Free-standing, High Areal Loading Sulfur-Carbon Nanofiber for Lithium-Sulfur Batteries
Lithium-sulfur (Li-S) batteries are promising next-generation lithium-ion batteries due to their high energy density and relatively inexpensive comprising materials. Nevertheless, the most critical issues inhibiting their commercialization are low sulfur loading, low volumetric density, and poor cyclability. Increasing the sulfur loading is challenging because of the insulating nature of sulfur; thus, materials such as carbon are often incorporated as a conductive medium. However, such a strategy will decrease the cell volumetric density. This work aims to increase the sulfur loading and volumetric density of Li-S batteries by utilizing highly electronically conductive carbon nanofibers (CNFs) as a sulfur host. CNFs were fabricated by electrospinning, and sulfur was incorporated into CNFs by vacuum filtration. The sulfur-CNFs electrode itself is free-standing; therefore, no current collectors is needed. Consequently, the sulfur carbon nanofibers (S-CNFs) electrode in our group achieves a high capacity and long cycle life with sulfur loading up to 5.2 mg cm-2. This excellent performance is mainly due to the electronic conductive pathway introduced by CNFs and MWCNTs framework. The porous structure of electrodes, in addition, enables more electrolyte penetration, which decreased its electrolyte-to-sulfur ratio to 10 mL/mg(sulfur), four times lower than that of the conventional slurry casted electrode. Our preliminary results of S-CNFs electrodes in coin cells and pouch cells demonstrate improved performance and cyclability with high sulfur loading. In pouch cells equipped with S-CNFs electrodes, they are able to operate at cycling rate up to 0.25C, and in coin cells, they could cycle at 0.5C rate for 200 cycles.