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 determined by the nature of the carbon materials and the distribution of sulfur within the composite electrode. The role of these major factors is poorly understood. The present study uncovers the factors that contribute to the internal resistance and rate capability of Ketjenblack-carbon based sulfur cathodes. Despite their 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. We find that a large contribution to the polarization resistance arises from inter-particulate contacts and charge-transfer processes occurring close to 30% depth of discharge. Insulating sheets of sulfur on the Ketjenblack particles require an additional electron-percolation pathway for reducing the polarization. Addition of a low-surface area carbon, Super-P, provides such an electron conduction pathway resulting in an impressive and stable capacity of 950 mAh g-1 at C/5 rate. We anticipate that the detailed analysis of impedance and the new learnings will provide the insight for improving the design of the sulfur cathode.