Understanding the Role of Carbon Mixtures on the Polarization of Sulfur Electrodes in Lithium-Sulfur Batteries

Abstract

The performance of the sulfur-carbon composite cathode in a lithium-sulfur battery is chiefly determined by the nature of the carbon materials, the total sulfur content, and the distribution of sulfur within the composite electrode. Despite years of research, the role of these major factors in achieving optimal cathode performance is not well-understood. The present study uncovers the factors arising from formulation and processing that contribute to the internal resistance and rate capability of Ketjenblack-carbon based sulfur cathodes. Despite its enormous surface area, Ketjenblack-carbon-based sulfur electrodes exhibit surprisingly poor discharge rate capability and a high polarization resistance, providing no more than 180 mAh g−1 at the C/20 rate and delivering a high specific capacity of 1200 mAh g−1 only at a very low discharge rate of C/50. Our studies uncovered that a large contribution to the polarization resistance that arises from inter-particulate contacts and charge-transfer processes occurred close to 30% depth of discharge whereupon sulfur formed thick insulating sheets on the Ketjenblack particles, requiring an additional electron-percolation pathway for reducing the polarization. By the addition of a low-surface area carbon, Super-P®, such an electron conduction pathway could be achieved; the internal resistance of the Ketjenblack cathode decreased by four times and the electrode delivered an impressive capacity of 950 mAh g−1 at C/5 rate. We anticipate that the detailed analysis of impedance and the new learnings from this study will provide the insight needed for improving the formulation and design of the sulfur cathode.