Design and Development of High-Loading Carbon-Sulfur Nanocomposite Cathodes with Drop-Casting Method
Authors: Chung Sheng-Heng , Yu Chien-Hsun 
Published: July 2021

Advanced rechargeable batteries that can be widely applied in different energy-storage systems require a high energy density, a stable cyclability, and low costs in materials and fabrications. The commercial lithium-ion batteries exhibit high discharge capacity with a long cycle life, which allows them to dominate the current rechargeable battery markets. With the continuous development, the lithium-ion battery chemistry would soon encounter the theoretical material limitations. The mature fabrication technique and materials science have promoted the lithium-ion battery cathodes to meet their theoretical charge-storage capacity values. Moreover, the lithium insertion compounds have to maintain a stable crystal structure by limiting the full electrochemical utilization of the active material during cycling. Furthermore, with an increasing demand for lithium-ion battery products, the high prices and limited availability of the raw materials impact the market and the development of future lithium-ion batteries.

To address these issues, a conversion-reaction cathode bypasses the limitation of low theoretical charge-storage capacity and limited electrochemical utilization garner significant attention. Among various conversion-reaction cathodes, the lithium-sulfur system is the most promising candidate for developing the post-lithium-ion technology with higher energy density at an affordable cost. This is primarily due to its naturally-abundant and low-cost sulfur cathode possesses a high charge-storage capacity of 1,672 mA∙h g-1, which makes the lithium-sulfur cells to approach a high energy density of about two to three times higher than that of current lithium-ion batteries. However, many obstacles limit the fast commercialization of lithium-sulfur technology. The low electronic conductivity values of sulfur and its end-discharged sulfides affect the electrochemical utilization of the active material and the development of a high-loading sulfur cathode for demonstrating a high energy density. In between sulfur and sulfides, the polysulfide intermediates generated during charge and discharge states are liquid-state active materials and have high solubility toward the liquid electrolyte. The dissolution of polysulfides would irreversibly diffuse out from the cathode and uncontrollably migrate in the cell, which causes the loss of active material from the cathode, the contamination of electrolyte, and the corrosion of the lithium metal anode. Consequently, the polysulfide issues result in poor electrochemical stability and further impact the high-loading cathode performance.

This presentation will focus on the use of a drop-casting method to fabricate a high-loading carbon-sulfur nanocomposite cathode. Subsequently, we apply the developed high-loading nanocomposite cathode for exploring the effect of carbons’ nanopore sizes toward the electrochemical performance of sulfur cathodes. First, the drop-casted carbon-sulfur nanocomposite cathodes are fabricated with various carbon-sulfur nanocomposites. Super P-Sulfur, Ketjenblack-Sulfur, Black Pearl-Sulfur, and Vulcan Black-Sulfur nanocomposites featuring the nonporous Super P carbon, the mixed micro/mesoporous Ketjenblack carbon, the microporous Black Pearl carbon, and the macroporous Vulcan Black carbon are synthesized by the melt-sulfur diffusion method. The carbon-sulfur nanocomposites are mixed with liquid electrolyte to form a paste, followed by drop-casting on the current collector. These carbon-sulfur nanocomposite cathodes attain and are fixed at a high sulfur loading and sulfur content of 4 mg cm-2 and 60 wt%, respectively. Our findings indicate that the Black Pearl-sulfur nanocomposite cathode exhibits the best overall electrochemical performances featuring the high rate capability from C/20 to C/3 rates, stable long-term cyclability, and high capacity retention of above 80% after 100 cycles. The Ketjenblack-sulfur nanocomposite cathode with mixed micro/mesoporous carbon shows similar performances, while relatively lower than those obtained by Black Pearl-sulfur nanocomposite. With no nanopores and with macropores, Super P-Sulfur and Vulcan Black-sulfur nanocomposites display short cycle life in 40 cycles and low electrochemical utilization of sulfur, respectively. As a result, a carbon-sulfur nanocomposite with microporous carbon as the host would give the excellent polysulfide retention and the active-material utilization. With a nonporous carbon substrate, the carbon-sulfur nanocomposite might encounter the diffusion of polysulfides from the cathode electrode during cycling. With a macroporous carbon, the carbon-sulfur nanocomposite might also face the polysulfide diffusion issue, while mainly the diffusion of polysulfides from the composite. All these results demonstrate that the drop-casting method is a promising solution to develop high-loading sulfur cathode. Moreover, the further optimization of the nanocomposite might improve the electrochemical performance of the resulting high-loading nanocomposite sulfur cathode.