graphite]. Sulfur loading varies between 60 and 73 wt %. Among these three carbon/S cathodes, beyond 100 cycles, the graphene/S cathode showed a discharge capacity of 840 mA h g –1 at 0.2 A g –1 current density and its average Coulombic efficiency was above 99.4%, demonstrating the best cycle stability and reversibility. While at a higher current rate, 1 A g –1, CNT/S reaches the best capacity of 518 mA h g –1 among these three cathodes, revealing excellent sulfur utilization under high rate conditions. The X-ray photo spectroscopy shows evidence for chemical bonding between graphene/CNTs surfaces and carbonyl, hydroxyl, and ether groups, resulting in well-confined polysulfides in the cathode side, which significantly restrain the movement of soluble polysulfide in the charging process and efficiently decreases the capacity fading of sulfur. This unique structure is a potential explanation for the outstanding electrochemical performance.