Trapping and Redistribution of Hydrophobic Sulfur Sols in Graphene-Polyethyleneimine Networks for Stable Li-S Cathodes

The lithium–sulfur battery is a promising next-generation rechargeable battery system which promises to be less expensive and potentially fivefold more energy dense than current Li-ion technologies. This can only be achieved by improving the sulfur utilization in thick, high areal loading cathodes while minimizing capacity fading to realize high practical energy densities and long cycle-life. This study reports a simple method to fabricate a high capacity, high loading cathode with one of the highest cycle-stabilities reported. It is demonstrated that sulfur sols formed by crashing dissolved elemental sulfur into water are trapped between graphene oxide sheets when flocculated with polyethyleneimine. Low temperature, hydrothermal treatment produces a conductive, partially covalent composite exhibiting outstanding cycle-stability. Using this method, sulfur can be uniformly distributed at fractions as high as 75.7 wt%. Electrodes with high areal sulfur loadings (up to ≈5.4 mg cm⁻²), prepared using these composites, lead to projected high cell level practical energy densities of 400 Wh kg⁻¹. The electrodes demonstrate negligible capacity loss over 250 cycles at 0.15 C and only 0.028% capacity loss per cycle over 810 cycles at 0.75 C. Eventual capacity fading is found to be linked to degradation of lithium-metal anode suggesting that the cathode material remains stable over even more extended cycling.