Confining Sulfur Species in Cathodes of Lithium-Sulfur Batteries: Insight into Nonpolar and Polar Matrix Surfaces
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To alleviate polysulfides shuttle effect in lithium-sulfur batteries (LSBs), the use of a functionalized carbon matrix with polar surface has been widely reported to chemically bind the soluble polysulfides. However, whether and how such a polar carbon surface affects the overall cathode performance, particularly the initial discharge corresponding to the reduction of cyclooctasulfur (S8), has not caught enough attention. By combining polar and nonpolar carbon matrix surfaces in different configurations through sandwiching sulfur species between two carbon matrix membranes, we found cells with dramatically different performance. The discharge process at different states, particularly the charge transfer resistances corresponding to nonpolar S8 and polar polysulfide intermediates and the final Li2S, were investigated. The experimental results, further supported by first-principles density functional theory calculations, indicate that the adsorption energy and barrier for electron transfer together affect the electrochemical performance of LSBs, and therefore a rational design that combines polar and nonpolar surfaces should be adopted.